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Influence of processing methods on the tensile and flexure properties of high temperature composites
COMPOSITES SCIENCE AND TECHNOLOGY Composites Science and Technology 64 (2004) 1763–1772 www.elsevier.com/locate/compscitech
Influence of processing methods on the tensile and flexure properties of high temperature composites M. Ramulu a
a,*
, P.B. Stickler b, N.S. McDevitt b, I.P. Datar b, D. Kim a, M.G. Jenkins
a
Department of Mechanical Engineering, University of Washington, P.O. Box 352600, Seattle, WA 98195, USA b The Boeing Company, Seattle, WA 98124, USA Received 4 March 2003; received in revised form 22 December 2003; accepted 31 December 2003 Available online 11 March 2004
Abstract In this study, the influence of processing method on the mechanical properties of high temperature polymer matrix composites was evaluated. Three material systems were studied: PEEK(poly-ether-ether-ketone), PIXA-M, and a hybrid titanium composite laminate (HTCL). Two manufacturing processes were considered for consolidation of each material system: autoclave curing and induction heating. The tensile and flexural properties of the induction-heated materials were compared with their counterpart using the autoclave curing. Post fracture analyses were performed using optical and scanning electron micrography. Similarities and differences due to processing methods are discussed with possible reasoning for each, based on results and the post fracture analysis. These results indicate that the induction-heating process has potential as a cost-effective alternative to the autoclave curing process for consolidation of these high temperature composites. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: A. Polymer matrix composite; Hybrid laminate; B. Mechanical properties
1. Introduction The next generation military and high-speed commercial aircraft require high-strength, lightweight structural materials capable of withstanding elevated temperatures [1]. Titanium, aluminum, and advanced composites are all being widely used today in many aerospace applications. Conventional polymeric matrix composites (PMCs) reinforced with carbon fibers possess high specific strength and stiffness, and superior fatigue performance compared to monolithic metallic alloys. Unfortunately, due to the nature of the polymeric matrix, some properties such as interlaminar strength may deteriorate significantly at elevated temperatures [2]. However, recently developed hybrid composites laminates such as aramid reinforced aluminum laminates (ARALL) have been shown to exhibit excellent fatigue and damage-tolerant properties due to a fiber bridging *
Corresponding author. Tel.: +1-206-543-5349; fax: +1-206-6858047. E-mail address: [email protected] (M. Ramulu). 0266-3538/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2003.12.008
mechanism provided by the fiber reinforced adhesive layers [3]. Crack growth resistance was demonstrated by transferring load from cracked plies to uncracked plies thus providing crack tip opening resistance through adjacent adhesive layers. This mechanism provided hybrid laminates with excellent fatigue-resistance and damage tolerance. PEEK (poly-ether-ether-ketone) and PIXA-M thermoplastic resins are candidate matrix materials being considered for elevated temperature engineering applications. In combination with graphite fiber reinforcement, these resin matrix formed high temperature thermoplastic composites have operating temperatures up to 177 °C for long duration applications. HTCL (hybrid titanium composite laminate) materials are a new type of hybrid material system, that holds promise for emerging aerospace applications. HTCL consists of thin sheets of titanium adhesively bonded with a high temperature polymer resin that contains high or intermediate modulus fibers. The use of the titanium helps overcome some of the traditional limitations of composites.
