Final Report AFF

Transcription

1 Final Reprt Prject Title: Cure Cycle Optimizatin in Plymer Cmpsites Grant# AFF Prgram Manager: PI: Dr. Charles Lee AFOSR/NL 801 N. Randlph St., Ste 732 Arlingtn, VA Phne: Fax: charles.lee(2),afsr.af.mil Prf. Madhu S. Madhukar 316 Perkins Hall The University f Tennessee Knxville TN Phne: Fax: mmadhuka(a),utk.edu Date Submitted: April 25, 2000 DTIC QUALITY INSPECTED /18 062

2 REPORT DOCUMENTATION PAGE Public reprting burden fr this cllectin f infrmatin is estimated t average 1 hur per respnse, including the time fr reviewing data needed, and cmpleting and reviewing this cllectin f infrmatin. Send cmments regarding this burden estimate r any trv this burden t Department f Defense, Washingtn Headquarters Services, Directrate fr Infrmatin Operatins and Reprts (070' 4302 Respndents shuld be aware that ntwithstanding any ther prvisin f law, n persn shall be subject t any penalty fr fa valid OMB cntrl number PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS. 1. REPORT DATE (DD-MM-YYYY) 4/25/ TITLE AND SUBTITLE 2. REPORT TYPE Final Reprt Cure Cycle Optimizatin in Plymer Cmpsites AFRL-SR-BL-TR-00-6^/j 3. DATES COVERED (Frm - T) 3/1/96-2/28/99 5a. CONTRACT NUMBER AF F b. GRANT NUMBER c. PROGRAM ELEMENT NUMBER lgthe ucing 02- irrently 6. AUTHOR(S) Madhu S. Madhukar 5d. PROJECT NUMBER 5e. TASK NUMBER 5f. WORK UNIT NUMBER 7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) The University f Tennessee Andy Hlt Twer Knxville TN SPONSORING / MONITORING AGENCY NAME(S) AND ADDRESS(ES) AFOSR/NL 801 N. Randlph St., Ste 732 Arlingtn, VA PERFORMING ORGANIZATION REPORT NUMBER SPONSOR/MONITOR'S ACRONYM(S) CYCL 11. SPONSOR/MONITOR'S REPORT NUMBER(S) 12. DISTRIBUTION / AVAILABILITY STATEMENT Apprved fr Public Release: Distributin Unlimited 13. SUPPLEMENTARY NOTES 14. ABSTRACT This wrk includes experimental and theretical study f the effect f cure cycles n the develpment f cure-induced stresses in thermset plymer cmpsites. The experimental part includes mdifying a newly develped test methd t mnitr the develpment f cure-induced stresses in thermset plymer cmpsites. Test results gave a clear understanding f the mechanisms f stresses develpment. The test methd was further mdified with a clsed lp feedback cntrl system t mdify the cure cycles t reduce cure-induced stresses. Test results shw that the mdified cure cycles reduce residual stresses in cmpsite laminates. The theretical part included the develpment f a mechanical ptimizatin mdel t study the effect f changing the cure cycle n the develpment 'f residual stresses in cmpsite laminates. The mdel was used t generate cure cycles that reduce residual stresses in thermset plymer cmpsites. Findings f the theretical mdeling were fund t agree well with thse f the experimental study. 15. SUBJECT TERMS 16. SECURITY CLASSIFICATION OF: a. REPORT b. ABSTRACT c. THIS PAGE 17. LIMITATION OF ABSTRACT 18. NUMBER OF PAGES 19a. NAME OF RESPONSIBLE PERSOh Madhu.S. Madhukar 19b. TELEPHONE NUMBER (include area cde) (865) Standard Frm 298 (Rev. 8-98) Prescribed by ANSI Std. Z39.18

3 Cure Cycle Optimizatin in Plymer Cmpsites Madhu S. Madhukar Mechanical and Aerspace Eng. The University f Tennessee,Knxville, TN (865) mmadhuka(a)utk.edu ABSTRACT This wrk cncerns the study f the effect f cure cycles n the develpment f cure induced stresses in thermset plymer cmpsites. The study includes experimental testing as well as theretical mdeling. The experimental part included mdifying a newly develped test methd t mnitr the develpment f cure induced stresses in thermset plymer cmpsites. Test results were shwn t be highly reprducible and gave a clear understanding f the mechanisms f stresses develpment as well as cancellatin. Gd qualitative agreement between test results and the independently measured plymer vlume change data was seen. The test methd was further mdified with a clsed lp feedback cntrl system t mdify the cure cycles t reduce cure induced stresses. Several cmpsite material systems were studied. Test results shw that the mdified cure cycles reduce residual stresses in cmpsite laminates cmpared t the cure cycles recmmended by materials manufacturers while maintaining the same glass transitin temperature. It was shwn that the mdified cure cycles may als enhance the matrix-dminated mechanical prperties, maintain better dimensinal stability, and reduce cure time. The theretical part f the study included the develpment f a mechanical ptimizatin mdel and a cmputer prgram, OPTICURE, t study the effect f changing the cure cycle n the develpment f residual stresses in cmpsite laminates. The mdel and the prgram were used t generate cure cycles that reduce residual stresses in thermset plymer cmpsites and hence enhance the dimensinal stability and mechanical prperties. Findings f the theretical mdeling were fund t agree well with thse f the experimental study.

4 1-1 INTRODUCTION The inhmgeneus structure f thermset plymer cmpsites causes cure shrinkage and thermal vlume changes during prcessing t prduce residual stresses. These stresses may be high enugh in epxy plymer cmpsites t cause micrdamages such as ply cracking and delaminatin which may affect their mechanical prperties and durability (1-3). In additin, these stresses result in gemetrical distrtin f cmpsite parts which increases as the part size increases. This ges against the current trend in aerspace and ther industries t develp large integrated structures t reduce the expensive cst f assembling small parts. Part assembly and fit up are knwn as the majr cst item f prducing tight-tlerance parts as thse used in stealth technlgy. Residual stresses in plymers and plymer cmpsites have been the subject f many previus studies. These stresses develp during cure prcess as well as during cldwn after cure is cmplete. Cntradictry findings and assumptins have been fund regarding the cntributin f cure shrinkage t the residual stresses. Als, little wrk was dne t mdify cure cycles t reduce residual stresses 1-2 EXPERIMENTAL STUDIES Several experimental methds have been used t measure residual stresses n plymers and plymer cmpsites. This includes duble beam methd [4-6], phtelasticity [7-10], cnfinement tubes with different gemetry [11-18], fiber ptics [19], piezresistivity [20], peel-ply [21], unsymmetric laminates [21-23], single fiber stress test [24], and thers [25]. Duble beam methd is simple and can be used t study the develpment f stresses in thin plymer films but nt in cmpsites r plymers under cnstraints. Using this methd, Dannenberg studied develpment f residual stresses in Epn 828 and 1002 epxies cured with different agents [4]. It was shwn that residual stresses increase with curing temperature and depend n the. curing agent. Als, it was shwn that stresses start t develp significantly when temperature drps belw glass transitin temperatures. Using similar test setup, Wang and Yu studied residual stresses in diglycidyl ether f bisphenl A [5]. They used different cure cycles and shwed that lwering cure temperatures may reduce stresses build up. Hwever, lnger cure time is needed at lwer cure temperatures. Using a film cat n quartz beam, Feger and his cwrkers studied the develpment f residual stresses in PMDA-ODA plyamic acide plyimide films [6]. They shwed that the remval f slvent and changes in misture cntent cause stresses t develp in the plyimide films. Results indicate that the plyimide films maintained abut 25% f ttal stresses in the vicinity f glass transitin temperatures and disappeared nly after heating t higher temperatures. Pawlak and Galeski [7] used three-dimensinal phtelasticity t study residual stresses in plymer matrix with spherical inclusins. It was shwn that residual stresses increase significantly as the space between inclusin decreases. Similar cnclusins were shwn by'thecaris and Paipetis [8]. Asamah and Wd used phtelasticity t study residual stresses in fiber reinfrced epxies [9]. They shwed that stresses in resin increases significantly as fiber spacing decreases. A tri-cn f matrix material is created between fibers which is subjected t high level f cnstraining by the fibers. The authrs used elastic finite element

5 analysis and cnfirmed their cnclusins qualitatively. Cunningham and his cwrkers studied residual stresses in resin materials between fibers in fiber reinfrced cmpsites inside a rigid uter shell [10]. The cnstrained resin was shwn t underg significant residual stresses. Shimb and his cwrkers measured the effect f mdifying epxide resin with a spir rth-ester n residual stresses [11]. They measured stresses by measuring strains in a ring surrunded by the curing plymer. It was fund that the mdified resin has a lwer glass transitin temperature and hence less build up f residual stresses. In anther paper, they mdified the plymer t reduce the plymer stiffness while maintaining the glass transitin temperature [12]. Reducing stiffness causes residual stresses t decrease. In bth cases, reductin f stresses is accmpanied by scarifying desirable prperties. In a later study, Shimb and his cwrkers studied the mechanism f stresses develpment in epxide resins [13]. It was shwn that cntributin f cure shrinkage t final residual stresses increases significantly when the cure is carried belw glass transitin temperature. Lee and Schile tried t simulate develpment f stresses in fiber reinfrced epxy cmpsites. They arranged glass rds inside a curing plymer and measured strains in the rds during cure and cldwn [14]. Als, they cnducted an elastic finite element analysis. They studied different epxy/hardener systems used in cmpsites. The results shw that the glass rds cnstraint the plymer vlume changes. Residual stresses due t thermal shrinkage during cldwn assuming elastic analysis were smaller than measured values. The difference was attributed t cure shrinkage. Different systems f epxy-hardener prduced different relative values f stresses due t cure shrinkage t cldwn thermal shrinkage. Plepys and his cwrkers studied the effect f degree f cnfinement f the plymer during cure n the develpment f residual stresses used in Epn 828 epxy cured with different curing systems [15,16]. Test data shw that when ne-dimensinal cnstraints exists, the cntributin f cure shrinkage t the final residual stresses is much smaller than that f thermal shrinkage. When three-dimensinal cnstraints were cnsidered, stresses due t cure shrinkage were significantly high and f cmparable values t that f thermal stresses during cldwn. Similar findings were shwn by Krthv and his cwrkers [17,18]. Accrding t the principles f mechanics, the behavir f plymer appraches pure elastic behavir under three-dimensinal cnfinement. In this case, stress relaxatin f stresses due t cure shrinkage may be ignred and cure shrinkage, which is usually f large value, will cntribute significantly t residual stresses. Interestingly, Plepys shwed that fr certain cure schedules, stresses due t cure vlume changes (thermal expansin during heating and cure shrinkage) after the cure is cmplete may be in the ppsite directin f stresses due t cure shrinkage and hence will reduce final residual stresses. Lawrence and his cwrkers used embedded fiber ptics t measure strains in cmpsites due t residual stresses [19]. It was shwn that significant strains are develped during prcessing f crss ply laminates made f HyE 6049U carbn/epxy cmpsites. Results indicate that strains start t build up during cure due t chemical shrinkage. These strains may cnstitute abut 25% f the ttal mechanical strains and hence residual stresses. Crast and Kim tried t utilize carbn fiber piezresistivity t measure residual

6 Stresses in cmpsites [20]. It was shwn that, as already established, that increasing cure temperature results in increasing fiber stresses as measured by fiber piezresistivity. In anther study, Crast and Kim measured stress free temperature in AS4/ graphite/epxy as an indicatin n residual stresses [21]. A definitin fr the stress free temperature is the temperature at which a freestanding unsymmetric laminate will be flat. The higher the residual stresses, the higher the stress free temperature will be. The unsymmetric laminates were cured at different temperatures and prduced different glass transitin and stress free temperatures. Test data shwed that stress free temperature is higher than bth curing temperature and glass transitin temperature. The difference between stress free temperature and cure temperature, abut 15 C, was attributed t cure shrinkage stresses. Stress free temperature was higher than glass transitin temperature with abut 8 C. It was expected that abve glass transitin temperature the sftening f the matrix will cause cmpsite t lse stiffness and remain flat at higher temperatures. Hwever, when the temperature was raised abve stress free temperature the directin f the laminate curvature was reversed indicating that the cmpsite maintained high stiffness. This can be explained by the fact the behavir f the plymer in cmpsites in the existence f fiber cnfinement may be different than that f neat plymers. Sme case studies have been cnducted t investigate the effect f cure cycle mdificatin n the cure induced residual stresses [22,23]. In these studies, the cure cycles were mdified arbitrarily t see the effect n residual stresses. In ne study [22], the tw dwell time temperature standard cycle fr IM6/3100 graphite/epxy cmpsite was mdified by intrducing a third dwell temperature. Different lcatins and duratin f the third dwell temperature were investigated (see Figure 1.1). One f the mdified cycles reduced the stresses by up t 25% while maintaining the mechanical prperties as that f the specimens cured with the standard cure cycle (cure cycle recmmended by the prepreg manufacturer). There were n criteria fr chsing the lcatin and duratin f the additinal dwell temperature. In anther study [23], the tw dwell cure cycle f 976 epxy was mdified by extending the first dwell temperature. As can be seen frm Figure 1.2 such extended cycles can reduce spring back f unsymmetric laminates by mre than 25%. While these studies shwed that mdifying cure cycles can reduce residual stresses in cmpsites, they are nt effective because f the trial and errr methds invlved. These experimental methds t mdify the cure cycles t reduce the residual stresses demnstrated the validity f such apprach. Hwever, they did nt prvide definite criteria t mdify the cure cycles. A new test methd was develped by Madhukar and his cwrkers which is based n curing the plymer arund a pretensined fiber [24]. After the plymer develps significant stiffness, vlumetric changes in the plymer will change fiber tensin which is a measure f the residual stresses. They demnstrated that changing the cure cycle f Epn 828 cured with mpda will change the cntributin f cure vlume changes t the residual stresses in the fibers.

