Design considerations for selectively reinforced titanium matrix composites

Transcription

1 ( ) PII: S Materials & Design, Vl. 18, N. 3, pp , Elsevier Science Ltd Printed in Great Britain. All rights reserved $ Design cnsideratins fr selectively reinfrced titanium matrix cmpsites U. Ramamurty Schl f Mechanical and Prductin Engineering, Nanyang Technlgical University, Singapre , Singapre Received 16 Nvember 1997; accepted 8 December 1997 The rle f cladding in the mechanical prperties f SiC-fiber reinfrced Ti-matrix cmpsites ( TMCs) has been examined. Experimental and analytical results btained n mdel clad panels are reviewed. A case study was cnducted by taking the example f TMC cnnecting rd which utilizes the cladding principle fr design f a high stiffness cmpnent. Experiments n specimens which mimic real cmpnents were cnducted in an attempt t identify design-limiting factrs. Results suggest that the elastic prperties and strength f the clad TMCs can be mdeled using mdified rules f mixtures. Fatigue life f the cmpnent was lwer than expected and a brittle reactin layer was identified as the surce fr crack initiatin under fatigue and hence lwered fatigue life Elsevier Science Ltd. All rights reserved. Keywrds: cmpsites; mechanical prperties; cladding; fatigue life; design Intrductin Silicn carbide fiber reinfrced titanium matrix cmpsites Ž TMCs. have several attractive mechanical prperties such as high specific mdulus and strength and excellent fatigue and creep resistance. As a result, there has been significant interest in these materials fr applicatins in aerspace prpulsin systems where cmpnents are subjected t temperatures up t 600 C. Cnsiderable research has been cnducted n the mechanical prperties f TMCs in rder t understand the micrmechanisms f defrmatin and crack grwth 1 4. Varius micrmechanical mdels were develped t predict their behavir under cmplex perating envirnments. In general, structural cmpnents made f TMCs are expected t be clad with mnlithic Ti allys fr varius reasns. A mnlithic Ti layer wuld facilitate machining tlerance and allw fr jining f TMC cmpnents t ther metallic cmpnents in the structure. Anther ptential use fr cladding is t reinfrce cmpnents lcally in regins f high stress cncentratin such as panels cntaining thrugh-thickness hles Ž Figure 1.. The purpse here is t alleviate the high degree f ntch sensitivity exhibited by TMCs 1. Thrugh judicius selectin f the reinfrcing material and its thickness, it is anticipated that the lad bearing capac- Tel.: ; fax: ; mram@ntu.edu.sg ity f the panel at the crss-sectin f the hle diameter can be brught up t the levels crrespnding t that far away frm the hle. In additin t these tw cases, in certain instances TMC itself is used as a selective reinfrcement within a mnlithic Ti structure in rder t enhance its stiffness and strength. In all the afrementined cases, cladding will decrease the perfrmance f the cmpsite because f the lwer stiffness, strength and fatigue crack grwth resistance f the Ti ally cladding vis-a-vis ` the cmpsite. Frm the design pint f view, it is necessary t understand the changes in the prperties with respect t the clad thickness. In the cntext f lcal reinfrcement t alleviate ntch-sensitivity, a simple methdlgy t predict the thickness f cladding required fr a given hle diameter is essential. In unclad TMCs, multiple matrix cracking ccurs upn cycling the lad 5. These matrix cracks are bridged by the fibers and this is effective in preventing catastrphic failure f the TMC cmpnent. Hwever, when the TMC is cladded with a mnlithic ally, it is pssible fr the matrix cracking t get entirely suppressed. Instead a single dminant crack may nucleate and grw leading t significant reductin in fatigue life f the cmpnent. Since mst f the clad TMC cmpnents will be subjected t fatigue lading, it is paramunt t understand their fatigue behavir. The bjective f this article is t address the rle f cladding n the mechanical prperties f TMCs, partic- Materials & Design Vlume 18 Number 3 183