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Compared to conventional PMCs, hybrid laminates exhibit: better distribution of point forces, better compression-after-impact (CAI) strength, and improved resistance to moisture, ultraviolet light, lightening strike, and abrasion. These properties make hybrid laminates very attractive for many applications [5]. Potential applications include aerospace structures, snowboards, faceplates for golf heads, and inserts for protective vests [6]. Several researchers have evaluated the properties of hybrid laminates. Veazie et al. [7] conducted uniaxial tensile tests for stress–strain response of HTCL, to study the effect of the titanium layer. The materials used for the study were Ti-15-3, b-21S, LARCTM -IAX and Carbon IM7. Burianek and Spearing [8] performed open hole tension tests on HTCL 2-6-2. HTCL 2-6-2 consists of titanium Ti-15-3 alloy, PIXA thermoplastic polyimide resin and IM7 intermediate modulus graphite fiber. The lay-up used was [Ti/0/90/02 ]. Li et al. [9] conducted a study on fabrication methods for hybrid titanium composite laminates. Two fabrication processes were evaluated, autoclave curing and the hot press forming. Fatigue damage mechanisms in HTCL have been investigated by Rhymer and Johnson [10]. A significant increase in fatigue damage tolerance was observed under both tension–tension and tension–compression loading for ply cracking. Fabrication cost is a primary concern for the use of these high temperature composites. By using induction heating to process these materials, significant cost savings may potentially be achieved. These savings are achieved due to the induction heaterÕs rapid heating and cooldown rates and reduced tool setup time. The current industry standard for processing PMCs is autoclave curing. Since the manufacturing process affects mechanical properties, the objective of this study was to investigate the trade-off between mechanical properties and fabrication cost and to determine if induction heating is suitable for consolidation of high quality PMCs.
2. Experimental methods 2.1. Materials Three material systems, differentiated by matrix type, were investigated in this study namely: PEEK, PIXA-M, and HTCL. PEEK is a semi-crystalline thermoplastic polymer consisting of linear aromatic chains. Because of its exceptional flammability, heat, chemical resistance, mechanical and electrical properties, this engineering polymer is now competing with conventional thermoset resins as a matrix for advanced fiber-reinforced composites. PIXA-M is an engineering thermoplastic that was evaluated for NASAÕs High Speed Research (HSR) programs to make HTCL panels for wing skins. HTCL is made from combining titanium foil with PIXA-M or
other thermoset systems to create a strong, high temperature composite. The PEEK panels used in this study were 16-ply quasi-isotropic flat panels with graphite fiber reinforcement and stacking sequence {(45, 0, )45, 90, 45, 0, )45, 90)2 }. The PIXA-M panels were 24-ply quasi-isotropic layups. Graphite fibers with the stacking sequence {(45, 0, )45, 90, 45, 0, )45, 90, 45, 0, )45, 90)2 }. The HTCL panels were made using two surface plies of titanium foil and eight plies of Graphite/PIXA-M with symmetric stacking sequence: {Ti, PIXA-M (0, 90, 0, 0, 0, 0, 90, 0), Ti}. 2.2. Processing Materials were consolidated by autoclave and induction heating methods shown schematically in Fig. 1 [4]. Autoclaving (Fig. 1(a)) is a combination of vacuum and pressure bag molding for fabrication with prepreg tapes. Typically, to cure and consolidate a composite panel using the autoclave process takes 27–32 h. The overall design of a typical induction processing system consists of the ceramic die contained in a phenolic box. A schematic diagram of the induction heating equipment and its elements are depicted in Fig. 1(b). The phenolic boards serve as the casting containment walls and pressure plates for the subsequent reinforcement procedure. Induction coils are cast into the die and provide electromagnetic energy as well as contain coolant to assist in the rapid cooling rates. To provide a post stresses compressive state to the ceramic die and to enable improved duability to the system, reinforcement rods are fixed to the interior of the phenolic box before the ceramic is cast and later tightened to the press. Forming, consolidation, and curing of the part take place in the part cavity. Within this part cavity, the electromagnetic energy is converted into thermal energy. During process pressurization, a strong metallic back is used as a stiff plate to keep the tools dimensionally accurate, and a mechanical constraint is used to keep the die halves together. The induction processing system also includes flexible coil connections to provide the ability to open and close the dies while the coils are connected and a power and coolant inlet/outlet in which the coils are connected to the power supply and the cooling tower system [4]. With the induction heating press, the process is complete in 1.5 h and there is no vacuum bag required. An abrasive water jet (AWJ) cutting method was used in trimming the composite tensile test specimens from the processed composite panels. All the specimens were fabricated by using: garnet abrasives, mesh size 80 with an abrasive flow rate of 10 g/s, pressure 240 MPa, traverse rate 2.5 mm/s and standoff distance 2.5 mm. Figs. 2(a) and 3(a) show photomicrographs of the sectioned PEEK composite material processed by the autoclave
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Fig. 1. Schematic of autoclave (a) and induction heating processes (b).