7 1-3 THEORETICAL STUDIES In earlier studies, stresses develped due t thermal shrinkage during cldwn were evaluated using linear elastic analysis [26]. Stresses due t cure shrinkage were neglected assuming that it will be relaxed during cure. In anther study, the effect f swelling due t misture absrptin was shwn t reduce residual stresses [27]. The authr indicated that residual stresses are higher than 50% f the transverse strength. Based n the elastic analysis, he cncluded that stress-free temperature is lwer than the curing temperature. Excluding the effect f misture effect which depend n envirnmental cnditins cnsidered, this cnclusin can nt be generalized in the view f experimental data discussed abve. In a later study by the authr, effects f residual stresses n ply cracking and warping in unsymmetric laminates were discussed [1]. It was shwn experimentally that stress free temperature is higher than curing temperature and the cldwn rate apparently has n effect n residual stresses. Als, data shws that changing gelatin temperature may reduce residual stresses. In a later study, White and Hahn studied the develpment f residual stresses thrugh the whle cure cycle [28]. It was shwn that transversal stiffness and strength, which are matrix dependent, develp nly after plymer degree f cure reached 0.83 in IM6/CYCOM 3100 graphite/bmi cmpsites. It was shwn that pstcure causes increase in the residual stresses which was attributed t shrinkage due t additinal cure and lss f misture r vlatiles. Other mdels were develped t estimate stresses develpment during cure cycle [29-35]. Mdeling f the mechanical prperties as a functin f degree f cure and temperature have been the mst difficult because f the viscelasticity invlved. Assumptins invlved in mdeling the mechanical prperties develpment in the cmpsite directly affect the validity f the predicted stresses. A mdel t evaluate residual stresses in plymer cmpsites predicted that chemical shrinkage cntributes less than 5% t final residual stresses in IM6/3100 graphite/bismaleimide cmpsites during cure [29,30]. The mdel included viscelastic effects t accunt fr stress relaxatin. It was assumed that lngitudinal stiffness increases abut 50% when the cure is cmplete while transverse mdulus increases significantly nly after certain degree f cure. Hwever, test data as shwn in Figure 1.3, indicates that the mdel underestimates the cntributin f chemical shrinkage due t verestimatin f stress relaxatin r underestimatin f mechanical prperties r cure shrinkage [22]. This can be recgnized cnsidering that fr thermsetting plymers, the cureinduced stresses by chemical shrinkage in sme cases may exceed 30% f final stresses [15-19, 25]. This is supprted by the fact that changing the cure cycle fr sme cmpsite systems (with the same cure temperature and cldwn rate) affects the final residual stresses significantly [22,23]. This is mainly attributed t the change in the cntributin f chemical shrinkage t the final stresses. A mdel was develped t evaluate cure stresses in thick cmpsites [31-35]. Stress relaxatin was nt cnsidered and resin mdulus was evaluated using a mixing rule depending n plymer degree f cure. As shwn abve, a threshld value f degree f cure must exist befre sme f the mechanical prperties are develped significantly [28]. Mechanical prperties f the cmpsite were estimated frm prperties f matrix and fibers using micrmechanics neglecting any interactin. As discussed abve, behavir f resin 1

8 may be significantly different than the behavir f resin with high fiber vlume fractins as in cmpsites. Full cure shrinkage was used t estimate stresses. Experimental results indicate that nly part f cure shrinkage may cntribute t residual stresses [4-18,28]. As expected high values f cure stresses were estimated. N experimental verificatin was presented t supprt the mdel. Weitsman develped a rigrus viscelastic ptimizatin mdel t maximize stress relaxatin during cldwn fr a given cling dwn time [36,37]. The mdel predicts 10% reductin in residual stresses in epxy-resin cmpsites if ptimum cldwn path is applied althugh this has nt yet been supprted by experimental data. Harper and Weitsman cnducted an experimental and theretical investigatin n the effect f cure shrinkage, misture, and cldwn path n residual stresses in plymer cmpsites during cldwn [38-40]. The presence f misture was shwn t enhance viscelastic behavir. Predictins f stresses due t misture change were in reasnable agreement with experimental data. Based n their ptimal cldwn path fr AS4/3502 graphite/epxy, a 10-15% reductin in residual stresses may be btained, hwever, it was nt seen experimentally. Using duble beam methd, Harper shwed that cure shrinkage may cntribute up t 25% f residual stresses in 3502 epxy [41]. Several mdels were develped t evaluate residual stresses in thermset resin [42-50]. Levitsky and Shaffer develped an elastic mdel t evaluate stresses in a slid sphere cast frm a thermset plymer during curing [42-43]. The frmulatin is rigrus, hwever results are nt directly applicable t cmpsites. Lange and his cwrkers studied stresses build-up in thermset films cured belw their ultimate glass transitin temperatures [44,45]. They used a Maxwell mdel t simplify viscelastic analysis. Duble beam methd was used t verify analysis predictins. It was shwn that cntributin f cure vlume changes t final residual stresses depends n amunt f resin shrinkage beynd gelatin and density f crsslinking. They indicated that stresses frm cure shrinkage vary frm 1 t 30 % f the final residual stresses depending n the plymer under cnsideratin and n cure cycle. Mallick and Krajcinvic develped a micrmechanical mdel t study stress and strain fields in resin slab [46]. They demnstrated the use f aggregatin-perclatin mdels t btain the cnstitutive equatins f the curing resin. Stresses were shwn t develp significantly nly after gelatin. A similar apprach was cnsidered earlier by Adlf and Martin [47-49]. Adlf suggested the use f time-cure superpsitin t accunt fr nnisthermal cure viscelasticity. They indicated that changing cure cycle fr a given plymer may affect significantly the cntributin f cure shrinkage t final residual stresses. Mnk and his cwrkers develped a viscelastic mdel fr residual stresses in spun plyimide films [50]. They used fundamentals f chemical reactins and macrmlecules t predict macr-prperties such as strains. Shrinkage due t evapratin was included in the analysis. Given the frmulatin apprach, many parameters were difficult t measure and/r verify. These parameters were estimated by fitting mdel predictins with test results btained using duble beam methd. Finite element analysis (FEA) was used t study develpment f residual stresses in cmpsites [51-56]. Jain and Mai used linear FEA t study develpment f residual stresses in T300/934 carbn/epxy cmpsite cylinders'including the effect f cure shrinkage (which was taken arbitrarily as 1.5%) [51,52].

9 Measured spring-in values fr channel sectins were fund t change with tling materials. The predicted results based n the assumed cure shrinkage value appear t agree with measured values. Sala and Di Landr used linear FEA and classical laminate thery t study residual stresses in T800/ carbn/bmi cmpsites [53]. In their frmulatin, they cnsidered stress build-up frm stress free temperature. In that study, stress free temperature was measured experimentally and was fund t exceed cure temperature by abut 5365 C. Fr a given measured stress free temperature, the elastic slutin was shwn t verestimate curvature f unsymmetric laminates. In a later study [54], the authrs reprted that based n the cmparisn between analytical and experimental results, the transverse mdulus f the cmpsite was fund t change with time and temperature as an indicatin f the presence f viscelasticity. Wang and Daniel develped a linear viscelastic mdel t study residual stresses and warpage in wven- glass/epxy laminates [55]. They cnsidered thermrhelgically simple mdel and time-temperature superpsitin in their frmulatin. Chemical shrinkage in additin t thermal expansins were cnsidered in calculating stresses. Because f the wven nature f the fabric bth transverse mdulus and lngitudinal mdulus were fund t exhibit time-temperature dependency. The viscelastic analysis was fund t yield satisfactry results when qualitatively cmpared t experimental data. Mhan and Grentzer used linear FEA t simulate prcessing and predict residual stresses develpment in glass/vinyl ester cmpsites [56]. They applied the methd t study stresses develpment during cure f a cmpsite nzzle. The authrs indicated that the cntributin f cure shrinkage is significant althugh it needs mre investigatin t be quantified. 1-4 PROBLEM STATEMENT Frm the abve discussin, it can be seen that a substantial amunt f research wrk was dne t investigate the develpment f residual stresses in thermset plymers and plymer cmpsites. Hwever, cntradictry assumptins and cnclusins still exist regarding the effect f majr parameters n the develpment f stresses. Fr example, the extent f cure shrinkage cntributin t residual stresses and effect f stress relaxatin are nt clear yet. In additin, very little wrk was dne t reduce these stresses withut scarifying desirable prperties r cnsiderable elngating the cure cycle. This necessitates a fundamental understanding fr the prcess f cure stresses develpment. This fundamental understanding can then be used t study the pssibility f reducing residual stresses. The apprach used here is mainly experimental using single fiber cure induced stress test (sectin 1.2) [24]. This new methd was develped t mnitr stresses develped in fibers during cure f a single fiber cmpsite. The methd was shwn t be bth simple and effective t study mechanisms f stresses develpment. In the current study, this methd was used t investigate stresses develpment in single fiber thermset plymer cmpsites. Different material systems were cnsidered t btain general understanding. Then the methd was mdified as necessary t ptimize cure cycles t reduce residual stresses. 4>

10 2. EXPERIMENTAL METHODS 2-1 INTRODUCTION A test prcedure t study the develpment f cure induced stresses in thermset plymer cmpsites was develped at the Cmpsite Materials Labratry at the University f Tennessee as discussed in Chapter 1 [24]. The methd was applied t a simple epxy system (Epn 828/mPDA system). The test prved t be a simple and effective methd t study the develpment f cure induced stresses. Preliminary results shwed hw plymer vlume changes during cure affect fiber stresses. Als, it was shwn that changing the cure cycle changes the stresses develped during cure. Hwever, the early versin f the test setup was basic and nt suitable t run tests n mre cmplicated plymer systems requiring higher cure temperature and mre unifrm heating. In additin, test cmpnents and prcedure were nt yet well develped t eliminate the effects f parameters ther than plymer vlume changes. This includes frictin between fiber and test cmpnents, degassing technique that takes lng time that may advance cure befre resin is pured arund fiber, shape f the mld which result in cnstraining the specimen during shrinkage, and nnunifrm temperature alng the specimen. Fr example, results f the early versin f the test setup fr AS4/ graphite/epxy cmpsite were nt always cnsistent. In the current research wrk, this test methd was mdified t reslve the abve prblems. The mdified test setup and the testing prcedure prduced cnsistent results fr several materials systems. The mdified setup and prcedure were used t study the effect f the cure cycles n the cure induced stresses. A clsed lp feedback cntrl system was als develped t cure cycles that reduce stresses. In this chapter, the mdified test methd and typical results are shwn and discussed. Feedback cntrl system and its applicatins are als described. 2-2 CURE INDUCED STRESS TEST (CIST) Figure 2.1 shws the cnceptual mechanism assumed fr the develpment f fiber stresses in CIST due t plymer vlume changes. The plymer is cured arund a prtin f a pretensined fiber. Once the plymer develps sufficient stiffness, thermal expansin in the plymer (as a result f heating) will result in decreasing fiber tensin. On the ther hand, shrinkage in the plymer (as a result f crsslinking r cling dwn) will result in increasing fiber tensin. The change in the fiber tensin will decrease with time because f stress relaxatin whenever the plymer exhibits viscelastic behavir. Crrespndingly, it can be seen that plymer shrinkage will result in cmpressive stresses in the fiber prtin embedded inside the plymer whereas the expansin will result in tensile stresses. All vlume change data presented here fr cmparisn were btained at Air Frce Research Labratry. The vlumetric dilatmeter used was GNOMIX, Inc. PVT Apparatus (Figure 2.2). Mre details abut the test setup and prcedure can be fund in reference [57]. Figure 2.3 shws the vlume change data f epxy during a tw dwell cure cycle recmmended by resin manufacturer (standard cure cycle). There is expansin during heating frm ambient temperature t the first dwell temperature and during heating frm 7