2 three different clad materials. With increasing clad thickness, the elastic mdulus, E and the ultimate tensile strength, u, decrease whereas the ductility in- creases. These trends can be ratinalized assuming that the fibers in the clad cmpsite are distributed unifrmly thrugh the crss-sectin with an effective fiber vlume fractin, f, given by f f Ž 1 t ˆ. Ž 1. where f is the fiber vlume fractin in unclad cmps- ite. Fllwing this apprach, the mdified rule f mixtures fr the elastic mdulus and the tensile strength f the clad cmpsites are given by 6 Figure 1 Schematic illustratin f the lcal reinfrcement cncept t alleviate ntch sensitivity ularly frm a manufacturing and design pint f view. This article is rganized in the fllwing manner. First, a brief review f wrk carried ut n mdel cmpsite panels are presented. Applicability f these results n real cmpnents is examined by taking a TMC cnnecting rd case as a practical example. Prblems that arise during prcessing f clad TMC cmpnents are highlighted. Backgrund Results btained n the mdel materials are briefly reviewed here as a backgrund. Further details can be fund in references 6 8. The mdel materials cmprised f a Ti-6Al-4V ally reinfrced unidirectinally with 32 vl.% f 6-ply SCS-6 SiC fibers. The thickness f the cmpsite, 2t, was 1.3 mm. The panels were clad n bth sides with the Ti-6Al-4V ally, with a relative clad thickness f ˆt t c t 0.5 r 1. Unntched prperties Figure 2 shws the tensile stress strain curves fr the E E Ž 1 te ˆ E. Ž 1 t ˆ. Ž 2a. m 1 t ˆ 1 t ˆ 2b Ž m. Ž. Ž. u u u u where E and Em are the Yung s mduli and u and u m are the strengths f unclad cmpsite and the clad material, respectively. Similar prcedures have been used t ratinalize the trends in residual thermal stresses, the yield stress and the fracture strain f the clad TMCs suggesting that the unntched tensile prperties are insensitive t the spatial distributin f the fibers 6,7. Ntched strength In the cntext f ntch sensitivity prblem, the purpse f the cladding is t lcally reinfrce the regin f stress cncentratin, as shwn in Figure 1. Fr a prescribed ntch size, 2 a, there exists a critical clad thickness, t c, which is needed t raise the ntched strength f the clad material up t the unntched strength f the TMC alne 8. This thickness can be cmputed n the basis f crack bridging mdel such as the Dugdale Barenblatt Ž DB. mdel which assumes that the tughness is derived frm the plastic defrmatin within a narrw plastic strip ahead f the cracktip The plastic strip is characterized by a bridging law which relates the stress within the bridging zne t Figure Tensile stress strain curves fr the three different clad cmpsites Materials & Design Vlume 18 Number

3 the lcal plastic defrmatin 9. Integral f the bridging law gives the steady-state fracture energy,. The trend in fracture energy with clad thickness can be described by a rule f mixtures relatin 8 Ž. Ž. Ž. t t t t 3 m c c where m and are the fracture energies f the cladding material and the unclad cmpsite, respectively. The DB law is characterized by a stress,, which is typically equal t the yield stress f the material and a critical displacement, u. The latter scales with the tensile fracture strain multiplied by the plastic zne width. The ntched strength is expected t vary with ntch size in accrdance with the relatin E cs exp. Ž 4. N 2 8a ž / Since the parameters and E are functins f the clad thickness, the critical clad thickness t alleviate ntch sensitivity can be calculated using Equatin Ž Results f such predictins are pltted in Figure 3 Fr hle sizes f 5 20 mm, the critical clad thickness nrmalized with respect t the unclad cmpsite thickness is predicted t be in the range f 1 2. It is anticipated that such data can prvide guidance in designing TMC cmpnents cntaining stress cncentratrs. Nte that a tw-level bridging law has t be invked s as t predict the ntch strength variatins fr lw values f ntch size Žwith respect t a characteristic bridging length. 