Fig. 2. Photomicrographs of autoclaved panels: (a) PEEK, (b) PIXA-M and (c) HTCL.
Fig. 3. Photomicrographs of induction-heated panels: (a) PEEK, (b) PIXA-M and (c) HTCL.
and induction heating methods, respectively. Note that the integrity of the materials is extremely high regardless of processing methods. Figs. 2(b) and 3(b) show the photomicrographs of the sectioned PIXA-M composite material processed by the autoclave and induction
heating methods, respectively. The material is of high quality in the case of the autoclave-processed material, however the induction-heated panel shows some voids and edge delamination. Figs. 2(c) and 3(c) show the photomicrographs of the sectioned HTCL composite
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material processed by the autoclave and induction heating methods. The induction-heated sample shows non-uniform fiber distribution and the presence of voids. 2.3. Testing The specimens were tested in accordance with ASTM D3039 [11] and ASTM D790 [12]. Three specimens of each material type and processing method were tested. For tensile testing a 500 kN test machine was used with a load cell capacity 250 kN. The test specimens were mounted in the grips of the test machine and monotonically loaded in tension while recording load. A constant cross-head speed displacement rate of 2 mm/ min was used. The applied force was measured by the load cell and the strain was determined from an extensometer. For flexural testing, a commercial servo hydraulic materials test system was used. The flexural test fixture has a support span of 40 mm, loading span of 20 mm and a deflection rate of 1 mm/min.
3. Results 3.1. Tensile tests As shown in Figs. 4(a) and 5(a), the tensile stress– strain curves for both autoclaved and induction-heated PEEK are linear up to failure, indicating brittle fracture. This type of sudden failure is typical for polymer matrix composites. Comparable stresses, strains, and moduli were observed. Regardless of processing methods, the tensile stress–strain curves shown in Figs. 4(b) and 5(b) for both autoclaved and induction-heated PIXA-M are linear until failure, again indicating brittle fracture. The panels showed a high degree of consistency between the test specimens. Comparable stresses, strains, and moduli were observed. In the case of HTCL, as shown in Figs. 4(c) and 5(c), there is a knee in the tensile stress–strain curves due to yielding of the titanium. Table 1 shows the mean and standard deviation for ultimate tensile strength, percent strain at failure, and tensile modulus of elasticity of the three PEEK, PIXAM, and HTCL specimens tested for each processing condition. Note that material properties for autoclaved and induction heated PEEK, PIXA-M and HTCL are reasonable consistent. A classical laminated plate theory analysis confirms this conclusion. Results of this analysis are given in Table 2. 3.2. Flexure tests Figs. 6 and 7 show the results from the flexural testing. As shown in Figs. 6(a) and 7(a), the force–displacement curves are very similar, which indicates the
Fig. 4. Tensile stress–strain response: autoclaved specimens. (a) PEEK, (b) PIXA-M and (c) HTCL.
first severe failure of the PEEK test specimen occurred close to 0.7 kN of force and 2 mm of displacement for both autoclaved and the induction-heated test specimens. In Fig. 6(b), the force–displacement curves show that the major failure happened close to 1.8 kN of load and 2 mm of displacement in the autoclaved case of PIXA-M, while the curves show in Fig. 7(b) indicates that the severe failure occurs at a force of 1.7 kN and a displacement of 1.5–2 mm in the induction-heated PIXA-M. The force–displacement curves shown in Figs. 6(c) and 7(c) for both autoclaved and induction-heated HTCL are linear until failure at about 0.6 kN of force and 4-4.8 mm of displacement. Table 3 summarizes the mean maximum force, deflection at maximum force, maximum stress, strain at maximum stress, and flexural modulus of elasticity for the PEEK, PIXA-M, and HTCL flexural test specimens. Again, three test specimens of each type were tested to calculate the statistics for each processing condition.