11 the first dwell temperature t the secnd dwell temperature. On the ther hand, there is shrinkage due t crsslinking during the temperature hlds and als upn cl dwn after cure is cmplete. Hence, there are tw types f vlume changes ccurring during the cure f thermset cmpsites; first type is the expansin during heating and the ther is the shrinkage due t the chemical reactin (crsslinking shrinkage) and during cling dwn (thermal shrinkage). Vlume changes prir t plymer gelatin are nt expected t cntribute t residual stresses since the melted plymer acts like a fluid with lw viscsity and stress relaxatin will impede stresses build up. Gelatin is defined by the pint at which infinite netwrk f mlecules is develped inside the plymer. Beynd gelatin, vlume changes prduce prprtinal stresses in the fibers and stresses may start t build up. Thrugh the rest f this reprt, the term effective vlume will be used t refer t the vlume changes that prduce stresses in the fibers. Effective expansin ccurs during any heating beynd the plymer gelatin, and effective shrinkage results frm plymer crsslinking beynd gelatin and during cl dwn. Effective expansin will prduce tensile stresses in the fiber prtin embedded inside the plymer, whereas effective shrinkage will prduce cmpressive stresses. This can easily be seen frm the test mechanism shwn in Figure 2.1. In typical cure cycles, gelatin temperature is clse t maximum cure temperature and hence effective expansin (which crrespnds t the increase in the temperature frm gelatin t the maximum cure temperature) is typically small, if any Test Setup Figure 2.4 shws a schematic diagram f CIST. Figure 2.5 shws a phtgraph f the test setup. The fiber is fixed t a rigid supprt at ne end, and the ther end is passed thrugh a cavity in a silicne mld and then glued t a lad cell (250 gm lad capacity and 0.01 gm sensitivity). A ceramic shield is used t frm a cure chamber arund the mld. The shield has tw narrw slts, ne at each end t pass the fiber. The sizes f these slts were kept t a minimum t reduce the heat lss at the ends and t avid significant variatin in the temperature inside the cure chamber. The dimensins f the cure chamber are apprximately 160 mm lng, 70 mm wide, and 76 mm high. A ceramic strip heater (frm Research Inc., Mdel 4184) is placed at the tp t apply the time-temperature cycle. A thermcuple is lcated beside the middle f the silicn mld t mnitr temperature. The thermcuple is cnnected t a data acquisitin system. A cmputer prgram was written t cntrl temperature and t cllect fiber tensin data thrugh the cure cycle. The prgram was fund t prduce a heating prfile within ±0.25 C f a given input time-temperature cycle. The dimensins f the silicn mld are 76 mm lng, 3.5 mm wide, and 2 mm high. The thickness f the base f the silicne mld was kept as small as pssible (abut 0.3 mm) t minimize the effect f silicne vlume changes and cnstraints applied by mld n the plymer n test results. After psitining the fiber, it is glued at bth ends..the glue is allwed t cure accrding t the recmmendatins f its manufacturer. Then, the fiber is pretensined t abut 5 gm. The lad cell reading shuld be stable fr at least 5 minutes befre puring the plymer arund the fiber t ensure prper fiber fixatin and the cure f the glue. Abut 1 gm f the resin is melted and degassed in a high temperature 8

12 syringe using a vacuum ven. The degassing temperature is chsen t be the minimum temperature that is high enugh t melt the resin and enable degassing while nt causing significant cure in the plymer. After degassing, the resin is injected inside the silicn mld arund the fiber and the time-temperature cycle is started. CIST results will be shwn as the fiber tensin data btained frm the lad cell reading. A cmparisn between fiber tensin data and stresses in fiber prtin embedded in the resin shws that any increase in the fiber tensin mre than its initial value (as a result f the resin shrinkage) crrespnds t the applicatin f cmpressive stresses n the fiber prtin embedded inside the plymer. On the ther hand, any drp in the fiber tensin belw its initial value (as a result f the resin expansin) crrespnds t the applicatin f tensile stresses n the fiber prtin embedded inside the plymer. Under n external lads, the matrix in the vicinity f the fiber experiences tensile r cmpressive stresses when the fiber experience cmpressive r tensile stresses, respectively Typical Results The material used in this sectin is AS4/ graphite/epxy cmpsite system. The standard cure cycle f cnsists f: heating the sample frm rm temperature t 116 C in 30 minutes; hlding the temperature at 116 C fr 60 minutes; raising the temperature frm 116 C t 177 C in 25 minutes; hlding the temperature at 177 C fr 240 minutes; fllwed by cl dwn t rm temperature. Figure 2.6 shws the fiber tensin data during such a cycle. The plymer vlume change data fr this cure cycle is shwn in Figure 2.3. A gd crrelatin between fiber tensin and vlume change can be seen. During the first part f the cure cycle fiber tensin remains cnstant because the viscsity f the resin is small. As the resin viscsity increases, the expansin during the heating ramp causes the fiber tensin t decrease and the crsslinking shrinkage causes the fiber tensin t increase. The crsslinking shrinkage during the secnd temperature dwell increases the fiber tensin. Finally, thermal shrinkage during cl dwn causes increase in the fiber tensin. The drp in the fiber tensin during the heating ramp crrespnds t the develpment f tensile stresses in the fiber prtin embedded in the plymer. The increase in the fiber tensin during the secnd dwell crrespnds t develpment f cmpressive stresses in the fiber prtin embedded in the plymer. The fiber tensin at the end f the cure, just befre cl dwn starts, is higher than the initial fiber tensin indicating the develpment f cmpressive stresses in the fiber prtin embedded in the plymer. These stresses are undesirable because they will be added t cmpressive stresses develped during cl dwn and hence increases residual stresses. In the CIST, the cmpsite system is essentially a single fiber in an infinite plymer. T simulate a mre representative fiber vlume fractin as in cmpsite laminates, an experiment was perfrmed with fiber cated with a thin layer f resin. The cated fiber was subjected t the standard cure cycle and changes in the fiber tensin were measured (Figure 2.7). The shape f the change f fiber tensin is the same as btained with the fiber running inside an infinite plymer (Figure 2.6) but with smaller values. This 9

13 indicated that the mechanisms that are identified in the single mdel cmpsites are present in cmpsite laminates as well Parameters f CIST T study the reprducibility f the test results and determine the effect f different parameters including initial fiber tensin and fiber embedded length, several CIST experiments were cnducted n AS4/EPON 828 graphite/epxy single fiber cmpsites. This cmpsite system was chsen because it is easy t wrk with and the duratin f the cure cycle is relatively shrt. The EPON 828 is a diglycidyl ether f bisphenl- A and was cured with 14.5 parts per hundred by weight f meta-phenylene diamine (mpda). The matrix was mixed, degassed under vacuum fr 10 minutes, and cured with different cycles. The typical cure cycle used fr this resin cnsists f: heating frm ambient temperature t 75 C in 5 minutes then the temperature is held at 75 C fr 2 hrs; then it is raised t 125 C and held at 125 C fr tw hurs fllwed by cl dwn t ambient temperature. Figure 2.8 shws typical CIST results fr Epn 828/mPDA epxy. The fiber tensin is unchanged fr the first 1.5 hurs at 75 C. Sme increase in the fiber tensin ccurs during the last 0.5 hur at 75 C. The fiber tensin drps rapidly when the temperature is increased frm the first dwell at 75 C t the secnd dwell at 125 C. Then, again there is an increase in the fiber tensin during the first half-hur at 125 C temperature hld. During cling t ambient temperature, the fiber tensin again increases and stabilizes at the cmpletin f cling. In Figure 2.8, the rapid heating frm rm temperature t the first dwell temperature just melts the resin withut allwing significant resin cure t ccur. Thus, it is expected that the resin just flw arund the fiber withut causing fiber stresses. During the temperature hld at 75 C, the viscsity f the resin increases with the advancement f resin cure. At that stage, vlume changes start t be effective and the cure shrinkage starts t increase fiber tensin. During the temperature ramp between first and secnd dwell temperatures bth thermal expansin and crsslinking shrinkage exists. Hwever, it appears that the rapid heating rate causes thermal expansin t dminate prducing net expansin. This expansin causes fiber tensin t drp. The chemical shrinkage during secnd temperature hld causes fiber tensin t increase again. Finally, thermal shrinkage during cl dwn increases the tensin. The changes in the fiber tensin shwn in Figure 2.8 are primarily due t plymer vlume changes because the effect f fiber expansin/shrinkage is cmparatively small. This can be seen frm CIST results shwn in Figure 2.9. In the set f tests, the silicn mld cavity was nt filled with resin and nly the fiber was running thrugh. The cure cycle was applied and the changes in fiber tensin were recrded. The test was repeated with different types f fibers. Results shw that fiber tensin des nt change significantly during the cure cycle because f fiber vlume changes. Figure 2.10 shws results f CIST under different values f initial fiber tensin. Results indicate that the initial fiber tensin has n effect n the changes in the fiber tensin during the cure cycle. Figure 2.11 (O

14 shws the effect f embedded fiber length n the variatin in the fiber tensin. The tensin in the free fiber length is prprtinal t the defrmatin at the end embedded in the plymer. This defrmatin increases as the embedded fiber length increases. Thus the fiber tensin in the free length increases as the embedded length increases. This can be seen frm CIST results in Figure In all cases, test results were highly reprducible. T further understand the effect f plymer vlume change and stress relaxatin n the fiber stress, the cure cycle was mdified as shwn in Figure A heating ramp frm 65 C t 149 C was divided int 3 equal steps f 16.7 C and ne last step f 33.3 C. Similar t the case f typical tw dwell cycle, there is n change in the fiber tensin during first heating ramp frm ambient temperature t the first dwell temperature. The fiber tensin remains unchanged until the end f the secnd temperature ramp suggesting that the plymer has lw viscsity and/r insignificant crsslinking shrinkage. The fiber tensin starts t increase rapidly during the secnd temperature hld indicating that the resin nw has significant stiffness. The fiber tensin drps during the third temperature ramp due t thermal expansin. Beynd this stage, the fiber tensin increases during the temperature hlds and drps during the fllwing temperature ramps. A cmparisn amng the change in the fiber tensin during different temperature steps in Figure 2.12 shws that after the resin has sufficient stiffness, the rate f the fiber tensin increase decays with advancement in the cure cycle. This is expected since the rate f the chemical reactin and hence the crsslinking shrinkage decreases after it reaches certain extent. Als, the drp in the fiber tensin due t thermal expansin during heating increases with the increase in the plymer cure. This is expected since as the degree f cure increases, the resin attains higher stiffness. The drp in the fiber tensin during the last temperature ramp (with 33.3 C temperature increment) is almst twice the drp in the ramp just befre it (with 16.7 C increment). This indicates that plymer stiffness reaches smewhat cnstant value beynd certain extent f cure. Results in Figure 2.12 suggest that the thermal expansin when develped twards the end f the cure cycle will prduce significant fiber stresses. Average stresses in the embedded part f the fiber during a cure cycle were calculated frm simple mechanics f materials. Assuming a unifrm fiber stress distributin alng the embedded fiber length and perfect bnd between the fiber and the matrix, the fiber stresses in the free part f the fiber, cr' f (t) (Figure 2.13), at time t, can be related t the fiber tensin as iwzm (1) where T(t) is the fiber tensin at any time t, T(0) is the initial fiber tensin, and A/\s the crss sectinal area f the fiber. The crrespnding strain (s'/t)) can be written as E, {t) jj!hm ( 2) 1 As Es where /is the Yung's mdulus f the fiber. The ttal defrmatin in the free fiber length (A'(0) is given by:!/

15 AV)=nzmi 2Ll ( 3) AfE f where L, is as shwn in Figure Since the fiber length is fixed during the test, the defrmatin in the embedded part equals that f the free part with a negative sign. Thus, the strain in the embedded length (e/t)) is given as s f (t) = - T(t)-T(0)2 Ll (4) AfEf L 2 where L 2 is the embedded fiber length as shwn in Figure Thus, stresses in the embedded fiber length can be written as cr f (t) = -2 'V A, \L-LJ The effect f the change in the fiber length due t temperature change was neglected in Equatin 5 since the thermal expansin cefficient f fibers is generally very small relative t that f the resin. The fiber stresses calculated frm Equatin 5 fr a single fiber graphite/epxy cmpsite are pltted in Figure The fiber stresses shwn in this figure are due t matrix vlume changes nly, i.e., the fiber stresses resulting frm the initial fiber tensin T(0) are nt included. An imprtant bservatin in Figure 2.14 is that when the temperature is raised frm the first dwell t the secnd dwell, the embedded fiber experiences a tensile stress peak (-1.2 GPa). T verify the results n cure induced fiber stress btained frm Equatin 5, independent tests were cnducted with different values f T(0). The average tensile strength f AS4 fibers ranged frm 2.5 t 3.0 GPa. Thus, if a fiber is prestressed t abut 1.4 GPa, the cure induced tensile stress (1.2 GPa) superimpsed n the fiber prestress value will cause the fiber stress t apprach its failure stress. Tests shwed that when CIST cnducted with the fiber prestressed t -1.4 GPa, a fiber crack was detected as shwn in Figure When the fiber was prestressed t less than 1.4 GPa, n fiber crack was detected. These bservatins suggest that, althugh the analysis used t calculate the fiber stress during the curing is based n sme grss assumptins (such as unifrm fiber stress alng the entire embedded length), it still agree quite well with the experimental bservatins. T verify that the fiber fracture ccurs within a shrt time interval when the temperature is raised frm 75 C t 125 C, tw additinal experiments were cnducted. In bth f these specimens, the fibers were pretensined t abut 70% f their expected failure lad. Such pretensin lads are expected t prduce fiber failure during the standard cure cycle. In ne f the specimens the test was terminated after the first temperature hld, i.e. befre starting the temperature ramp. The specimen was remved and immediately examined under an ptical micrscpe. N fiber crack was detected in this specimen. The ther specimen was remved at the end f the heating ramp and was immediately examined under the micrscpe. This specimen did shw a fiber crack as that shwn in Figure This suggests that the fiber failure ccurred during the heating ramp frm 75 C t 125 C. (5) \2-