4. Hwever, this prcedure t predict the critical clad thickness requires cmplicated numerical calculatins. Fatigue crack grwth The fatigue crack grwth characteristics f bth unclad and clad TMCs were similar t thse f unclad TMCs 2. Ntably, fatigue cracks initiated readily at the ntch rt and grew steadily thrugh the matrix accmpanied by fiber bridging 8. The main effect f fiber bridging was t cause reductin in the crack grwth rate with increasing crack length. At high stress levels, the nset f fiber fracture in the wake f the crack leads t acceleratin f crack grwth and subsequent fracture. The fatigue resistance increases with increasing clad thickness, when cmpared n the basis f fixed stress amplitude Žcalculated n the basis f the cmpsite thickness alne. 8. Crack-bridging mdels develped by McMeeking and c-wrkers have been successfully used t simulate the fatigue crack grwth respnse in TMCs In these mdels, the fibers are assumed t be frictinally cupled t the matrix thrugh a cnstant interfacial sliding resistance,. The bridging law is btained by shear lag analysis 12. Such mdels are adapted fr the clad TMCs by assuming that the fibers are evenly distributed thrugh the thickness f clad structure 8. Heat-tinting experiments shwed that the crack frnt was essentially straight thrugh the thickness f the clad structure indicating that there is n crack trapping assciated with the higher fatigue resistance f the TMC. This renders supprt fr the effective fiber vlume fractin apprach. The crack grwth simulatins are perfrmed using relevant material prperties and the values f Ž MPa., measured using fiber bundle pull-ut tests 8. Gd crrelatins were btained between the experimental and predicted fatigue crack grwth rates. This shws that the fatigue crack grwth characteristics f clad TMCs can be mdeled much in the same way f unclad TMCs. Fiber fracture threshld During fatigue cracking, there exists a critical stress amplitude, th, belw which the crack grwth rate appraches a steady-state value, independent f crack length Abve th, fiber failure ccurs in the crack wake leading t fracture. This th represents a threshld belw which the fatigue life f a cmpnent cntaining a stress cncentratr is infinite. The bridging mdel can als be used t predict the th. The Figure 3 Predicted variatin f the critical clad thickness as a functin f ntch r hle size Materials & Design Vlume 18 Number

4 value f is given by the relatin 8 th a th th ž / ab ž f S Ž 1 R. / ž f S Ž 1 R. / f f where 3 2 Ž. 5a fe EŽ1 2. f, Ž 5b. Ž f E A m is evaluated using effective fiber vlume fractin and the effective mdulus, A is a rthtrpy factr f rder unity, Sf is fiber bundle strength, is the Pis- sn s rati Ž 0.2. and ab is a characteristic bridging length defined by a DS 6 Ž 6. b f with D being the diameter f the fibers. Results f the abve fiber failure mdel are pltted in Figure 4. Here, the size f the hle Ž r stress cncentratr. is nrmalized with respect t a bridging length scale, ab A series f threshld lines are btained fr varius clad thicknesses. This figure illustrates that by cladding, it is pssible t increase the threshld fr fatigue fracture f TMCs. Unntched fatigue Experiments cnducted n dg-bne type f specimens shw that the fatigue life f clad cmpsites is significantly lwer than that f the unclad TMCs 15. This is because in the clad TMCs, multiple matrix cracking is suppressed and fatigue life was dictated by the nucleatin and grwth f a single dminant