Thermal aging effect on the mechanical properties of polyester fiberglass composites

Transcription

1 J. Mater. nviron. Sci. 6 () (5) Belaid et al. ISSN : 8-58 CODN: JMSCN Theral aging eect on the echanical properties o polyester iberglass coposites S. Belaid *, S.F. Chabira, P. Balland, M. Sebaa, S. Belhouideg Mechanics laboratory, UniversityAar Telidji, Laghouat, Algéria. SYMM laboratory,univ. Savoie,, F Annecy, France. Received 9 Jan 5, Revised 5 Sept 5, Accepted 6 Sept 5 * Corresponding Author. -ail: bl4sali@gail.co Abstract In this work, the echanical properties o polyester iberglass, which is subjected to accelerated theral aging, have been evaluated or dierent periods (, 6, 9 and days). The aging teperature was ixed to 8 C. It has been observed that the echanical properties o this aterial are strongly aected by theral aging. As the aging tie increases, the elastic odulus decreases o about 5% and the stress at break decreases o approxiately %. This can be explained by the changes undergone by the icrostructure o the atrix and by the iber-atrix debonding and iber degradation. The experiental results obtained by tensile test perored on test saples or the studied aterial are then copared with those got by nuerical siulations. Keywords: Theral aging, echanical properties, tensile test, polyester iberglass coposite. Introduction Current knowledge concerning the aging eect on the echanical behavior o polyester iberglass coposites leads to oversized nesign and thereore expensive.coposite aterials are oten presented as the aterials o the uture in reason o their potential or innovation that oer. reinorced polyester glass iber coposites are increasingly used in several areas such as aerospace, autootive, shipbuilding and oshore oil [].These aterials present any advantages; low weight, high stiness-to-weight and strength-to-weight ratios. Unsaturated polyester resins reinorced with glass ibers are widely used or the construction o large structures (i.e. Pools, tanks, pipes, containers, car bodies, roos...) in aggressive environents (cheical, huidity, theral). Properties o iber reinorced polyer are relatively well known by scientists and engineers, but there are still any concerns about their durability and their perorance under severe environental conditions [].Several environental actors can cause the ageing that occurs within ibers reinorced polyers, such as oisture [ 5], water absorption [6 8] and elevated teperature [9, ].The environental eect on two dierent epoxy resins with woven glass reinorceent was studied by Sookay et al. [].The echanical behavior o iber reinorced polyer coposites can be strongly inluenced by the teperature and aterial icrostructure []. Fiber reinorced polyer coposites are sensitive to teperature variations as a result o induced theral stresses between the ibers and polyer atrix [] which arises due to their distinct theral expansion coeicients. At elevated teperatures, dierential theral expansion o iber and atrix ay lead to the oration o icro cracks at the iber/polyer interace [4]. The iber atrix interace also becoes sensitive to aggressive reactions under the exposure o high teperature environent, which can lead to the degradation o both o the ibers and the atrix [5]. The polyester iber glass coposite studied in this paper will be used in the anuacture o bath tubs and tanks. To peror this study the aterial has been aged in oven at teperature about 8 C, or an overall duration o days. The change o the echanical properties has been checked every days (onthly interval). The elastic odulus and the Failure stress have been easured and copared. Then, the experiental results have been copared with those obtained with nuerical siulations. 795

2 J. Mater. nviron. Sci. 6 () (5) Belaid et al. ISSN : 8-58 CODN: JMSCN. xperiental procedures.. Materials The studied coposite aterial is an orthotropic lainate. It is protected ro the surrounding environent by a pigented resin layer: the gelcoat UROL CP. This lainate consists o 6 plies stacked iberglasspolyester. The used resin is unsaturated orthophthalic polyester SIRSTR FS 99LP. It is diluted to styrene, oderately reactive and has a low viscosity. Its properties are given in Table. The role o the resin is to tie the ibers (cohesion role), and to ensure the transission o the stresses. Furtherore, the atrix ensures the strength o the aterial in the transverse direction o the reinorceent. The woven ibers provide a echanical strength to the aterial (traction, copression, atigue...) as shown in igure. The reinorceent is a planar abric o iber glasses (woven iber glasses o type) as cross taeta o basis weight 5g/, those properties are given in Table. The catalyst is a peroxide solution, called PMC5 (solution with 5% by weight o ethyl ethyl ketone peroxide). Two types o speciens were prepared: saples with % o catalyst and saples with % o catalyst. With the addition o the catalyst, hardening at roo teperature takes only a ew hours. Table shows the properties o the resin ater the addition o catalyst. Figure :Scheatic representation o woven abric architecture [7] Table :Mechanical properties o polyester resin lasticity odulus [Pa] Shear odulus [Pa] Poisson ratio [-] Density [g/c ] Table :Mechanical properties o iberglass lasticity odulus [Pa] Shear odulus [Pa] Poisson ratio [-] Density [g/c ] Table : Mechanical properties o polyester resin ater the addition o catalyst Flexural odulus [Pa] Barcol hardness Flexural strength [MPa] Teperature o delection under load (LDT) [ C] lastic behavior o the aterial The advantage o the polyester iberglass coposites and coposite aterials in general is that they are designed and diensioned such that the echanical stresses are applied in the iber direction. It ollows that the propertie are dierent according to the loading direction, thus the aterial is anisotropic. The studied aterial is orthototropic. For an orthotropic lainate aterial, only six independent elastic constants are needed to describe the elastic behavior. I the isotropic axis is in the direction, the independent elastic paraeters are: three Young odulus (, and ), two Poisson s coeicients (ν and ν) and a shear odulus ( ). The ollowing relationships ust be satisied: 796