16 Fr a given upper cure temperature limit, there may be many cure cycles which cmplete the cure. The resulting residual stresses will nt be the same since different cycles mean different amunts f effective vlume changes and different extents f stress relaxatin. The fllwing example shws hw changing the cure cycle can change cure induced stresses. Figure 2.16 shws the change in the fiber stresses during a mdified cure cycle. The heating rate during the heating ramp frm the first t the secnd dwell temperatures was reduced arbitrarily t allw fr mre balance between chemical shrinkage and the thermal expansin. Als, the slw heating rate allws fr mre stress relaxatin. Mrever, the gradual heating applies mre thermal expansin twards the end f the cure cycle which causes thermal expansin t be effective. The mdified cycle results in mre unifrm prfile fr the fiber stress. The maximum value f tensile stress was lwer than that in the typical tw dwell cure cycle. T further verify the results f Figure 2.16, anther CIST was cnducted with the fiber prestressed t 1.8 GPa. N fiber cracks were detected when cure cycle shwn in Figure 2.16 was used. The final value f tensile stresses in the fiber at the end f the cure, just befre cl dwn, was higher than that in the standard cure cycle. As discussed earlier, tensile stresses that develp during cure cunteract part f the stresses develped due t the thermal shrinkage and thus reduce the final residual stresses. In this case, the new cycle is expected t prduce less residual stresses after cl dwn is cmpleted. This can be seen by cmparing final value f fiber stresses at the end f each cycle (Aa in Figures 2.14 and 2.16). 2-3 MODIFICATION OF CURE CYCLES USING TRIAL AND ERROR METHOD T study the effect f changing the cure cycle n fiber tensin prfile, several tests were perfrmed n AS4/ graphite/epxy. The bjective was t find a mdified cure cycle that minimizes the stress develpment in the fiber during cure. Such a cure cycle prduces ultimately a flat fiber tensin curve. The parameters are, initial heating temperature and the subsequent heating rate/s. Maximum temperature was set as that f the standard cure cycle. Trial and errr apprach was used t explre the existence f such a cycle. Figure 2.17 shws crrespnding fiber tensin prfile during ne f the intermediate cycles. It is clear that changing the cure cycle affect the variatin in the fiber tensin. The trial and errr was cntinued until the cycle shwn in Figure 2.18 was btained. The btained cure cycle almst cmpletely eliminates the variatin in the fiber tensin during cure. The cure cycle was terminated when the temperature reached the highest cure temperature hld and the fiber tensin remained cnstant fr 30 minutes during temperature hld. The new cure cycle prduces an almst flat fiber tensin curve and it is shrter than the standard cure cycle. Figure 2.19 shws vlume change data during such a cycle. There is a gd agreement between plymer vlume change and fiber tensin. There is n change in the fiber tensin during the initial part f the cure cycle where the resin viscsity is lw. As the plymer viscsity increases, the thermal expansin cmbined with stress relaxatin act t cancel stresses due t crsslinking shrinkage and thus fiber tensin remains cnstant. T check the cmpleteness f resin cure, the glass transitin temperature f the specimens cured using the standard and the mdified cure cycles were measured. Results indicate that the glass transitin 13

17 temperature is almst the same, Table 2.1. Als, the T g remained unchanged fr specimens cured fr 30 and 60 minutes lnger at the highest temperature in the mdified cycle. This indicates that the mdified cure cycle satisfies the cure requirement while keeping cure induced stresses t a minimum. Table 2.1 Glass transitin temperature fr epxy specimens cured using different cure cycles. Cure Cycle Standard Cycle Mdified Cycle Mdified Cycle + 30 min. Mdified Cycle + 60 min. Glass Transitin Temperature 170 C 171 C 172 C 171 C In the abve mdified cycle, it was intended t cancel stresses develped during cure cycle. Anther apprach is t develp tensile stresses inside the cmpsite during cure and maintain these stresses until the cl dwn. These tensile stresses will cunteract part f the cmpressive stresses resulting frm the thermal shrinkage. T develp tensile stresses during cure, the effective thermal expansin shuld be increased. This can be dne by develping mre thermal expansin after the plymer attains high viscsity. This implies that cure shuld be carried at relatively lw temperature fr lng time befre heating t a higher temperature t cmplete the cure. Thermal expansin that ccurs during this part f the cure cycle will prduce larger tensile stresses than if the heating was applied early in the cure cycle. Figure 2.20 shws typical results f epxy when this apprach is cnsidered. This cure cycle was chsen arbitrarily with the first dwell extended fr three hurs (versus 1 hr in the standard cure cycle). The drp in the fiber tensin during the heating ramp between first and secnd dwell temperatures in this cycle is larger than that in the standard cycle (Figures 2.6 and 2.20). As a result f this large drp, the fiber tensin at the end f the cure cycle, prir t cling dwn, was still smaller than the initial fiber tensin (AF in Figure 2.20). This decrease in the fiber tensin crrespnds t the develpment f net tensile stresses in the fiber running inside the plymer. These tensile stresses will ffset part f the cmpressive stresses prduced during cl dwn and hence reduce the final residual stresses. It shuld be nted that this type f cycle is lnger than the standard cycles. The parameters in this apprach include, first dwell temperature, number f dwells, duratin f these dwells, and heating rates between dwells. The prper chice f these parameters can result in cycles with lwer final residual stresses but the cure cycle will be lnger. If

18 2-4 MODIFICATION OF CURE CYCLES USING CLOSED LOOP FEEDBACK CONTROL SYSTEM (CLFS) Effects f residual stresses generated during the cure are reflected in the mechanical prperties and dimensinal stability f the cmpsite parts. Different cure cycles result in different patterns f vlume changes in the plymers during cure. Thus, changing cure cycles can affect residual stress develpment in cmpsites. In sectin 2-3, CIST was used t determine mdified cure cycles which reduced cure induced stresses in single fiber cmpsites. The apprach was t use trial and errr t btain such a cycle. Althugh the trial and errr apprach answered the questin regarding the existence f such cycles, it was nt practical t use with different cmpsites systems since many trials may be invlved. This calls fr the develpment f a feedback cntrl system. The idea is t reschedule the vlume changes in the plymer and utilize the stress relaxatin t minimize the variatin and the values f the cure induced stresses. As mentined befre, vlume expansin and shrinkage ccur in the plymer during different perids f the cure prcess. Befre the plymer gelatin, stress relaxatin is very effective and fiber stresses are nt likely t build up. As the plymer appraches gelatin, the increase in the viscsity causes relaxatin f stresses t take lnger times and stresses may start t build up due t cure shrinkage. These stresses can be reduced by causing thermal expansin in the plymer. In the current study, the CLFS was develped t determine mdified cure cycles that reduce cure shrinkage stresses fr varius cmpsite systems. In this feedback system, the unrelaxed part f stresses develped due t plymerizatin shrinkage is simultaneusly canceled by prprtinal thermal expansin. Since effective expansin prduces tensile stresses, it will cunteract a fractin f the cmpressive stresses due t shrinkage, and thus it reduces final residual stresses. Therefre, it is desirable t increase the effective thermal expansin. Effective thermal expansin is nt a cnstant fr a given resin. It depends mainly n gelatin temperature. The bjective f this research wrk is t develp a simple prcedure t reschedule cure cycle t prduce maximum cancellatin f stresses due t effective shrinkage with effective expansin and stress relaxatin Optimizatin Prcedure A cmputer prgram was develped t autmatically adjust the heating rate during cure t minimize the variatin in the fiber tensin. The methd is based n searching fr a heating path that results in a maximum cancellatin f the unrelaxed stresses. Thus, when the CLFS detects an increase in fiber tensin (as a result f crsslinking shrinkage in the resin), it will increase the temperature such that the resulting thermal expansin is enugh t cancel the stresses prduced by the chemical shrinkage. On the ther hand, a decrease in fiber tensin implies excessive thermal expansin in the resin due t rapid heating. In such cases, the prgram will start t decrease heating rate prprtinally. The check fr the change in the fiber tensin is dne at certain time intervals that are lng enugh t allw fr sme stress relaxatin. The test is iy

19 cnsidered cmplete when the fiber tensin des nt change fr a certain perid during a temperature hld at the maximum cure temperature. This perid (typically frm 20 t 40 minutes) varies accrding t the resin type. A blck diagram f the CLFS is shwn in Figure Optimizatin Parameters The time interval and the heat increment applied by CLFS determine the heating rate at a particular mment in the cure cycle. Time interval is chsen by the user. The lnger the time interval, the mre stress relaxatin is allwed (fr same temperature increment). Hwever, lnger time interval means lnger cure cycle. Fr mst plymers at high temperature, stress relaxatin is inevitable but its extent may vary depending n time and temperature. Stress relaxatin can be utilized tgether with effective thermal expansin t minimize stresses due t chemical shrinkage. In practical cure cycles, mst f thermal expansin is cnsumed during initial heating and hence there is a small amunt f effective thermal expansin. In such cases, stress relaxatin shuld be allwed as much as pssible (withut significantly prlnging the cure cycle) tgether with increasing thermal expansin t balance shrinkage stresses. The time interval shuld nt be shrter than that required t. allw the effect f the heat increment n matrix thermal expansin t ccur cmpletely. The heat increment is prprtinal t the change in the fiber tensin frm a reference value, which is taken as the initial fiber tensin. The prprtinality cnstant that determines the extent f heat increment crrespnding t a certain change in the fiber tensin can be determined apprximately by running the CIST with a cure cycle in which temperature is increased in steps. In this study, the initial value f this cnstant was taken as the rati f the temperature increase during the ramp between the first and secnd dwells and the crrespnding drp in the fiber tensin f the standard cure cycle (AT/AF in Figure 2.6). The prprtinality cnstant was fund t change with the degree f cure. The larger the degree f cure, the larger the value f the prprtinality "cnstant". This may be attributed t the decrease in the cntributin f stress relaxatin at higher degree f cure t cancel the change in stresses as the degree f cure increases. In the CLFS, nly apprximate initial value is required, then the prgram uses cntinuusly measured fiber tensin and temperature change data t adjust this rati t accmmdate fr the pssible effects f degree f cure and temperature. The test starts with raising the temperature frm rm temperature t an initial temperature after which the feedback system starts t cntrl the heating rate. The prgram scans the lad cell reading 1000 times every minute and recrds their average, which is then cmpared t a reference value t determine the change in the fiber tensin. A threshld value f the fiber tensin change, belw which n adjustment in the heating rate is applied, was taken as 0.05 gm t accunt fr the pssible small envirnmental fluctuatins Typical Results Figure 2.6 shws the fiber tensin prfile during the standard cure cycle f AS4/ graphite/epxy cmpsites. A ntewrthy bservatin is that the final value f the fiber tensin prir t cl dwn is higher than the initial fiber tensin value. As discussed in earlier sectins, this net increase in the fiber ft

20 tensin crrespnds t the develpment f cmpressive stresses in the fibers. These stresses will be added t the stresses prduced during cl dwn resulting in mre residual stresses. In the fllwing discussin, results f the mdified cure cycles btained by CLFS are presented. Figure 2.22 shws the cure cycle btained frm CLFS fr the AS4/ graphite/epxy. Recall that the feedback system cnstantly adjusts the heating rate t keep the changes in the fiber tensin t a minimum. The fiber tensin curve is almst flat. Figure 2.23 shws the vlume change data btained frm the vlumetric dilatmeter fr the epxy fr a cycle in which the btained mdified cure cycle was apprximated with linear segments. The resin vlume change curve is almst flat after abut 1.5 hurs. It is believed that the plymer vlume change befre 1.5 hurs has minr effect n the fiber tensin because plymer viscsity is lw and, hence, stress relaxatin is very effective t cancel develped stresses. After 1.5 hurs, the thermal expansin appears t balance the crsslinking shrinkage t keep the plymer vlume almst cnstant. The initial temperature beynd which the cmputer starts t change the heating rate was 138 C which was taken frm the trial and errr results. Starting at a lwer temperature will increase the duratin f the cure cycle. On the ther hand, starting frm higher temperature will reduce the ttal amunt f thermal expansin that may be needed t the balance the crsslinking shrinkage. The effect f initial temperature will be discussed in Chapter 3. It shuld be nted that the ttal duratin f the mdified cure cycle was shrter than the standard cycle. Table 2.2 shws the Glass Transitin Temperature (Tg) btained using TMA fr epxy specimens cured using the standard and the mdified cure cycles! The values f the T g fr the standard and the mdified cycles are almst the same. The mdified cure cycle was extended by 30 minutes t study the effect f such increase. The extensin f the mdified cure cycle des nt increase the T g and hence the cure was cnsidered cmplete. Table 2.2 Glass transitin temperature fr epxy specimens cured using different cure cycles. Cure Cycle Standard Cycle Mdified Cycle Mdified Cycle + 30 min. Glass Transitin Temperature 170 C 171 C 172 C 2-5 SUMMARY A new test methd t mnitr the develpment f cure induced stresses in thermset plymer cmpsites was mdified t study several materials systems. Test results were shwn t be highly reprducible. Gd qualitative agreement between test results and independently measured plymer vlume change data was seen. n

21 The test methd was mdified with a clsed lp feedback cntrl system t mdify the cure cycles t prduce less cure induced stresses. The mdified cure cycles were fund t prduce less cure induced stresses cmpared t standard cure cycles while maintaining same glass transitin temperature and in sme cases shrter cure time. l&