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In the tensile testing, photomicrographs of failed autoclave cured and induction-heated test specimens are shown in Figs. 8 and 9, respectively. Delamination, fiber pullout, and fiber fracture are the predominant failure modes regardless of manufacturing method for the PEEK specimens. Note from the photomicrographs that both autoclaved and induction-heated PIXA-M test specimens showed severe delamination, fiber pull out, and fiber fracture. In the case of autoclave cured and induction heated HTCL test specimens, delamination of the titanium plies and fiber breakage are the primary failure mechanisms. However, the induction-heated test specimens show severe matrix and titanium layer delamination, whereas the autoclaved cured test specimens show more local delamination damage. For flexural test results, photomicrographs of as tested autoclaved and induction-heated test specimens are shown, respectively, in Figs. 10 and 11. Fiber fracture and localized delamination are the major failure mechanisms of the PEEK regardless of the manufacturing method. The failure of the PIXA-M is caused by fiber breakage and delamination both in autoclaved and induction-heated test specimens. In the case of the HTCL specimens, the delamination between the titanium and the PIXA-M is the predominant test failure mechanisms in HTCL regardless of the manufacturing method.
4. Discussion
Fig. 5. Tensile stress–strain response: induction-heated specimens. (a) PEEK, (b) PIXA-M and (c) HTCL.
Based on the results from the tensile tests, the following general observations may be made. The failure modes in the PEEK and PIXA-M test specimens were indicative of brittle fracture. No yielding of the material
Table 1 Summary of average PEEK, PIXA-M and HTCL tensile properties Material type
Mean maximum failure force (kN)/SD
Mean ultimate tensile strength (MPa)/SD
Mean percentage strain at failure (%)/SD
Mean tensile modulus (GPa)/SD
PEEK (A) PEEK (I) PIXA-M (A) PIXA-M (I) HTCL (A) HTCL (I)
21.95 0.41 23.01 1.36 34.30 1.68 33.24 1.61 24.80 1.02 26.59 1.10
857.96 19.36 884.34 60.79 862.79 32.86 836.27 38.73 1409.10 76.82 1697.9 69.55
1.49 0.03 1.60 0.11 1.50 0.04 1.52 0.02 1.43 0.12 1.42 0.08
57.20 1.58 55.96 1.40 57.36 1.41 55.18 2.78 109.12 3.61 121.8 7.21
Cure method: (A) autoclaved; (I) induction heated. Three samples per condition.
Table 2 Comparison of predicted and actual mean tensile moduli Material
Predicted tensile modulus (GPa)
Actual mean tensile modulus (GPa)
Predicted vs. experimental (%)
PEEK (A) PIXA-M (A) HTCL (A) HTCL (I)
58.2 58.2 115 115
57.2 57.4 109 122
1.7 1.4 5.5 6.1
Cure method: (A) autoclaved; (I) induction heated.
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Fig. 6. Force–displacement response: autoclaved specimens: (a) PEEK, (b) PIXA-M and (c) HTCL.
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Fig. 7. Force–displacement response: induction-heated specimens: (a) PEEK, (b) PIXA-M and (c) HTCL.
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Table 3 Summary of mean flexural PEEK, PIXA-M and HTCL properties Cure method
Mean maximum failure load (kN)
Mean deflection at maximum load (mm)
Mean maximum stress (GN/m2 )
Mean strain at maximum stress (mm/mm)
Mean flexural modulus (kN/m2 )
PEEK (A) PEEK (I) PIXA-M (A) PIXA-M (I) HTCL (A) HTCL (I)
0.80 0.03 0.78 0.03 1.94 0.06 1.69 0.03 0.52 0.02 0.55 0.02
2.67 0.05 2.80 0.05 2.20 0.09 1.89 0.35 4.22 0.30 4.28 0.32
1.01 0.10 1.04 0.10 1.06 0.05 0.94 0.03 1.28 0.13 1.71 0.25
0.021 0.003 0.022 0.001 0.027 0.002 0.023 0.002 0.024 0.001 0.021 0.001
53.4 1.0 55.5 4.5 44.7 3.5 46.2 2.5 92.2 3.2 90.3 1.0
Cure method: (A) autoclaved; (I) induction heated. Three samples per condition.