3 J. Mater. nviron. Sci. 6 () (5) Belaid et al. ISSN : 8-58 CODN: JMSCN ν ij ν ji i,j,, () i ν j () ν The copliance atrix can be written as ollows (orula ()) (oigt s notation): ν ν S ν ν ν ν () Figure deines the principal axes or a typical woven iber reinorced laina. Axis is along the iber length and represents the longitudinal direction o the laina; axes and represent the transverse in-plane direction and the thickness direction respectively. Figure : Lainate reerence axes... Preparations o the coposites The unidirectional coposites with crossed plies were prepared by contact olding. The volue raction o iberglass is 4% and the volue raction o resin is 6%. The aluinu old suraces are 5x 5 ²; they are coated with an unoldingmold WIZ-F57 CN. Six iber sheets o diension 8 x 5 ² are pre-cut in a roll or each test tube to develop. The olding ethod consists in sequentially depositing on one ace o the old a resin layer, then a reinorcing layer(see igure ).The surace o the ould is thoroughly cleaned to be ready or the use, by reoving any dust and dirt ro it.the ipregnation is ade o the reinorceent by a anual operation using a roller or brush (doubling and rolling) to reove the larger bubbles. This operation is repeated several ties to obtain the desired thickness o the test tube. So, in our case 6 plies ( and 9 to the ibers directions) were superposed.the against old is put in place without any claping. The hardening is done at roo teperature. A Plates with thickness o. are obtained..4. Aging conditions The studied aterial is placed directly ater post-hardening in an oven at 8 C to accelerate the aging phenoenon. The overall duration o aging tie is days. The sapling is done every days to control the change o the echanical properties. The echanical tests are run ater the saple is reoved ro the oven, once the saple has reached an equilibriu teperature, but not too long ater to avoid uncontrolled structural evolution. 797

4 J. Mater. nviron. Sci. 6 () (5) Belaid et al. ISSN : 8-58 CODN: JMSCN Figure :Preparation o polyester iberglass coposites..5. Tensile testing The tensile tests were perored on an INSTRON 5569 achine at a ixed crosshead speed o.in -. The speciens are cut into rectangular shapes, according to the standard ISO574-4 or coposite aterials, with the ollowing diensions: Width 5, length 5 and thickness. (See igure 4). The Modulus is calculated ro the slope o the stress strain curve. Figure 4: Tensile test pieces o glassiber polyester.. Results and discussion.. Young odulus deterination The echanical properties o the unaged lainates ade in the laboratory are deterined, this is required to characterize the initial properties o the aterial. Figure 5 and Figure 6 show a representative stress strain curve o the studied aterial or saples with % and % o catalyst, respectively.it is ound that the aging tie (at 8 C) aects the echanical properties.there is no plastic doain. 4.. Stress [MPa] % -unaged % - days % -6 days % -9 days % - days..%.5%.%.5%.% Strain [%] Figure 5: xperiental curve o the tensile test or polyester iber glass coposites with % o catalyst. Young s odulus () is easured during the tensile test according to the direction (see Figure ). It is deterined ro the slope o the linear part o the stress strain curve. The Young s odulus or the coposites 798