crack. Suitable mdeling methdlgy t predict fatigue life f clad TMCs is nt develped yet. In many instances, cracks nucleate at the surface f the cladding and grw twards the cmpsite as illustrated in the inset f Figure 5. This is a cnsideratin particularly in selectively reinfrced panels such as jet engine fan blades 16. In this scenari, a crack might grw thrugh the thickness f the sample if the stress n the fibers is nt high enugh t meet the fiber failure criterin, Equatins Ž 5a. and Ž 5b.. Such a situatin wuld lead t multiple matrix cracking and graceful failure f the specimen. On the ther hand, if the fiber failure criterin is met as the crack grws, the specimen will fracture catastrphically. The fiber failure criterin will depend n the clad thickness and the applied stress level. These effects are illustrated in Figure 5 where experimental results btained n edgentched specimens were presented. The stress amplitude Ž calculated n the basis f cmpsite alne. was 600 MPa in bth the cases. The fllwing feature is ntewrthy. Fr the thin cladding case, the crack stps t grw further after an extensin f 1.4 mm. This length is equal t the thickness f the fiber reinfrced sectin. Fr the thicker cladding case n the ther hand, the specimen catastrphically fractured after the crack had extended by apprx. 1.2 mm. These experimental results clearly suggest that a higher clad thickness might be detrimental when the fatigue cracks riginate frm the surface. Applicatins t cmpnent design: the cnnecting rd case Mti atin An example f a preliminary design f a hllw actuatr pistn rd is shwn in Figure 6. The mtivatin behind this design is t explit high specific stiffness f Figure 4 Fatigue threshld maps fr fiber failure during crack grwth, fr varius clad thicknesses, pltted as a functin f nrmalized hle r ntch size 186 Materials & Design Vlume 18 Number

5 Figure 5 Crack extensin vs. number f fatigue cycles fr the edge-ntched clad specimens subjected t a cmpsite net sectin stress amplitude f 600 MPa Figure 6 Preliminary design f a hllw nzzle-actuatr pistn rd utilizing the TMC Žcurtesy D. P. Walls and R. Tucker, United Technlgies, Pratt and Whitney. the cmpsite. Anther such example is a cnnecting rd in a high-temperature engine. One end f the cnnecting rd is expected t be threaded Žfr cnnecting with the crss-head. where as the ther end will be flat with a central circular hle. The latter end will be cnnected t a crank shaft with the aid f a cnnecting pin, as shwn schematically in Figure 7. Making the cnnecting rd whlly frm the TMC is nt practical, particularly frm a machining pint f view. Such cmplicated cmpnents are anticipated t be fabricated in the fllwing way. First a cmpsite f desired dimensins will be manufactured. This cmpsite is incrprated int an apprpriate Ti ally tube and then the whle assembly will be ht isstatically pressed Ž HIPed.. The rd thus prduced will be machined int a required gemetry. Althugh, the abve prcedure f manufacturing cmpnents invlves mre prcessing steps than machining a cmpnent frm a billet which is entirely cmpsite, the frmer prcess is cnsiderably simpler and cst effective. In certain instances, manufacturing Figure 7 A schematic f the clad TMC cnnecting rd which utilizes the pin cnnecting mechanism might nt be pssible in any ther way. Fr example, a threaded end r a cmpnent with crss-sectinal transitins Ž frm circular t rectangular crss-sectin. cannt be machined with cmpsite alne. Machining with adequate tlerances t prevent cntact between the cutting tl and the fibers is an additinal cnsideratin which favrs cladding rute. Experiments The bjective f the mechanical experiments n rds was t simulate perating cnditins and identify pssible prblems. Validatin f the results btained n mdel materials is an additinal mtivatin. Fr this purpse, a 16 mm diameter cmpsite rd, 183 mm in length, with a tapered end was fabricated. The cmpsite cmprised f 32 vl.% sigma SiC fibers Materials & Design Vlume 18 Number