22 3. EFFECT OF CURE CYCLE ON CURE INDUCED STRESSES 3-1 INTRODUCTION In chapter 2, CIST methd was presented. Sample results were demnstrated and it was shwn that the test methd can be used t mnitr the cure induced stresses in single fiber thermset plymer cmpsites. The test methd was mdified with a clsed lp feedback cntrl system (CLFS) t alter the time-temperature schedule t reduce the cure induced stresses. In this chapter, results f applying CIST and CLFS t different thermset systems are shwn. The CIST was used t study the develpment f fiber stresses during the cure f several thermset plymers using cure cycles recmmend by resin manufactures. The resins included were and 934 epxies, tughened epxy, LTM-45 ELD and X lw cure temperature epxies, and bismaleimide (BMI). Fibers mainly were AS4 graphite and, fr cmparisn, S2 glass in sme cases. The CLFS was used t mdify such standard cure cycles t reduce cure induced stresses. The results were analyzed and cure cycles were classified accrding t their effect n the cure induced stresses. The cure cycles btained frm the CIST were applied t cure cmpsite laminates in vacuum-ht-press t demnstrate the applicability f such cure cycles t prcessing f cmpsites. 3-2 STANDARD CURE CYCLES In this sectin, fiber tensin data are shwn fr different materials during their standard cure cycles. Vlume change data whenever available are als shwn fr verificatin and 934 Epxies Vlume change data during standard cure cycle f epxy was shwn in Figure 2.3. Typical fiber tensin data fr AS4/ graphite/epxy single fiber cmpsite was shwn in Figure 2.6. Fr cmparisn purpse, the test was repeated with S2 glass fibers instead f the AS4 graphite fibers. The resulting fiber tensin prfile is shwn in Figure 3.1. The trends f the results are similar fr the tw different fibers. The values f the fiber tensin changes are different which may be attributed t the difference in the fiber dimensins, mechanical prperties, and interface prperties. Standard cure cycle f 934 epxy resin cnsists f: heating frm rm temperature t 116 C in 30 minutes; hlding at 116 C fr 60 minutes; raising temperature frm 116 C t 177 C in 25 minutes; hlding at 177 C fr 240 minutes fllwed by cling dwn t rm temperature. Figure 3.2 shws the vlume change data f this resin during such a cure cycle. There is crsslinking shrinkage during the first dwell temperature indicating the nset f cure. The crsslinking shrinkage increases significantly during the secnd dwell. Figure 3.3 shws the variatin in an AS4 fiber tensin during such a cure cycle. The trend f the results is similar t that epxy. Hwever, the values f the fiber tensin change are different fr these tw \P

23 resins as shuld be expected because f the differences in the vlume change characteristics f the resins and fiber/matrix interface prperties. A cmparisn between Figures 3.2 and 3.3 shws that vlumetric changes ccurring during the first dwell have n effect n fiber stresses. In this case, cure induced stresses are gverned by thechemical shrinkage during the secnd dwell nly. There is a gd agreement between vlume change data (Figure 3.2) and fiber tensin data (Figure 3.3), i.e. after the plymer has achieved relatively high viscsity, the thermal expansin causes a drp in the fiber tensin where as shrinkage causes an increase. Figure 3.3 shws that the value f the fiber tensin prir t the nset f the cl dwn (pint B) is larger than that f the initial tensin (pint A) indicating the develpment f net cmpressive stresses in the fiber due t cure shrinkage Tughened Epxy Standard cure cycle.f tughened epxy cnsists f: heating frm rm temperature t 180 C in 30 minutes; hlding at 180 C fr 360 minutes fllwed by cling dwn t rm temperature. Vlume changes f this resin during the standard cure cycle are shwn in Figure 3.4. There is vlume expansin during the initial heating frm the rm temperature t the maximum cure temperature. The crsslmking shrinkage dminates the vlume changes during the temperature hld at 180 C. Figure 3.5 shws fiber tensin prfile during the same cycle. Clearly, thermal expansin during the initial heating des nt change fiber tensin due t lw plymer viscsity during this stage. As the resin starts t cure, its stiffness develps during the temperature hld at 180 C and chemical shrinkage causes fiber tensin t increase. The rate f increase in the fiber tensin diminishes gradually as the cure appraches cmpletin. Finally, there is an increase in the fiber tensin during cl dwn due t thermal shrinkage BMI Fr the BMI resin, the standard cure cycle is divided int tw parts: initial cure and pst cure. Only the first part will be presented here (sectin 3-4 shws results fr an alternative cure cycle fr BMI including pst cure). This cycle cnsists f: heating the sample frm rm temperature t 121 C in 35 minutes; hlding at 121 C fr 45 minutes; raising temperature frm 121 C t 177 C in 25 minutes; hlding at 177 C fr 360 minutes fllwed by cling dwn t rm temperature. Figure 3.6 shws the vlume change f BMI during the first part f the standard cure cycle. Figure 3.7 shws the fiber tensin prfile fr AS4 in BMI matrix during the same cure cycle. The trends f the results are similar t thse f the epxy resins shwn abve. A cmparisn between Figures 3.6 and 3.7 shws that there is a gd crrelatin between plymer vlume change and fiber tensin change during cure, i.e. nce the plymer has develped significant stiffness, an increase r decrease in the plymer vlume prduces a decrease r increase, respectively in the fiber tensin. 20

24 3-2-4 X and LTM-45 ELD Lw Cure Temperature Epxies The standard lw temperature cure cycle fr X resin cnsists f: heating frm 30 C t 66 C at 1.5 C/min; hlding at 66 C fr 840 minutes; cling t 30 C at 2.5 C/min; heating t 177 C at 1.5 C/min; hlding fr 120 minutes; and cling t 30 C at 2.5 C/min. The standard lw temperature cure cycle fr LTM-45 ELD cnsists f: heating frm 30 C t 60 C at 0.5 C/min; hlding at 60 C fr 960 minutes; cling t 30 C at 1.5 C/min; heating t 177 C at 0.25 C/min; hlding fr 120 minutes; and cling t 30 C at 1.5 C/min. In a different set f experiments, these tw resins were cured using an autclave cure cycle that used fr epxy. That cycle cnsists f: heating frm rm temperature t 116 C in 30 minutes; hlding at 116 C fr 60 minutes; heating frm 116 C t 177 C in 25 minutes; hlding at 177 C fr 240 minutes; and cling t rm temperature in 50 minutes. This cycle was prpsed, by the industry as an alternative t cut cure time f the standard lw temperature cure cycles whenever autclave cure is justified. Figure 3.8 shws the vlume change data f X during a lw temperature cure cycle. Cure shrinkage ccurs during the 66 C hld and cntinues during the 177 C hld. Figure 3.9 shws the variatin in the fiber tensin during the same cycle. There is a slight gradual increase in fiber tensin during the 66 C hld due t cure shrinkage. During cl dwn frm 66 C t 30 C at the end f the 66 C hld, anther increase in the fiber tensin ccurs due t thermal cntractin. As the resin is heated t pst cure temperature, 177 C, the fiber tensin decreases as a result f the thermal expansin. The tensin drps belw the initial tensin value at the end f the heating ramp t 177 C. Sme increase in the fiber tensin can be seen during the 177 C hld time due t remaining cure shrinkage and stress relaxatin as discussed belw. Finally, fiber tensin increases during cl dwn frm 177 C t rm temperature. Figure 3.9 shws that fiber tensin drps belw its initial value during heating t pst cure temperature. It shuld be nted that stress relaxatin causes decay in any change in fiber tensin (increase r decrease) frm its initial value. In case f fiber tensin drp belw its initial value, stress relaxatin causes fiber tensin t increase again twards its initial value. Therefre, the increase in the fiber tensin during the 177 C hld in Figure 3.9 is due t a cmbinatin f chemical shrinkage and stress relaxatin (since fiber tensin is belw its initial value). During the abve cycle (Figure 3.9), curing at a relatively lw temperature fr a lng time increases the viscsity f the resin. This causes the thermal expansin ccurring during heating t pst cure temperature, at 177 C, t be effective expansin. The term effective vlume changes was defined in Sectin 2-2. This effective expansin causes a drp in the fiber tensin belw its initial value. This drp is larger than the increase in the fiber tensin during the 177 C hld (cmpare thermal expansin during the 177 C ramp t that f the cure shrinkage during the 177 C hld in Figure 3.8). The final value f the fiber tensin befre cl dwn is still smaller than the initial fiber tensin. This indicates that net tensile stresses are develped in the fiber prir t cl dwn. These tensile stresses will cancel part f the cmpressive stresses develped 2/

25 during the cl dwn and thus reduce residual stresses. In this particular case, stress free temperature will be belw the pst cure temperature. Stress free temperature was defined in Chapter 1. Figure 3.10 shws the vlume change data f X during the standard autclave cure cycle f epxy. A large amunt f shrinkage ccurs during the first temperature hld and additinal shrinkage ccurs during the secnd hld. Figure 3.11 shws the crrespnding fiber tensin variatin. The shrinkage during the first temperature hld causes a significant increase in the fiber tensin. The thermal expansin during the secnd temperature ramp decreases this tensin. A slight increase in fiber tensin can be seen during the secnd hld. Finally, cl dwn shrinkage increases fiber tensin. Figure 3.11 shws that the final value f the fiber tensin prir t cl dwn is mre than its initial value. In this typical autclave cycle, heating t the relatively high first temperature hld in shrt time just melts the resin and causes thermal expansin but des nt develp tensile stresses in the fiber. This causes the chemical shrinkage t dminate the vlume change and its effect will finally be added t cl dwn shrinkage causing mre residual stresses. Figure 3.9 shws that when the lw temperature cure cycle is applied, the fiber is in a tensile stress state just befre cl dwn. These tensile stresses will ffset sme f the cmpressive stresses that ccur during cl dwn resulting in less final residual stresses. On the ther hand, when typical autclave cure cycle is applied, the fiber is in a state f cmpressive stress prir cl dwn. These cmpressive stresses cmbined with stresses develped during cl dwn prduce higher residual stresses. A cmparisn between the tw cycles suggests that the autclave type cure cycle will result in mre residual stresses than the lw temperature cure cycle. Figure 3.12 shws the vlume change data f LTM-45 ELD epxy during its standard lw temperature cure cycle. Figure 3.13 shws the crrespnding fiber tensin prfile. Cmpared t X resin (Figure 3.9), mre stresses are develped during the 60 C hld. There is nt much change in the vlume during the 177 C hld. This suggests that the cure was almst cmpleted during the slw heating ramp frm 30 C t the pst cure temperature. Fiber tensin data agrees with the vlume changes data shwing nly a slight increase during the pst cure temperature hld (which may be due t stress relaxatin). Similar t X343-59, the effective thermal expansin develped during heating t pst cure temperature lwered the fiber tensin belw the initial value until the end f the cycle, i.e. fiber is under tensile stresses prir t cl dwn. Figure 3.14 shws the vlume change data f the LTM-45 ELD epxy under the autclave type cycle. The behavir f LTM-45 ELD under this cycle is similar t X under the same cycle, where mst f the cure ccurs during the first temperature hld. Figure 3.15 shws the crrespnding fiber tensin prfile. Cmpressive stresses are develped'in the cmpsite prir t cl dwn. Hence, similar t X343-59, lw temperature cure cycle is expected t prduce less residual stresses than the autclave type. 3-3 MODIFIED CURE CYCLES As discussed in Chapter 2, a clsed lp feedback cntrl system (CLFS) was develped t btain mdified cure cycles that reduce cure induced stresses. The methd is based n searching fr a heating path 22

26 that results in maximum cancellatins f the unrelaxed stresses. Typical results f applying the methd were demnstrated in Sectin Results f applying the CLFS t different cmpsite systems are given belw and 934 Epxies Fiber tensin data fr an AS4 fiber in epxy during standard cure cycle was shwn in Figure 2.6. Results f CLFS fr AS4 fibers were shwn in Figure 2.22 where a mdified cure cycle was shwn t minimize change in the fiber tensin during cure. The standard cure cycle was repeated again with S2 glass fibers instead f AS4 graphite fibers and results are shwn in Figure 3.1. Figure 3.16 shws the mdified cure cycle btained fr the glass fiber in epxy resin as btained frm CLFS. The btained cure cycle is similar t the mdified cycle f the graphite fiber/ epxy system. The slight differences are attributed t the differences in the interface prperties. Since the methd is based n reducing the strains thrugh rescheduling f vlume changes, the fiber prperties d nt seem t have much influence n the mdified cycle. Figure 3.17 shws a mdified cure cycle btained frm the feedback cntrl system fr the AS4/934 graphite/epxy system with the initial temperature taken as 138 C. By cmparing the mdified cycle with the standard cure cycle (Figure 3.3) we see that the mdified cycle reduced fiber stresses during cure. Hwever, the mdified cycle is lnger than the standard cycle because the cure cycle was extended by CLFS t allw fr mre stress relaxatin f stresses due t chemical shrinkage. Als, results in Figure 3.17 indicate that a significant part f the thermal expansin was used early in the cure cycle (as can be seen frm the rapid heating during first 60 minutes beynd initial heating). Applying thermal expansin early in the cure cycle is nt recmmended frm stress cancellatin pint f view since stresses due t chemical shrinkage are mre likely t relax in shrt time during this perid withut much temperature increase needed. Cnsuming thermal expansin during the early part f the cycle may cause chemical shrinkage t dminate the vlume changes twards the end f the cycle where stress relaxatin may nt be very effective t cancel stresses in shrt time. This suggested intrducing a temperature hld befre the feedback starts. Anther cycle was generated using CLFS with lwer initial temperature t increase the available thermal expansin (rather than extending the cure cycle t have mre stress relaxatin) and with a temperature hld. The apprach f lwering the initial temperature and intrducing the temperature hld leads t a shrter cure cycle with minimum cure-induced stresses as shwn in Figure The initial temperature was taken as 132 C. The new mdified cycle results in an almst flat fiber tensin curve. Als, the duratin f this cycle is within that f the standard cure cycle. Stresses develped during the temperature hld beynd the initial heating are small because f rapid stress relaxatin. 23