Fig. 8. Photomicrographs of fractured edge surface of autoclaved (tensile test): (a) PEEK, (b) PIXA-M, (c) HTCL and (d) HTCL.
Fig. 9. Photomicrographs of fractured edge surface of induction heated (tensile test): (a) PEEK, (b) PIXA-M, (c) HTCL and (d) HTCL.
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Fig. 10. Photomicrographs of fractured edge surface of autoclaved (flexural test): (a) PEEK, (b) PIXA-M, (c) HTCL and (d) HTCL.
Fig. 11. Photomicrographs of fractured edge surface of induction heated (flexural test): (a) PEEK, (b) PIXA-M, (c) HTCL and (d) HTCL.
was observed. Polymer matrix composite laminates typically do not yield because of the brittle nature of the carbon fibers. However, the HTCL material showed evidence of yielding as indicated by the knee in the stress–stain curve caused by yielding of the titanium plies. This observation is consistent with other researchers [3]. The nature of failure of the inductionheated test specimens is similar to that of the autoclaved test specimens, in the cases of PEEK, PIXA-M, and HTCL. Comparable stresses, strains and moduli were observed. The experimentally determined results for both processing methods of test specimens are consistent with that of predictions.
After comparing the photomicrographs of the autoclaved panel and induction-heated PEEK and PIXA-M panels of Figs. 8 and 9, it can be observed that there are very few differences. Severe delamination accompanied by fiber pullout and fracture seem to be the predominant failure mechanism in both cases. The severe delamination and fiber pullout indicate existence of out-of-plane or interlaminar stresses and a combination of mode II and mode III failure. The fracture mechanism seems to progress with interlaminar stresses affecting the integrity of the test specimens and then finally, fiber fracture (mode I failure) causing the ultimate failure of the material. For the HTCL material the fracture surface of the
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induction-heated specimens showed severe matrix and titanium layer delamination damage, whereas the autoclaved cured specimens showed more local delamination. The autoclaved cured specimens exhibited better bonding of the titanium layer with the matrix material and overall, better consolidation. The results of the tensile and flexural testing show only slight differences in all cases including the PEEK, PIXA-M and HTCL test specimens. The properties of the test specimens are consistent in each case regardless of the manufacturing methods. Also, from the photomicrographs of the tested PEEK, PIXA-M and HTCL test specimens shown in Figs. 10 and 11, the PEEK and PIXA-M fracture mechanisms are fiber fracture and local delamination, while the primary failure mechanism of the HTCL is the delamination between the titanium and the PIXA-M. Composite materials are by definition heterogeneous. The individual laminas that constitute a continuous fiber reinforced composite laminate are anisotropic. Small changes in stacking sequence or processing parameters may alter the mechanical properties. The current industry ‘‘standard’’ is to use autoclave curing for consolidation of polymer composites. However, fabrication cost is a primary concern for the utilization of these developmental composites. Using induction heating to process these composite materials may lead to significant cost savings. These savings are achieved due to the induction heaterÕs rapid heating and cool down rates.
5. Conclusions In the case of PEEK and PIXA-M, fracture mechanisms are found to be fiber fracture and local delamination, while the primary failure mechanism of the HTCL was the delamination between the titanium and the PIXA-M. This may be attributed to material variations between the panels and temperatures variations in the manufacturing processes. The differences in tensile and flexural properties between the autoclaved and induction-heated panels, in all materials, were less than 10%. Based on this preliminary evaluation the induction-heating process has potential as a cost-effective alternative to autoclave curing process for consolidation
of these materials with only limited effect on mechanical properties.
Acknowledgements The authors would like to acknowledge The Boeing Company for providing the experimental test panels and for supporting graduate education at the University of Washington.