6 Table 1 Cnstituent prperties Prperty Cmpsite Ti-6Al-4V ally Elastic mdulus, E Ž GPa Pissn s rati, Yield stress, Ž MPa. y Tensile strength, Ž MPa. u Strain t fracture, u Ž % Fracture energy, Ž kj m Fiber vlume fractin, f Ž % Figure 9 Schematic f the fatigue specimen tested. Pints A thrugh E refer t lcatins f 0 90 strain-gages Figure 8 Neutrn radigraph f a HIPed Ti ally can cntaining the TMC rd Ž D 100 m. with Ti-6 Al-4V ally as a matrix. This cmpsite rd was placed inside a Ti-6Al-4V ally tube and then this whle assembly was HIPed. A cylinder; diameter 50 mm and length 30 cm; was prduced and supplied by United Technlgies, Pratt and Whitney. A neutrn radigraph f the cylinder is shwn in Figure 8. Cnstituent prperties f the cmpsite and the Ti ally are summarized in Table 1. The purpse f the tapered end in cmpsite is t facilitate a circular t rectangular crss-sectin transitin in the cmpnent. Hwever, such a tapered end culd lead t degradatin in mechanical prperties. Fr example, the tip f the cmpsite can act as a stress cncentratr, leading t fatigue crack initiatin and 188 Materials & Design Vlume 18 Number grwth thrugh the mnlithic layer at that crss-sectin. T examine this, a fatigue specimen with the tapered end in the middle was fabricated. Schematic f the specimen is shwn in Figure 9. Five 0 90 straingages were n the specimen t measure the elastic mdulus and Pissn s rati at different lcatins. The specimen was subjected t a cyclic stress amplitude f 300 MPa, the lad rati f minimum t maximum stress in the fatigue cycle being 0. The chice f this stress amplitude was mtivated by the discussins with designers wh indicated the maximum perating stress amplitude t be arund 300 MPa. Tests were cnducted using an MTS servhydraulic testing machine with hydraulic grips. Flat cmpsite specimens with a clad thickness f 2 mm were machined and fatigue crack grwth tests were perfrmed by intrducing ntches in the cladding. The fatigue stress amplitude was 300 MPa. The purpse f this test was t check if cracks riginating in cladding were able t penetrate thrugh the cmpsite with cncmitant bridging. An actual cnnecting rd Žmachined accrding t the preliminary designs and supplied by Pratt and Whitney. was als tested until failure in mntnic lading. Micrstructural and failure analysis was cnducted using ptical and scanning electrn micrscpy. Micrstructural analysis A crss-sectin Ž in the tapered part. f the cnnecting rd is shwn in Figure 10. The cmpsite and the cladding appear t have bnded very well with n

7 Figure 10 ally A crss-sectinal view f the TMC rd clad with a Ti discernible bundary between them. Nte that the fibers are arranged in a radial pattern, particularly in the uter regins f the cmpsite. A higher magnificatin micrgraph frm the uter regin f the cmpsite shws that the fiber t fiber distance in the hp directin being much shrter than that in the radial directin, Figure 11() a. Such inhmgeneties in fiber packing can alter the mechanical behavir f the cmpsite. Detailed finite element studies are necessary t address such effects. In sme instances, the fibers can be seen tuching each ther as shwn in Figure 11() b. Very clse packing f the fibers will lead t a transitin in the lad sharing mechanisms Žfrm glbal t lcal lad sharing. reducing the tensile strength f the cmpsite 15. Furthermre, a clse packed array f brken arrays might act as nucleatin site fr fatigue crack initiatin. Prir t cmpsite manufacturing, the fibers are cated with tw 0.5 m layers f graded C, fllwed by 1 m f TiB 2. These features are illustrated thrugh a high magnificatin micrgraph f the interface regin in Figure 12. The purpse f C is t facilitate easy debnding and sliding f the fibers which is required fr glbal lad sharing as well as fiber bridging during fatigue crack grwth. The TiB2 cating serves as a diffusin barrier between the matrix and the fiber. During high temperature prcesses such as cnslidatin r HIPing, the TiB2 reacts with the matrix t frm TiB needles, which are 1 m lng. In the taper sectin f the cmpsite, hwever, the fiber ends were nt cated, which lead t a reactin between the fibers and the matrix during HIPing. Thickness f this reactin layer is apprx. 