27 Tughened Epxy Figure 3.19 shws ne f the mdified cycles btained frm the CLFS. Stresses develped during cure are significantly less than that develped in the standard cycle. Hwever, it was nt pssible t cancel all the stresses develped during cure. Fr epxy resin, the rate f the chemical reactin is nly significant at high temperatures. The initial temperature was chsen as 149 C, which is higher than that used with ther resin. In this case, the amunt f thermal expansin available at the later part f the cure cycle t cunteract the chemical shrinkage will be relatively small. This can be seen frm Figure 3.19 where the maximum temperature was reached befre the cure was cmpleted (fiber tensin still increasing). This causes chemical shrinkage t finally dminate and prduce an increase in the fiber tensin during the temperature hld at 180 C. At this late stage in the cure cycle, stress relaxatin time is lng and stresses were nt relaxed (as can be seen, the fiber tensin prir t cl dwn is still higher than the initial tensin). An alternative apprach utilized was t increase the time interval used in the CLFS t allw fr mre stress relaxatin. In additin, the temperature was kept cnstant fr 3 hurs after the initial temperature t allw fr mre stress relaxatin and increase the effective thermal expansin (by lwering gelatin temperature). Stresses develped at early part f the cure cycle are mre likely t relax during cure than thse stresses develped at the later part. Figure 3.20 shws the results f the secnd mdified cycle. It was pssible t cancel mst f the stresses due t chemical shrinkage. In additin, the duratin f the new mdified cure cycle is still within that f the standard cure cycle BMI Figure 3.21 shws a cure cycle and the crrespnding fiber tensin prfile btained frm the CLFS. The initial temperature was taken as 138 C. The fiber tensin prfile during the mdified cure cycle is much unifrm cmpared t that develped during the standard cycle. Figure 3.22 shws the vlume change data f the BMI during the mdified cycle. The cure cycle was apprximated with linear segments. A gd crrespndence between fiber tensin and vlume change data can be seen after the viscsity f the plymer has increased. T study the effect f the initial temperature, up t which the specimen is heated befre enabling the CLFS, tests with different initial temperatures were cnducted. Figure 3.23 shws a mdified cycle in which the initial temperature was 143 C. The new cycle resulted in a mre unifrm fiber tensin prfile during cure. Hwever, cmpared t the mdified cycle shwn in Figure 3.21, the new cycle is lnger. Starting at high temperature reduces the ttal thermal expansin available t cancel shrinkage stresses. In this case, slwer heat rate has t be applied by the feedback t allw fr mre stress relaxatin. The slwer heating rate resulted in lnger cure cycle t cmplete the cure. A cmparisn between the tw mdified cycles shws the imprtance f chsing the initial temperature since it affects the cmbinatin f thermal expansin and stress relaxatin required t cancel the chemical shrinkage. The T g data fr BMI resin cured using the different cure cycles are shwn in Table 3.1. Fr few experiments, the mdified cure cycle was extended t study the effect f cure time increase. The value f M

28 T increases as the cure cycle is extended indicating that the cure is nt cmpleted. As was mentined earlier, the BMI resin requires pst cure at high temperature t cmplete the cure. Because the resin cure and hence shrinkage cntinues until the end f the cycle, Figures 3.21 and 3.22 shw a steady cntinuus decrease in the resin vlume and crrespnding increase in the fiber tensin twards the end f the cure cycle. Table 3.1 Glass transitin temperature fr BMI specimens cured using different cure cycles. Cure Cycle Standard Cycle Mdified Cycle Mdified Cycle + 30 min. Glass Transitin Temperature 165 C 153 C 163 C X and LTM-45 ELD Lw Cure Temperature Epxies The CLFS was used t mdify the cure cycles f single fiber cmpsites with X and LTM-45 ELD resins and AS4 fibers t reduce cure induced stresses. It shuld be nted that a majr assumptin in determining the required thermal expansin t cancel unrelaxed stresses is that the applied temperature increase will nt change the plymer chemical reactin rate greatly. Fr lw cure temperature epxies, the chemical reactin is very sensitive t the increase in temperature. Hence, the temperature increment applied by the feedback cntrl may greatly increase the rate f the reactin causing mre chemical shrinkage. T cancel the additinal shrinkage, the feedback cntrl will apply mre temperature. The new temperature increment will further increase the reactin rate and generate mre shrinkage and s n. In such cases, equilibrium at each pint may nt be pssible. In this situatin, cancellatin f stresses may be achieved by allwing mre stress relaxatin rather than heating. Hwever, increasing time intervals t allw fr mre stress relaxatin will extend the cycle duratin and may lead t vitrificatin preventing cmpletin f the cure. Vitrificatin slws dwn the chemical reactin significantly and as a result the CLFS will stp raising the temperature (since there is n shrinkage t cancel). In such cases, the cure cycle will be stpped befre the cure is cmplete. Fr X resin, Figure 3.9, mst f stresses were develped during the pst cure, s the feedback cntrl was enabled nly during this part f the cure cycle. The bjective f the feedback cntrl here is t apply the heat incrementally in such a way that the fiber tensin remains cnstant at a certain value until the resin is cured. This kind f incremental heating prevents effective cure shrinkage frm develping cmpressive stresses in the fibers during cure. Als, applying the temperature incrementally prevent large decay f tensile stresses due t thermal expansin by the stress relaxatin. Reducing stress relaxatin f tensile stresses shuld finally reduce residual stresses. The lng hld at lw temperature was applied as is. Then the temperature was increased linearly t 130 C. At this pint, feedback cntrl was enabled. This temperature was high enugh t start cntinue the 2->

29 chemical reactins and als t reach the 177 C limit in reasnable time. Figure 3.24 shws the btained cure cycle and the crrespnding fiber tensin. As can be seen, cntrlling the temperature thrugh the feedback cntrl des nt cancel shrinkage stresses cmpletely and the curve is nt flat. As discussed abve, the increase in the temperature appears t als increase the chemical shrinkage thrugh enhancing the reactin rate. This will result in net increase in the fiber tensin instead f canceling it. As the reactin advances, its rate appears t decrease allwing the thermal expansin t start canceling the chemical shrinkage. Hwever, the 177 C limit is reached befre the tensin returns t the initial value. The cmpletin f the cure was checked by measuring Tg. The btained cycle gives 171 C versus 174 C fr the standard lw temperature cure cycle. The difference appears t be statistically insignificant since the highest temperature 177 C is applied in bth cases fr similar time. Figure 3.25 shws the case where feedback cntrl was enabled at arbitrarily chsen lwer temperature (110 C). Better cntrl ver the reactin was pssible resulting in an almst flat curve. S starting at lw temperature gives the advantage f slwer reactin rate but the reactin may stp befre the cure is cmpleted because f vitrificatin. It shuld be nted that starting at higher temperature is expected t increase the reactin rate and cmplete it in shrter time but ffers less cntrl as was seen when the feedback cntrl was enabled at 130 C C. Other ptimizatin runs were perfrmed t find autclave type cure cycles with minimum residual stresses. Figures 3.26 and 3.27 shw the btained fiber tensin and resulting vlume change, respectively. Again, at ne stage in the cure cycle there is a regin in which the feedback cntrl des nt cancel stresses because the chemical reactin rate als increases greatly as the temperature increases. Hwever, the final value f the fiber tensin is almst identical t initial value. This indicates that there are n cure induced stresses develped during such a cycle in the cmpsite. Such an ptimized cycle will prduce less residual stresses than typical autclave cure cycle shwn in Figure 3.4 in which final value f the fiber tensin is mre than the initial tensin value. The increase in the fiber tensin abve the initial value indicates cmpressive cure stresses in the cmpsite. These cmpressive stresses will be added t the cmpressive stresses develped during cl dwn prducing mre residual stresses. Glass transitin temperature fr such a cycle is 176 C which means that the reactin is cmplete Using the ptimized autclave type cycle (Figure 3.26), the resin can be cured in a shrter time than fr the ptimized lw temperature cure cycle, Figure Hwever, a cmparisn between the tw cycles shws that the ptimized lw temperature cure cycle has the advantage f develping tensile stresses in the cmpsite during cure versus almst zer stresses in the ptimized autclave type cycle. These tensile stresses will decrease the final residual stresses in the cmpsite. The feedback cntrl was als used t btain an ptimized lw temperature cure cycle fr LTM-45 ELD resin with AS4 fibers. Similar t the case with X343-59, during the feedback cntrl, the temperature increase als increases the reactin rate significantly preventing canceling chemical shrinkage stresses cmpletely. After sme trials, the cycle shwn in Figure 3.28 appears t decrease stresses during the first 2t>

30 stage f the cure cycle. The pst cure was used as is since there is n significant variatin in fiber tensin. Figure 3.29 shws the crrespnding vlume change data. The ptimized cure cycle shwn in Figure 3.28 prduces almst zer stresses during the first part f the lw temperature cure cycle. This may be helpful t maintain dimensinal stability f the parts during this stage (befre pst cure at higher temperature). Als, when this ptimized cycle is cmpared with typical lw temperature cure cycle (Figure 3.13), the typical cycle results in cmpressive stresses at the end f the first stage f the cycle (as indicated by the increase in the fiber tensin prir t cl dwn frm 60 C t 30 C). These cmpressive stresses will reduce the tensile stresses that will be develped during the heating ramp t pst cure temperature. As discussed earlier, tensile stresses help t reduce residual stresses. The ptimized cycle has the advantage f almst zer stresses develped during the first stage. Thus, it keeps mre tensile stresses in the cmpsite (which wuld be used t cancel stress due t shrinkage during the first stage) and hence less final residual stresses Applicatins Results f the CIST and CLFS indicate that the mdified cycles can effectively reduce stresses prduced during cure f single fiber mdel cmpsites. Hwever, cmpsite laminates cntain high fiber vlume fractin. Als, during manufacturing there may exist gradients in the temperature and degree f cure acrss laminate thickness especially in thick parts. These factrs and thers shuld be cnsidered t mdify cure cycles fr cmpsite laminates t reduce residual as will be addressed in chapter 4. In this part f the study, the mdified cure cycle as btained frm the CLFS was applied t cmpsite laminates withut any changes. The curvature f unsymmetric laminate was used as a measure f the ttal residual stresses. The material used was /AS4 graphite/epxy prepreg frm Hercules. The cmpsite laminates were cured in a vacuum ht press. Six unsymmetric 1X10 inches [0V90 4 ] strips were cured using the standard cure cycle (Figure 2.2) and anther six strips were cured using the mdified cycle as btained frm the CLFS (Figure 2.22). The same values f the pressure, vacuum, and cl dwn rate were used in bth the cure cycles. The specimens were made narrw t limit the effect f the curvature in the 90 directin. N machining n the specimens was dne t avid any stress relaxatin. The dimensinless curvature f the specimens was calculated as fllws,, 8/D (6) Dimensinless Cuvature = z =- 4D 2 +L 2 where D, L, and t are the midpint deflectin, prjected length f the specimen, and thickness f the specimen. A phtgraph f tw representative specimens cured using the tw cycles is shwn in Figure 3.30 (a). The mdified cycle reduces the specimen curvature. A plt f the dimensinless curvature (Equatin 6) shws that the mdified cycle reduces the curvature f the unsymmetric laminates by abut 14% (Figure 3.30 (b)). The mdified cycle was als shwn t cmplete the cure as measured by the glass 27

31 transitin temperature. Additinal mdificatin f the cure cycle t accunt fr the effect f pssible temperature and cure gradients and fiber vlume fractin in the laminates may further reduce the final residual stresses. 3-4 EFFECT OF POST CURE AND COOL DOWN RATES Stresses may develp in fibers during cure, pst cure, and cl dwn. In the abve sectins, results f CIST and CLFS were shwn mainly fr stresses develp during cure. In this sectin, results f CIST fr fiber tensin changes during pst cure and cl dwn are presented. The materials used in this sectin are AS4 fibers in BMI. Standard cure cycle fr BMI was presented in sectin In this sectin, an alternative cure cycle will be used. This cycle cnsists f: heating the sample frm rm temperature 177 C in 30 minutes; hlding at 177 C fr 360 minutes; cling dwn t rm temperature in 100 minutes. The pst cure cnsists f: heating frm rm temperature t 205 C in 30 minutes; hlding at 205 C fr 360 minutes; cling dwn t rm temperature in 100 minutes. Figure 3.31 shws the variatin in the fiber tensin during such a time-temperature cycle. During the first part f the cure cycle, the fiber tensin increases due t chemical shrinkage and during cl dwn due t thermal shrinkage. During pst cure temperature hld, there is a slight increase in the fiber tensin. This increase is the resultant f tw ppsite actins. The first is the decrease in fiber tensin because f heating abve previus cure temperature t pst cure temperature (frml77 C t 205 C). The secnd is the increase in fiber tensin due t additinal cure at high temperature. It seems that the increase in the fiber tensin due t additinal cure shrinkage and thermal expansin due t additinal heating are almst f equal value and as a result, the fiber tensin is almst unaffected. The final value f the fiber tensin after cl dwn frm psture (pint B) is similar t that btained after cl dwn frm curing at 177 C (pint A). Similar findings were reprted by White and Hahn fr cmpsite laminates subjected t pst cure [22]. T study the effect f cl dwn rate n final fiber tensin, after cl dwn frm pst cure, the specimen was heated t the same temperature (205 C) and cled t rm temperature at the same previus rate. Same value f final fiber tensin was btained (pint C). In the next step, the specimen was heated t the same temperature (205 C) fllwed by cl dwn at slwer rate. The final fiber tensin at rm temperature (pint D) is apprximately 7% lwer than the previus recrded values. T verify that this reductin is due t the mre stress relaxatin allwed by slwer heating rate, the specimen was heated again fllwed by cl dwn at same rates as frm pst cure twice. The reductin in the fiber tensin disappeared when cl dwn rate was increased again (pints E and F). The test was repeated and similar results were recrded. Results indicate that slwer cl dwn rate may reduce fiber stresses due t cl dwn shrinkage. It shuld be nted that these results are fr BMI and can nt be generalized. Als, because f the dminant plymer vlume fractin in CIST (a single fiber in infinite plymer), plymer viscelasticity may be mre effective than it will be in case f cmpsites with high fiber vlume fractin. Cnstraints applied by fibers n plymer reduce the extent f viscelasticity as seen in resins. -7<?