5 J. Mater. nviron. Sci. 6 () (5) Belaid et al. ISSN : 8-58 CODN: JMSCN with % and % o catalyst decrease progressively with aging tie as shown in Figure 7. These curves show the inluence o the catalyst concentration and the aging tie on the elastic odulus. Aging tie tends to decrease the Young s odulus while the catalyst concentration iproves it. Above 55 days o heat treatent, an abrupt decrease o the slope is observable or both catalyst concentrations.the initial Young s odulus is 8.5 Pa or the coposite with % o catalyst and 8. Pa or the one with %. When the saple oves ro the unaged stage to aged one (i.e. days at 8 C), the Young s odulus decreases by 55% or the coposite with % o catalyst and 47% or the coposite with % o catalyst. In act, at the end o the aging protocol the Young s odulus becoes.7 Pa or saple with % and 4.4 Pa or the saple with % o catalyst. It can be thus considered that the catalyst tends substantially to iproves the Young s odulus o the aterial. Stress [MPa] 4... % -unaged 8. % - days 6. % -6 days 4. % -9 days. % - days..%.5%.%.5%.% Strain [%] Figure 6 : xperiental curve o the tensile test or polyester iber glass coposites with % o catalyst. Young s odulus [Pa] Aging tie [Day] Figure 7: Young s odulus variation according to aging tie at 8 C or polyester iberglass coposites with % o catalyst and % o catalyst.. volution o the ailure stress Figure 8 shows the variation o the ailure stress versus aging tie or polyester iberglass coposites with % and % o catalyst. In both cases the ailure stress decreases with aging tie. In general, the ailure stress o the atrix is lower than that o the ibers. Indeed, the breaking by cracking occurs earlier or the resin than or the glass ibers. In achine direction, the tensile ailure stress cannot be calculated ro the ixtures law, because the deoration o the atrix at break is greater than that o the ibers. However in the elastic doain o the ibers, the aterial reains hoogenous.when the breaking o a iber occurs, the load is transitted by the interace by shearing to the atrix, broken ibers retain partially their ability to carry the load or a short distance ( to ties the iber diaeter, critical length).when the average length o the broken ibers is below the critical 799 % o catalyst % o catalyst

6 J. Mater. nviron. Sci. 6 () (5) ISSN : 8-58 CODN: JMSCN Belaid et al. length, the breaking o the aterial occurs, with a loosening o the ibers. With ageing ( days, 8 C) the ailure stress decreases by % or the saples with % o catalyst and by % or the saples with % o catalyst. Beore aging the ailure stress is o about MPaorthe saples with % o catalyst and 4MPa or the saples with % o catalyst. For the last stage o aging, the ailure stress decreases till 9 MPa or the saples with % o catalyst and MPa or those o %. As or the odulus, it can be considered that the catalyst tends to slightly iprove the ailure stress o the aterial. Failure stress [MPa] Aging tie [Days] % o catalyst % o catalyst Figure 8 : Failure stress variation according to to aging tie at 8 C or polyester iber glass coposites with % o catalyst and % o catalyst.. SM icrographs The scanning electron icrographs (SM) o iberglass coposite polyester are shown in Figure 9. The teperature causes a degradation o the glass iber polyester coposite, by iber-atrix debonding and iber degradation. The eects o the theral aging will be well observable i the aterial is exposed to relatively high teperatures, and it can be considered as the ain cause o the atrix ailure at long-ter. Figure 9 : SM icrograph o a saple aged during 9 days at 8 C 4. Nuerical odeling and nuerical siulations 4. Material Characterization The aterial elastic properties o the lainate o test speciens are deterined through the law o ixtures. These properties are Young s oduli ( in direction, in direction, in direction ), Poisson s ratios (ν, ν, and ν ), inplane shear odulus ( ) and transverse shear oduli ( and ) as reerred in Figure. The elastic constants o the unidirectional coposite are calculated using the law o ixtures by the relations o equation (4) [6]. 8

7 J. Mater. nviron. Sci. 6 () (5) Belaid et al. ISSN : 8-58 CODN: JMSCN 8 (4) Where the indices and denote atrix and iber, respectively. Ater calculating, the elastic constants o the unidirectional coposite and the elastic constants o the woven abric coposite aterial are estiated using the relations o equation (5) [7] and the results are listed in table 4. 4 (5) Where and denote unidirectional iber and woven iber, respectively. Table 4: elastic properties o woven abric coposite lainate lastic odulus [Pa] lastic odulus [Pa] Poisson ratio ν [-] Poisson ratio ν [-] Shear odulus [Pa] Nuerical siulations The study o tensile was perored in D on a rectangular plate. For syetry reasons, the odel consists to a quarter o the plate. Thus, the perpendicular displaceents to the syetry plane are locked. The ANSYS 4.5code, and Abaqus 6. based on FM, are used or the nuerical siulations. For the ANSYS the structure is eshed with SOLID45 eleents. The SOLID45 is used or the -D odeling o solid structures and the nuber o eleents is about 44.The eleent is deined by eight nodes having three degrees o reedo at each node: translations in the nodal x, y, and z directions. Figure shows the eshed structure.