2 3 fiber diameters, Figure 13. Energy dispersive X-ray analysis shwed that the reactin prducts are silicides and carbides f Ti. Since the reactin prducts are highly brittle in nature, it is expected that they will be detrimental t the mechanical behavir f the cmpsite. Figure 11 Ž. a A SEM micrgraph shwing a rectangular pattern in the fiber packing. Ž b. An example f the fibers tuching each ther Figure 12 Elastic prperties High magnificatin image f the fiber matrix interface Elastic mdulus and Pissn s rati measured at varius lcatins are listed in Table 2. The effective fiber Materials & Design Vlume 18 Number

8 elastic mdulus and Pissn s rati s are given by E Ž 1 t ˆˆ. tem E Ž 8a. 2 Ž 1 t ˆ. and Ž 1 t ˆˆ. t m, Ž 8b. 2 Ž 1 t ˆ. Figure 13 An ptical micrgraph f the TMC Ti ally interface in the taper area shwing the reactin zne Table 2 Summary f elastic prperty measurements Lca- Cmpsite Clad Elastic Pissn s Effective fiber tin radius, thickness, mdulus, rati, vlume fractin, r Ž mm. t Ž mm. EŽ GPa. f Ž %. c c A B C D E vlume fractin fr the circular gemetry is given by 2 f f Ž 1 t ˆ. Ž 7. where ˆt is defined as the rati f clad thickness, t c,t the cmpsite radius, r c. Using simple rule f mixtures and the effective fiber vlume fractin, the effective respectively. Predictins f the abve mdel are cmpared with the experimental results in Figure 14. Here, instead f nrmalized clad thickness Žsince ˆt with r 0. c, we use the radius f the cmpsite fr predic- tins. Frm Figure 14, it is seen that there is a gd agreement between experimental results and mdel predictins. This result shws that the lngitudinal elastic prperties f fiber cmpsites are insensitive t spatial variatin in fiber distributin, validating the cnclusins drawn n mdel panels. Fracture strength The lad-displacement diagram fr the cnnecting rd supplied by Pratt and Whitney is shwn in Figure 15. Initial nn-linearity in the P curve is due t lad train cmpliance. The specimen failed when it was laded t 190 kn and the failure ccurred in the rectangular crss-sectin, just abve the transitin Ž Figure 16(). a. The cmpsite diameter at this lcatin is 7 mm, indicating that the fracture has ccurred in the cmpsite taper. The strength calculated n the basis f this crss-sectin is 1188 MPa. This cmpares favrably with the predictin f 1170 MPa f strength at this crss-sectin using rule f mixtures ŽEquatin Ž 2b. mdified fr circular gemetry.. This calculatin was perfrmed using strength f 1590 MPa fr the cmpsite, a value measured by Weber et al. 3 n a 32 vl.% sigma SiC fiber reinfrced Ti-6Al-4V cmpsite panel. The strength f the matrix, 1030 MPa, was measured using indentatin tests. Figure 14 Experimental and predicted trends in the Elastic mdulus and Pissn s rati at varius lcatins in the TMC Ti ally rd 190 Materials & Design Vlume 18 Number

9 Figure 15 Experimental lad-displacement curve fr a real TMC cnnecting rd Figure 16 Ž. a A picture f the brken TMC cnnecting rd. Ž. b A higher magnificatin view f the fracture surface Examinatin f the fracture surfaces revealed that the fracture riginated at the brittle cmpsite clad interface and grew radially utwards, Figure 16() b. Inside the cmpsite sectin the fracture mrphlgy was similar t that bserved in TMCs, with fiber matrix debnding and pull-ut, Figure 17() a. Hwever, fracture in the reactin layer is highly brittle with n fiber pull-ut, Figure 17() b. An additinal feature that is ntewrthy is the radial cracking that was bserved in-between the fibers that are clsely packed in hp Materials & Design Vlume 18 Number