32 3-5 CLASSIFICATION OF CURE CYCLES The results presented in the abve sectin suggest that the majr factrs affecting such cntributin are the amunt f effective shrinkage and amunt f effective expansin generated and extent f stress relaxatin allwed. Effective shrinkage and expansin. Fr a given thermset plymer and a given maximum cure temperature, the lcatin f the gelatin pint is the gverning factr that determines the amunt f effective thermal expansin. The lwer the gelatin temperature, the larger the effective thermal expansin. Effective cure shrinkage prduces cmpressive stresses in the fibers. The unrelaxed part f these stresses will be added t the cl dwn stresses increasing the residual stresses in the cmpsite. The lcatin f the gel pint can be lwered t allw fr mre effective thermal expansin. The extent f relaxatin f the cure induced stresses depends mainly n time allwed and temperature after gelatin befre cl dwn starts. The cure time beynd gelatin can be extended by slwing dwn the heating rate. This may allw fr mre stress relaxatin. Thus, it may be pssible t increase the summatin f the effective thermal expansin and stress relaxatin t vercme the stresses generated by the effective shrinkage. Depending n the balance between the three factrs, the cure cycles can be classified int three types: Type I: In this case the stresses generated by effective shrinkage are larger than the cmbined effect f the effective thermal expansin and stress relaxatin. Figure 3.32 (a) shws a schematic f such a cycle as mnitred by CIST. The tensin prir t the nset f cl dwn is larger than the initial tensin indicating net cmpressive fiber stresses due t effective shrinkage. Type II: In this case the stresses generated by effective shrinkage are equivalent t the cmbined effect f the effective thermal expansin and stress relaxatin. Figure 3.32 (b) shws a schematic f such a cycle as btained by CLFS. The tensin prir t the nset f cl dwn is the same as the initial tensin indicating zer net fiber stresses due t cure vlume changes. It shuld be nted that the balance can be generated thrugh the whle cure cycle r just prir t nset f cl dwn, i.e. allw fr increase r decrease in the fiber tensin and then cancel these changes thrugh the cycle. Type III: In this case the stresses generated by effective shrinkage are smaller than the cmbined effect f the effective thermal expansin and stress relaxatin. Figure 3.32 (c) shws a schematic f such a cure cycle as mnitred by CIST. The tensin prir t the nset f cl dwn is smaller than the initial tensin indicating net tensile fiber stresses due t effective expansin. Mst f the standard cure cycles belng t Type I. This is attributed t the rapid heating t the maximum cure temperature r using shrt dwell at lw temperature. In mst f these cycles the gelatin temperature is very clse t the maximum cure temperature which allws fr a small amunt f effective thermal expansin. Type II cycle can be generated using CLFS as explained abve. Type III cycles can be btained by lwering the gelatin temperature t increase effective expansin and allwing mre stress relaxatin. It shuld be nted that increasing the effective thermal expansin by increasing the cure temperature will nt help in terms f the final residual stresses since this will increase the cl dwn shrinkage. S, Type III cycle is basically generated by lwering the gel pint and allwing fr mre stress relaxatin. Results fr 29

33 tw typical resin systems, and epxies, are presented here. Hwever, the methd can be applied t ther resifi systems Epxy Figure 3.33 (a) shws a Type I cure cycle f epxy with crrespnding fiber tensin changes in an u AS4 graphite fiber. This cycle is the standard cure cycle recmmended by resin manufacturer. The fiber tensin prir t cl dwn is higher than the initial tensin indicating net cmpressive fiber stresses. Figure 3.33 (b) shws a Type II mdified cycle which was btained using CLFS. The change in the fiber tensin thrugh the cure cycle is clse t zer. The duratin f the mdified cycle is shrter than the standard cycle while prducing the. same glass transitin temperature as was shwn befre in Sectin These results were shwn befre and presented here fr cnvenience. Figure 3.33 (c) shws a Type III cure cycle. The cure was carried ut at a lw temperature fr lnger time t reach gelatin at lwer temperature. The effect f thermal expansin is larger than that in the standard cure cycle as can be seen by cmparing the drp in the fiber tensin crrespnding t the increase in the temperature frm the first t the secnd dwell temperature. The fiber tensin prir t cl dwn is larger than the initial fiber tensin indicating net tensile fiber stresses. These stresses will reduce the residual stresses develped during cl dwn by thermal shrinkage. The duratin f this cycle is lnger than the standard cure cycle. A cmparisn between the three types f cycles shws that Type II cycle is the shrtest and prduces less residual stresses than the standard cure cycle. Type III cycle results in minimum residual stresses hwever it is the lngest. The three types f cycles prduce the same glass transitin temperatures (170, 171, and 170 C C respectively) Tughened Epxy The standard cure cycle f (Figure 3.34 (a)) is a Type I cycle. The CLFS was applied t btain Type II and III cycles. The bjective in Type II cycle is t have a balance between effective shrinkage and the summatin f effective expansin and stress relaxatin. In this case there will be n cure induced stresses and residual stresses will that prduced nly during the cl dwn. Fr Type III cycle, the bjective is t increase the summatin f the effective thermal expansin and stress relaxatin mre than the effective shrinkage. Standard cure cycle f is a Type I cycle (shwn in Figure 3.24 (a) fr cnvenience). A Type II was develped using CLFS as shwn in sectin (shwn in Figure 3.24 (b) fr cnvenience). The CLFS was applied t prduce Type III cycle with mre effective thermal expansin and stress relaxatin. The initial heating temperature was taken as 118 C. Hwever, at this lw temperature a lnger temperature hld is required t advance the cure befre increasing the temperature (such that thermal expansin will be effective). Figure 3.34 (c) shws the Type III cure cycle. The final fiber tensin prir t cl dwn is lwer than the initial fiber tensin. This indicates the develpment f tensile stresses in the fiber prir t cl -20

34 dwn. These tensile stresses will cancel part f the cl dwn stresses and hence this type f cycle will prduce less residual stresses than the Type I and II cure cycles. Hwever, Type III cycle is cnsiderably lnger than the ther tw types f cycles Applicatins In this part f the study, the three types f cycles fr tughened epxy are applied t cure cmpsite laminates and residual stresses are cmpared. The cmpsite system used here is IM7/977-3 graphite/tughened epxy prepreg. The cure cycles are used t cure unsymmetric [0790 ] cmpsite laminates. The curvature f the cured laminates was used as an indicatr fr residual stresses. The laminates were cut and laid up manually and cured in the same lcatin f the single fiber cmpsite specimen t reduce the variatin between the cure parameters between CIST and CLFS and the laminates. The dimensins f the specimens are 3/8x3 inches and the lay up is [0,/90 2]. The cure cycles are shwn in Figure 3.34 (a), (b), and (c). The same cl dwn rate and pressure schedule were used in the three cure cycles. The specimens were made narrw t limit the effect f the curvature in the 90 directin. N machining n the specimens was dne t avid any stress relaxatin. Specimens with different layup were tried, hwever thicker specimens tend t have delaminatin which may affect the cmparisn. Curvature f the cured specimens was calculated using Equatin 6. A phtgraph f representative specimens cured using the three different cycles is shwn in Figure 3.35 (a). The mdified cycles reduced the specimen curvature. A plt f the dimensinless curvature is shwn in Figure 3.35 (b) fr the different specimens. Cmpared t the standard cure cycle, the first mdified cycle (Type II) reduced the curvature by abut 16% while the secnd mdified cycle (Type III) reduced it by abut 19%. The cure was cmpleted in all cases since the glass transitin temperatures in all three cases were statistically same (abut C). 3-6 EFFECT OF MODIFIED CURE CYCLES ON MECHANICAL PROPERTIES The bjective here is t investigate the effect f changing the cure cycle n the mechanical prperties f the cmpsite laminates which is a majr cncern fr structural applicatins. The prperties affected mst by the change in the cure cycle are thse which are matrix dependent. The inplan shear strength and shear mdulus were chsen as representative mechanical prperties. The cmpsite system used in this part f the study is IM7/977-3 graphite/tughened epxy prepreg. Symmetrical specimens (10.5X10.5 inches; (45 2, ,45 0 2)) were cut and laid and cured in a vacuum-htpress. The cured laminates were then cut t 1X10 inches strips and tested in tensin in a universal testing machine (MTS machine). Strain gauges were used t measure strains in 0 and 90 directins. Lad and strain data were cllected at clse intervals t allw fr the determinatin f the stiffness. Six specimens f each cycle were tested and shear strength and shear mdulus were calculated. Figure 3.36 (a) shws the shear strength data fr the specimens cured with the three types f cycles. The Type II cycle increased the shear strength by abut 15%. The increase in the shear strength may be due t the reductin in the residual stresses and micdamages caused by residual stresses. Type III cycle reduced 5/

35 the failure shear stress by abut 25%. This may be due t the lng heating beynd cure f the specimens, which is similar t pst curing that was shwn t reduce shear strength in epxy cmpsites [58]. Figure 3.36 shws that the three cure cycles results in apprximately the same shear mdulus. Amng the three types f cycles, Type II cycle appears t prduce the best results. While the duratin is clse t the standard cure cycle, it reduces the residual stresses and enhances the mechanical prperties. 3-7 SUMMARY AND CONCLUSION In this chapter, results f a new apprach t mnitr and reduce cure induced stresses in thermset plymer cmpsites were presented. The apprach utilizes a new test setup and a feedback cntrl system. The methd was applied t study the cure induced stresses in several thermset plymer cmpsites cured using the cure cycles recmmended by prepreg manufacturers. Mdified cure cycles that reduce cure induced stresses were demnstrated. It was shwn that the mdified cure cycles nt nly reduce residual stresses in cmpsite laminates but might enhance the matrix-dminated mechanical prperties as well, while maintaining the glass transitin temperature as thse f the standard cure cycles. A general classificatin f the cure cycles accrding t their cure induced stresses was presented and demnstrated with tw examples. The classificatin can be used t cmpare qualitatively between different cure cycles t reduce cure induced stresses. 2&

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40 56- Mhan, R. and T. H. Grenizer "Prcess Simulatin in Thermset Cmpsites fr Cure Respnse and Stress Predictin," J. f Reinfrced Plastics and Cmpsites, 14, Russell, J. D. "Analysis f Viscelastic Prperties f High-Temperature Plymer Using a Thermdynamic Equatin f State," M.Sc. Thesis, University f Daytn, Lng, E. R., S.A. Lng, W.D. Cllins, S.L. Gray, and J.G. Funk "Effect f Pstcuring n Mechanical Prperties f Pultruded Fiber-Reinfrced Epxy Cmpsites and the Neat Resin," SAMPE Quarterly, 21(2), 9-16

41 Cl 182 C C2 C3" -J 160 C J 149 C cd Ö f u IM6/CYCOM3100 graphite/bmi Time (min) (a) Cure cycles T C^ 30 u 1 Ö 20 - U a i i B Q i IM6/CYCOM3100 graphite/bmi i 1 I 1 Cl C2 C3 Cure Cycle (b) Dimensinless curvature Figure 1.1 Cure cycle mdificatin t reduce residual stresses. Adapted frm White, S. R. and H. T. Hahn "Cure Cycle Optimizatin fr the Predictin f Prcessing-Induced Residual Stresses in Cmpsite Materials," J. f Cmpsite Materials, 27(14),

42 ( C) Of=l O 150 u ca «100 u Tl P 50 H T300/976 graphite/epxy ti Time (min) (a) Cure cycles ca PQ a, CO T300/976 graphite/epxy Degree f Cure, cc p 100 (b) Spring-back Figure 1.2 Anther methd fr cure cycle mdificatin t reduce residual stresses Adapted frm Sarrazin, H., B. Kim, S-H. Ahn, and G. S. Springer "Effect f Prcessing Temperature and Layup n Springback," J. f Cmpsite Materials, 29(10),

43 40, 30 a I 20 - CO 05 «a " i 1 In KTzi Experimental Mdel IM6/CYC0M 3100 graphite/bmi I- I In Cure Temperature ( C) 149 Figure 1.3 Cmparisn between test results and mdel predictins Adapted frm White, S. R. and H. T. Hahn "Cure Cycle Optimizatin fr the Predictin f Prcessing-Induced Residual Stresses in Cmpsite Materials," J. f Cmpsite Materials, 27(14),

44 Pretensined Fiber Lad Cell ' X Fixed i Supprt (a) Plymer shrinkage increases the fiber tensin. Lad Cell i : Fixed i Supprt (b) Plymer expansin decreases the fiber tensin. Figure 2.1 Mechanism f cure induced stress test.

45 PUMP SHUT OFF VALVE RELIEF VALVE" GAUGE Figure 2.2 Details f the PVT apparatus.

46 Temperature u S-4 Vlume Change i i 100 p, B Resin: Epxy Time(hr) 7 Figure 2.3 Variatin in plymer vlume during standard cure cycle.