References [1] Veazie DR, Grover RO, Bryant GI. Titanium layer influence of the strength of a hybrid titanium composite laminate. In: Proceedings of the First NASA NANURC Conference, Greensboro, NC. March 31–April 2, 1996. [2] Johnson WS, Li E, Miller JL. High temperature hybrid titanium composite laminates: an early analytical assessment. Appl Comp Mater 1996;3:379–90. [3] Li E, Johnson WS. An investigation into fatigue of a hybrid composite laminate. J Comp Technol Res 1998;20(1):3–12. [4] McDevitt SN. Effects of processing methods on the flexural properties of high temperature composites. Masters Thesis, University of Washington, Department of Mechanical Engineering; 1999. [5] Li E. Residual tension strength study of fatigued open-hole titanium–graphite hybrid composite laminates. American Institute of Aeronautics and Astronautics, AIAA-98-1960; 1998. p. 2098– 105. [6] Fanning JC. Properties and potential non-aerospace applications of TIMETALâ15-3 hybrid titanium composite laminates. The Minerals, Metals and Materials Society; 1998. p. 353–60. [7] Veazie DR, Badir AM, Grover RO. Titanium ply effects on the behavior of a hybrid thermoplastic composite laminate. J Thermoplastic Comp Mater 1998;11(5):443–54. [8] Burianek DA, Spearing SM. Fatigue damage in titanium–graphite hybrid laminates. American Institute of Aeronautics and Astronautics, AIAA 98-1959; 1998. p. 2088–97. [9] Li E, Johnson WS, Lowthere SE, St. Clair TL. An evaluation of two fabrication methods for hybrid titanium composite laminates. In: Hooper SJ, editor. Composite materials: testing and design, vol. 13, ASTM STP 1242, American society for Testing and Materials; 1997. p. 202–14. [10] Rhymer DW, Johnson WS. Fatigue damage mechanisms in advanced hybrid titanium composite laminates. Int J Fatigue 2002;24(9):995–1001. [11] ASTM D3039/D3039M. Standard test method for tensile properties of polymer matrix composite materials; 1995. [12] ASTM D790-86. Standard test method for flexural properties of unreinforced and reinforced plastics and electrical insulating materials; 1998.
Influence of processing methods on the tensile and flexure properties of high temperature composites M. Ramulu a
a,*
, P.B. Stickler b, N.S. McDevitt b, I.P. Datar b, D. Kim a, M.G. Jenkins
a
Department of Mechanical Engineering, University of Washington, P.O. Box 352600, Seattle, WA 98195, USA b The Boeing Company, Seattle, WA 98124, USA Received 4 March 2003; received in revised form 22 December 2003; accepted 31 December 2003 Available online 11 March 2004
Abstract In this study, the influence of processing method on the mechanical properties of high temperature polymer matrix composites was evaluated. Three material systems were studied: PEEK(poly-ether-ether-ketone), PIXA-M, and a hybrid titanium composite laminate (HTCL). Two manufacturing processes were considered for consolidation of each material system: autoclave curing and induction heating. The tensile and flexural properties of the induction-heated materials were compared with their counterpart using the autoclave curing. Post fracture analyses were performed using optical and scanning electron micrography. Similarities and differences due to processing methods are discussed with possible reasoning for each, based on results and the post fracture analysis. These results indicate that the induction-heating process has potential as a cost-effective alternative to the autoclave curing process for consolidation of these high temperature composites. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: A. Polymer matrix composite; Hybrid laminate; B. Mechanical properties
1. Introduction The next generation military and high-speed commercial aircraft require high-strength, lightweight structural materials capable of withstanding elevated temperatures [1]. Titanium, aluminum, and advanced composites are all being widely used today in many aerospace applications. Conventional polymeric matrix composites (PMCs) reinforced with carbon fibers possess high specific strength and stiffness, and superior fatigue performance compared to monolithic metallic alloys. Unfortunately, due to the nature of the polymeric matrix, some properties such as interlaminar strength may deteriorate significantly at elevated temperatures [2]. However, recently developed hybrid composites laminates such as aramid reinforced aluminum laminates (ARALL) have been shown to exhibit excellent fatigue and damage-tolerant properties due to a fiber bridging *
Corresponding author. Tel.: +1-206-543-5349; fax: +1-206-6858047. E-mail address: [email protected] (M. Ramulu). 0266-3538/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2003.12.008
mechanism provided by the fiber reinforced adhesive layers [3]. Crack growth resistance was demonstrated by transferring load from cracked plies to uncracked plies thus providing crack tip opening resistance through adjacent adhesive layers. This mechanism provided hybrid laminates with excellent fatigue-resistance and damage tolerance. PEEK (poly-ether-ether-ketone) and PIXA-M thermoplastic resins are candidate matrix materials being considered for elevated temperature engineering applications. In combination with graphite fiber reinforcement, these resin matrix formed high temperature thermoplastic composites have operating temperatures up to 177 °C for long duration applications. HTCL (hybrid titanium composite laminate) materials are a new type of hybrid material system, that holds promise for emerging aerospace applications. HTCL consists of thin sheets of titanium adhesively bonded with a high temperature polymer resin that contains high or intermediate modulus fibers. The use of the titanium helps overcome some of the traditional limitations of composites.