8 J. Mater. nviron. Sci. 6 () (5) ISSN : 8-58 CODN: JMSCN Belaid et al. (a) (b) Figure :(a) The eshed structure with SOLID45 eleent, (b)traction siulation results by abaqus. Figure shows the coparison between experiental results and those obtained by nuerical siulation. It depicts the stress according to the strain or an unaged polyester iber glass with % o catalyst.a dierence between the nuerical siulation and experient was observed. This dierencee is relatively low or the siulation with ANSYS and Abaqus. The results obtained by the two nuerical odels that are presented in the ollowing table, see reasonable, despite the deects acquired by the coposite during the anuacturing process, particularly air bubbles and uncertainties experiental devices used. Table 5: Coparison o the ultiate properties obtained by siulation (ABAQUS and ANSYS) and experientally ANSYS siulation strain stress,%,% 9,9975,% 8,45,%,5% 9,9875,7% 4,758,5%,6% 49,9687,8% 4,64,7%,95% 74,95,45% 8,4477,9%,7% 99,975,7% 59,8557,%,59% 4,9,9% 9,7,%,67% 4,67,5% % ABAQUS Siulation % xperienta strain stress Strain,5% 4,469,% al % unaged Stress 8, 4, 4,5 56,7 7,9 89, 5,,5 Stress [MPa] xperiental % - unaged Nuerical Ansys.%.5%.% Strain [%].5% Figure : nuerical and experiental coparison or unaged saples with % o catalyst 8

9 J. Mater. nviron. Sci. 6 () (5) Belaid et al. ISSN : 8-58 CODN: JMSCN Conclusion This paper presents a echanical characterization o polyester iberglass coposites therally aged at a teperature o 8 C well above the Tg (5.7 c ) o the atrix. A sapling has been perored every days and or each aging stage a tensile test has been carried out. The variations o the elastic odulus and the ailure stress have been investigated on two types o saples, the irst with a catalyst concentration o % and the second with %. It has been ound that the echanical properties o the polyester iberglass coposite are strongly aected by the theral aging. The young's odulus and the stress sensibly decreased due to the eect o teperature. The Young s odulus decreased signiicantly with aging tie (it goes ro 6% ater days to 55% ater days or the saple with % o catalyst and ro 5% ater days to 47% ater days with % o catalyst). This, leave us consider that it is o prie iportance to protect polyester iberglass coposite structures against exposure to high teperatures to avoid undesirable loss o perorances in their environent o use. In the present work a coparison between stress-strain curves obtained experientally with those siulated by nuerical odels or enlarged saples have shown a good result. In the orthcoing counications it will be presented the sae type o work, but this tie or aged saples and physico-cheical characterizations will be carried out to ake possible to link the changes o the echanical properties to that o the icrostructure. Creep experient can be regarded as a good approach to characterize echanically the aterial. It will bring additional inoration thanks to other echanical greatnesses, which will show the progressive aterial degradation. This technique is already developed in our laboratory by one o our colleagues working on polyethylene il degradation. Acknowledgeents-The authors would like to acknowledge the assistance rendered by Pr Laurent TABOUROT (Univ.Savoie, SYMM, F Annecy, France.) during the bench work o this research. Reerences. Blau P. J., ASM Hand book, Materials Park, ASM International, 8 (99) 45.. Bakis C.., Bank L.C., Brown.L., Cosenza., Davalos J.F., Lesko J.J., Machida A., Rizkalla S.H., Triantaillou T.C., J. Copos.Construct.6 () 7.. Zaar A., Bertocco F., Schjødt-Thosen J., Rauhe JC.,J.Copos.Sci.Technol.,7 () Aniskevich K., Aniskevich A., Arnautov A., Jansons J., J. Copos. Struct., 94 () Jiang X., Kolstein H., Bijlaard FSK., J.Coposites.Part B, 45 () Ouqir S. A., Bloo P. R., Toner B. M., L Azzouzi M., J. Mater. nviron. Sci., 6 (8) (5) J. Marzbanrad, A. Paykani, A. Akar, M. hajar J. Mater. nviron. Sci., 4 () () K. Singh J. Mater. nviron. Sci.,4 () () ngindeniz M., Zureick A., J. Copos. Construct, (8) 55.. Wang Y., Meng J., Zhao Q., Qi S., J. Mater. Sci.Technol., 6 () Sookay N.K., Kleperer C.J., erijenko.., J.Copos. Struct., 6 () 49.. Ray BC.,J.Colloid Interace Sci, 98, (6).. tches J., Potter K., Weaver P., Bond I., J. Copos A: Appl. Manu. Sci., 4 (9) Dorsal L.T., Rich M.J., Lloyd P.F., J. Adhesion, 6 (98). 5. Bockenheier C., Fata D., Possart W.,J.Appl. Poly. Sci., 9 (4) inson J.R., Sierakowski R.L., The behavior o structures coposed o coposite aterials, (8) Akkeran R., J.Coposites.Part B, 7 (6) 8. (5) ; 8