47 CO u Tab c c O Qß C3 -a s s u 3 HP

48 Figure 2.5 Cure induced stress test Eüü m

49 Temperature 200 AT öß lyjimj.infill» t nh^> Fiber: AS4 Resin: Epxy Time (hr) Figure 2.6 Variatin in the fiber tensin during the standard cure cycle f epxy. Temperature 200 Fiber: AS4 Resin: Epxy Fiber Tensin 1 '»www^fr^»' T u (!) Ul U CX S OJ H Time (hr) Figure 2.7 Fiber tensin during the standard cure cycle fr a single fiber cated with a thin layer f epxy.

50 Temperature 140 Fiber: AS4 Resin: EPON 828/mPDA 2 3 Time (hr) 20 Figure 2.8 Fiber tensin change during a typical tw dwell cure cycle.

51 Time(hr) Figure 2.9 Effect f temperature change n the fiber tensin (n plymer).

52 9 /-N 8 ÜO Ö 140 Temperature r " / i L=50 mm" 120 i i i S"~" / 100 _ U i i W cd H fi 7 fe fi L=25jHa, /^1 11. *" *i i ' in» _ 60 Fiber Tensin ^\ 5 H W ^ \ 40 \f v ^ } Time(hr) if Figure 2.10 Effect f embedded fiber length n the variatin in the fiber tensin during cure. 2 3 Time(hr) 20 Figure 2.11 Effect f initial fiber tensin n the variatin in the fiber tensin during cure.

53 w Ö Ö H u 9- <u Fiber: AS4 Resin: Epn 828/mPDA Epxy 1 Time (hr) Temperature u <u i a. S <u H Figure 2.12 Effect f changing cure cycle n fiber tensin in AS4/Epn 828 single fiber cmpsites.

54 T(0) PH T(Qj ^ Time = 0 1 Li -Li D CTf T(t) JH a f JH b' CTf T(t) " Time = 0 Figure 2.13 Schematic f the mdel used t calculate fiber stress during cure.

55 a OH w> CO Ö S u cd u E-i 2 3 Time(hr) Figure 2.14 Fiber stresses during a typical tw dwell cure cycle.

56 Figure 2.15 Cure induced stresses prduced fiber fracture in a pretensined fiber. Temperature 140 Fiber: AS4 Resin: EPON 828/mPDA Time(hr) 20 Figure 2.16 Variatin in fiber stresses during the mdified cure cycle.

57 00 VO - (3D c JO iu Temperature ^ \ u 0).2 "vi 3 C ifmk^*^ Fiber Tensin M a. S u cs - Fiber: AS4 Resin: Epxy H Time (hr) Figure 2.17 Fiber tensin during an intermediate cure cycle btained using trial and errr mdificatin methd. 200 Temperature Time (hr) Figure 2.18 Fiber tensin during a mdified cure cycle f epxy btained using trial and errr mdificatin methd.

58 Temperature O 4> I i_ <U Time (hr) Figure 2.19 Vlume changes during the mdified cure cycle f shwn in Figure 2.18.

59 Temperature 200 ÖD C c 4 H <u >»>Ma«;nnfii Fiber: AS4 Resin: Time (hr) Figure 2.20 Variatin in fiber tensin during anther mdified cure cycle f epxy.

60 Start Enter the initial temperature and time increment Heat t the initial temperature Hld the temperature cnstant r increase linearly Time increment Read fiber tensin and cure temperature Hld the temperature cnstant Increase the temperature prprtinally -N- J Figure 2.21 Flw chart f the Clsed Lp Feedback System (CLFS) used t btain cure cycles with reduced cure induced stresses.

61 e C -> H l-c A4 3 -r..»»v ^ >v *. Fiber: AS4 Resin: Epxy Fiber Tensin u 1002 c3 ft H 50 Time (hr) Figure 2.22 Fiber tensin data during a cure cycle mdified using CLFS. Temperature 200 1) /t c U 3 I2 > 5 -- \, Vlume Change Resin Epxy * % 150 u 1-1 li u a, E t> H 0 -* Time (hr) Figure 2.23 Vlume change data during a cure cycle mdified using CLFS.

62 Temperature 200 Glass Fiber Fibesr: AS4 Graphite and S2 Glass Resin: Epxy Fiber Tensin V u ll S-i <u a, 50 E u H 1 3 Time(hr) 4 5 Figure 3.1 Variatin in the fiber tensin during the standard cure cycle f epxy.

63 C c3 O z e "S > 0 - Resin: 934 Epxy Temperature "V. ^lume^hange 150 u U 50 s U E- n ' Z Time (hr) Figure 3.2 Vlume change f 934 epxy during the standard cure cycle ^ u <D 1- + IOOI PH Fiber: AS' Resin: 934 Epxy 50 Time (hr) L 0 Figure 3.3 Variatin in the fiber tensin during the standard cure cycle f 934 epxy.

64 Temperature Vlume Change U u i Q. B <u -2 - L 0 Resin: Epxy Time (hr) *-» Figure 3.4 Vlume change f epxy during the standard cure cycle. Temperature u 100 I 5 -!<%'IM I Mil Fiber: AS4 Resin: Epxy 50 CL, 4 J Time (hr) Figure 3.5 Fiber tensin data during the standard cure cycle f epxy resin. 0

65 200 Temperature A Vlume Change 150 u u + l a u a, Time (hr) Figure 3.6 Vlume change data f BMI resin during the standard cure cycle. Temperature Fiber Tensin 200 r 150_ u li Fiber: AS4 Resin: BMI 50 s H 2 - L Time (hr) Figure 3.7 Variatin in the fiber tensin during standard cure cycle f BMI.

66 <D DO u s " > -1 Vlume Change Temperature U 100 I cd 50 1) e H -2-3 Resin: X Time (hr) Figure 3.8 Vlume change f X during the standard lw temperature cure cycle. 200 v Fiber: AS4 Resin: X U CO u OH - 50 E Time (hr) Figure 3.9 Variatin in the fiber tensin during the standard lw temperature cure cycle f X epxy.

67 7 6 Temperature 200 <L> Ö ca ja U u s 3 " > ' Resin: X U 100 B TO <L>, S 50 u H 3 4 Time (hr) Figure 3.10 Vlume change f X during an autclave type cure cycle. 12 Temperature Fiber Tensin Fiber: AS4 Resin: X Time (hr) Figure 3.11 Variatin in the fiber tensin during an autclave type cure cycle f X

68 200 Vlume Change Temperature Resin: LTM-45 ELD 50 H Time (hr) Figure 3.12 Vlume change f LTM-45 ELD epxy during the standard lw temperature cure cycle. b.2 6 Vi C H 5 fc-l <u g4 8 - / Fiber Tensin Temperature Fiber: AS4 Resin: LTM-45 ELD u 1003 a s 50 H Time (hr) Figure 3.13 Variatin in the fiber tensin during the standard lw temperature cure cycle f LTM-45 ELD epxy.

69 Time (hr) Figure 3.14 Vlume change f LTM-45 ELD during an autclave type cure cycle B Temperature Fiber: AS4 Resin: LTM-45 ELD Fiber Tensin i t D u + 100^ 3 03 i-i 4), Time (hr) Figure 3.15 Variatin in the fiber tensin during an autclave type cure cycle f LTM-45 ELD epxy.

70 7 -- öc 6 -- e.2 'GO <u -> H Temperature»i "V ^ -V *>i+m ** I' I III»^ Fiber: S2 Resin: Epxy Fiber Tensin U -- l 3 cd 50 u Time(hr) Figure 3.16 A mdified cure cycle fr S2/ glass/epxy system.

71 Temperature C Fiber Tensin 7 "fymh^h Vl'i >!»i' I^W»4 ^- ih'wtfh-yr^-ni Fiber: AS4 Resin: 934 Epxy 100 ^ 1 50 H Time (hr) Figure 3.17 A mdified cure cycle fr AS4/934 graphite/epxy.

72 Temperature U Fiber Tensin 100 a cd OH Fiber: AS4 Resin: 934 Epxy Time (hr) Figure 3.18 Anther mdified cure cycle fr AS4/934 graphite/epxy ^7 i < VI C <D E- <a 6 - Temperature Fiber Tensin CXH 5 - Fiber: AS4 Resin: Epxy r 2 -i r 4 6 Time (hr) -~r 8 10 Figure 3.19 A mdified cure cycle fr AS4/977-3 graphite/epxy.

73 Temperature H i i i > Fiber: AS4 Resin: Epxy Fiber Tensin u u s <L> H 4 -* 4 6 Time (hr) 10 Figure 3.20 Anther mdified cure cycle fr AS4/977-3 graphite/epxy.

74 8 7 Temperature e.2,-* Fiber Tensin U I u,1-4 Fiber: AS4 Resin: BMI U CL s CD 50 H Time (hr) Figure 3.21 A mdified cure cycle fr AS4/ graphite/bmi c CO u s 3 2 > " 1': 1 TL Temperature / ^*«Vlume Change ^TH Resin: BMI I I \ \ 1 V U I. a. E u H Time (hr) Figure 3.22 Vlume change data f BMI during the mdified cure cycle shwn in Figure 3.21.

75 200 Ö O H 6-4- Temperature a Fiber Tensin h>* Ht^ Um i" 'i 'ii ^P^^J^iHV^'ifrM*» V» 100 Fiber: AS4 Resin: BMI 50 a S H 2 J Time (hr) Figure 3.23 Anther mdified cure cycle fr BMI with higher initial temperature.

76 H "S ^ 5 -\ c jj 3^ A Fiber Tensin Temperature u s Cu 6 D PH 2 1 H Fiber: AS4 Resin: X Time (hr) Figure 3.24 Variatin in the fiber tensin during a mdified lw temperature cure cycle fr AS4/X graphite/epxy ^ 6 - g 5H CO A Fiber Tensin * i i i a ' *i» 150 u u i-i 100 ^ 03 t-i (D f Fiber: AS4 Resin: X OH Time (hr) Figure 3.25 Variatin in the fiber tensin during anther mdified lw temperature cure cycle fr AS4/X graphite/epxy

77 7-5 - A C O 4 Temperature Fiber Tensin U ea E 2 Fiber: AS4 Resin: X OH S O Figure Time (hr) Variatin in the fiber tensin during a mdified autclave type cure cycle fr AS4/X graphite/epxy. 200 Öfl 1 J3 u s " > Resin: X U u CO 50 <L> OH a <u H Time (hr) Figure 3.27 Vlume change f X during the mdified cure cycle shwn in Figure 3.26.

78 Üfl e »-I cn Ö 11 s* <D Fiber: AS4 Resin: LTM-45 ELD Temperature 200 T E Time (hr) Figure3.28 Variatin in the fiber tensin during a mdified lw temperature cure cycle fr LTM-45 ELD s * U c JS -1 1 CJ E 1-2 > 0 - * ü#=~ Vlume Change " ft* 1 Temperature Resin: LTM-45 ELD U l a 50 ca %-, <a cu E w H 12 Time (hr) Figure 3.29 Vlume change f LTM-45 ELD epxy during the mdified cure cycle shwn in Figure 3.28.

79 wmmi«am Cured with Mdified Cycle Cured with Standard Cycle (a) Phtgraph. Standard Cure Cycle Mdified Cure Cycle (b) Curvature data. Figure 3.30 Curvature f unsymmetric AS4/ graphite/epxy laminates cured with different cure cycles.

80 (D) anubradraaj, m +-* "t O TT in m ü J-i Ü X) t«<u ö e CO CO 9 IS DO O»n in < c O a H c CD CO O <D I (N l/-> ca 13» ^ c CD u. c ID ca " Ü cti C+H w l-h 00 PL, ( ) uisusx J3C l!d[ 93

81 Stress Relaxatin Expansin Shrinkage Time (a) Type I cure cycle Expansin Stress Relaxatin Shrinkage I + ü u ja Temperature J^^* i i ' i i i / AS=0 J Fiber Tensin i 1. i Time (b) Type II cure cycle l I u! H Stress Relaxatin Shrinkage Expansin Time (c) Type HI cure cycle Figure 3.32 Schematic shwing the effect f the cure cycle selectin n the cntributin f cure vlume changes t fiber stresses.

82 Temperature 200 a- Type I cure cycle. 36 a.2 H i 4 yfci/ i i fc> <f Fiber: AS4 Resin: Epxy Fiber Tensin 150 ü <u l-l 3 lts I-c <u n, ^n «> DU f_ Time(hr) b- Type II cure cycle. J - 00 I 6 H Temperature / Fiber Tensin' J, U i Fiber: AS4 Resin: Epxy 50 I Time(hr) Temperature r 200 c- Type III cure cycle. D06 Fiber: AS4 Resin: Epxy Time (hr) Figure 3.33 Different types f cure cycles fr AS4/ graphite/epxy.

83 r200 a- Type I cure cycle c GO Ö H l-l Fiber: AS4 Resin: Epxy U U c3 (U H Time (hr) 200 Temperature b-type II cure cycle >M I I I I Fiber: AS4 Resin: Epxy 4 6 Time (hr) 10 Temperature 200 c- Type III cure cycle CO e H j Resin: Epxy 150 U <u *H <D OH S 4) H _. 12_, Time (hr) Figure 3.34 Different types f cure cycles fr AS4/974-3 graphite/epxy.

84 Standard Cycle Mdified Cycle #1 Mdified Cycle # 2 (a) Phtgraph. 16% -19.2% 3 CO 3 u.2 2 en C 1 Figure Standard cycle Mdified cycle # 1 Mdified cycle # 2 (Type I) (Type II) (Type III) (b) Curvature data Curvature f unsymmetric IM7/977-3 graphite/epxy laminates cured using different cure cycles.