10 Fatigue life The fatigue specimen failed after cycling fr 4, 64, 746 cycles, far less cmpared t tens f millins f cycles that this type f cmpnent is expected t last. Fracture ccurred in the tapered part f the cmpsite, where the diameter f the cmpsite is 6.5 mm. Nte that this is apprximately the same lcatin where fracture ccurred during mntnic lading. Fractgraphic bservatins reveal that the fracture riginated at the brittle reactin layer and grew radially utwards in a symmetric manner, Figure 18() a. The specimen fractured after a stable crack grwth f 1 mm. Mrphlgical fracture features f brittle fracture in the reactin layer, cleavage and striatins in the stable crack grwth regime and ductile dimple fracture in fast fracture regime are shwn in Figure 18( b,c,d ), respectively. A crude estimate f the fatigue life, N f, can be made using the relatin 17 2 Nf Ž m 2. CŽ. m m Ž 9. Žm 2. 2 Žm 2. 2 a a f where a and af are initial and final crack lengths C and m are Paris-law cefficients and is the gemetry factr which depends n the crack shape. Fr the crack 17 gemetry bserved, is 0.64, a 0.2 mm Ž size f the reactin layer. and af is 1.5 mm. Fr the Ti-6Al- 12 Ž V ally C and m are MPa m. 18 cycle and 3.4, respectively. Using these values, a fatigue life f cycles is predicted which is in the same rder f magnitude as the experimental value. This calculatin, albeit very crude, illustrates the detrimental effect that an initial prcessing flaw can have n the fatigue life f clad TMCs. Experiments shwed that the cracks that nucleate at the surface will nt be able t penetrate thrugh the cmpsite. Instead, delaminatin alng the cladding cmpsite interface was bserved, as shwn in Figure 19. This bservatin validates the cnclusin frm the strength and fatigue tests that the drawback in this particular design f cnnecting rd is the taper in the cmpsite that was intrduced fr the allwance f a transitin frm circular crss-sectin t square crss-sectin. Figure 17 Fracture mrphlgy in Ž. a the regin f interfacial reactin between fibers and the matrix and Ž b. in the interir f the cmpsite directin, Figure 17() c. This is prbably because f the high strain cncentratin in the thin ligament f matrix material in between the fibers. Because f such cracking, a brken fiber can lead t fracture initiatin in the fiber next t it Ž indicated by the arrw in Figure 17(). c. If this phenmenn ccurs sequentially, it may lead t decreased strength f the cmpsite. 192 Materials & Design Vlume 18 Number Cncluding remarks This wrk addressed the rle f cladding n the mechanical prperties f TMCs. Hwever, many f the cnclusins shuld in principle be applicable t ther fiber reinfrced cmpsites as well. Frm a design pint f view, results presented in this wrk shw that the unntched prperties such as elastic mdulus, Pissn s rati, ultimate tensile strength, etc., can be estimated using an effective fiber vlume fractin apprach with a reasnable accuracy. Frm the lcal reinfrcement perspective, cladding can be used as a means t alleviate ntch sensitivity exhibited by TMCs and the required clad thickness can be calculated using

11 Figure 18 Ž. a Fatigue fracture features shwing regins f stable and unstable crack grwth regins. Ž b., Ž c. and Ž d. are higher magnificatin views f lcatins 1, 2 and 3, respectively simple micrmechanical mdels. The majr drawback f cladding appears t be the reductin in fatigue life as cladding prmtes a single dminant flaw instead f multiple matrix cracking. This cnclusin can be drawn frm the experimental results n the mdel materials as well as the TMC cnnecting rd. The latter results als indicate that the designers as well as manufacturers shuld pay clse attentin t the fatigue life f the cmpnent, particularly when designing cmpsite cladding transitins. Care shuld be taken t prevent flaws such as a brittle reactin layer which are detrimental t the fatigue life f the cmpnent. Future prcessing methdlgies shuld take care t prevent reactin between fiber ends and the matrix material. Intrductin f an intermediary layer which prevents the reactin is a pssibility. This layer shuld have gd strength and bnd strngly with the matrix as well as the fibers. It will be beneficial, particularly fr enhancing the fatigue life, if the taper in the cmpsite is remved. A hemispherical end fr the cmpsite might prevent stress cncentratin and thus increase fatigue life. Acknwledgements This wrk was carried ut at the High-Perfrmance Cmpsites Center, University f Califrnia, Santa Barbara, USA. The authr wuld like t acknwledge Prfs. Frank Zk and Fred Leckie fr their encuragement and Mr. Kirk Fields fr his assistance during the mechanical testing f TMC rds. Drs. D. P. Walls and R. Tucker f United Technlgies, Pratt and Whitney, are gratefully acknwledged fr prviding the TMC rds tested in this wrk. References 1 Janssn, S., Deve and H., Evans, A.G., The anistrpic mechanical prperties f a Ti matrix cmpsite reinfrced with SiC fibers. Metallurgical Transactins, 1991, 22A, Walls, D.P., Ba, G. and Zk, F.W., Mde I fatigue cracking in a fiber reinfrced metal matrix cmpsite. Acta Metallurgica Materials, 1993, 41, Weber, C.H., Chen, X., Cnnell, S.J. and Zk F.W., On the tensile prperties f a fiber reinfrced titanium matrix cmpsite-i. Unntched behavir. Acta Metallurgica Materials, 1994, 42, Cnnell, S.J., Zk, F.W., Du, Z.Z. and Su, Z., On the tensile prperties f a fiber reinfrced titanium matrix cmpsite-ii. Influence f ntches and hles. Acta Metallurgica Materials, 1994, 42, Bakuckas, J.G., Jr., Jhnsn, W.S. and Bigelw, C.A., Fatigue damage in crss-ply titanium metal matrix cmpsites cntaining center hles. Trans ASME 1993, 115, 404 Materials & Design Vlume 18 Number

12 6 Ramamurty, U., Zk, F.W. and Leckie, F.A., Effects f cladding n the tensile prperties f titanium matrix cmpsites. Materials Science and Engineering, 1996, A214, Ramamurty, U., Zk, F.W. and Leckie, F.A., Rle f cladding in the ntched tensile prperties f a titanium matrix cmpsite. Metallurgical Transactins, 1997, 28A, Ramamurty, U., Dary, F.C. and Zk, F.W., A methd fr measuring residual strains in fiber reinfrced titanium matrix cmpsites. Acta Materials, 1996, 44( 8 ), Dugdale, D.S., Yielding f steel sheets cntaining slits. Jurnal f Mechanical Physics f Slids, 1960, 8, Barenblatt, G.I., The mathematical thery f equilibrium f crack in brittle fracture. Ad ances in Applied Mechanics, 1962, 7, Bilby, B.A., Cttrell, A.H. and Swinden, K.H., The spread f plastic yield frm a ntch. Prceedings f the Ryal Sciety, A 1963, 272, McMeeking, R.M. and Evans, A.G., Matric fatigue cracking in fiber cmpsites. Mechanics f Materials, 1990, 9, Begley, M.R. and McMeeking, R.M., Fatigue crack grwth with fiber failure in metal-matrix cmpsites. Cmparati e Science and Technlgy, 1995, 53, Ba, G. and McMeeking, RM., Fatigue crack grwth in fiber-reinfrced metal matrix cmpsites. Acta Metallurgica Materials, 1994, 41, Ramamurty, U., Fatigue in selectivity fiber-reinfrced titanium matrix cmpsites. Manuscript in preparatin 16 Dre, A.L., M Phil Thesis. The University f Birmingham, Suresh, S., Fatigue f Materials. UK: Cambridge University Press, Rsenfield, A.R., An analysis f reprted fatigue crack grwth data with special reference t Ti-6Al-4V. Engineering Fracture Mechanics, 1977, 9, 509 Figure 19 An ptical micrgraph shwing delaminatin alng cmpsite cladding interface fr edge-crack started at the surface 194 Materials & Design Vlume 18 Number