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Compared to conventional PMCs, hybrid laminates exhibit: better distribution of point forces, better compression-after-impact (CAI) strength, and improved resistance to moisture, ultraviolet light, lightening strike, and abrasion. These properties make hybrid laminates very attractive for many applications [5]. Potential applications include aerospace structures, snowboards, faceplates for golf heads, and inserts for protective vests [6]. Several researchers have evaluated the properties of hybrid laminates. Veazie et al. [7] conducted uniaxial tensile tests for stress–strain response of HTCL, to study the effect of the titanium layer. The materials used for the study were Ti-15-3, b-21S, LARCTM -IAX and Carbon IM7. Burianek and Spearing [8] performed open hole tension tests on HTCL 2-6-2. HTCL 2-6-2 consists of titanium Ti-15-3 alloy, PIXA thermoplastic polyimide resin and IM7 intermediate modulus graphite fiber. The lay-up used was [Ti/0/90/02 ]. Li et al. [9] conducted a study on fabrication methods for hybrid titanium composite laminates. Two fabrication processes were evaluated, autoclave curing and the hot press forming. Fatigue damage mechanisms in HTCL have been investigated by Rhymer and Johnson [10]. A significant increase in fatigue damage tolerance was observed under both tension–tension and tension–compression loading for ply cracking. Fabrication cost is a primary concern for the use of these high temperature composites. By using induction heating to process these materials, significant cost savings may potentially be achieved. These savings are achieved due to the induction heaterÕs rapid heating and cooldown rates and reduced tool setup time. The current industry standard for processing PMCs is autoclave curing. Since the manufacturing process affects mechanical properties, the objective of this study was to investigate the trade-off between mechanical properties and fabrication cost and to determine if induction heating is suitable for consolidation of high quality PMCs.
2. Experimental methods 2.1. Materials Three material systems, differentiated by matrix type, were investigated in this study namely: PEEK, PIXA-M, and HTCL. PEEK is a semi-crystalline thermoplastic polymer consisting of linear aromatic chains. Because of its exceptional flammability, heat, chemical resistance, mechanical and electrical properties, this engineering polymer is now competing with conventional thermoset resins as a matrix for advanced fiber-reinforced composites. PIXA-M is an engineering thermoplastic that was evaluated for NASAÕs High Speed Research (HSR) programs to make HTCL panels for wing skins. HTCL is made from combining titanium foil with PIXA-M or
other thermoset systems to create a strong, high temperature composite. The PEEK panels used in this study were 16-ply quasi-isotropic flat panels with graphite fiber reinforcement and stacking sequence {(45, 0, )45, 90, 45, 0, )45, 90)2 }. The PIXA-M panels were 24-ply quasi-isotropic layups. Graphite fibers with the stacking sequence {(45, 0, )45, 90, 45, 0, )45, 90, 45, 0, )45, 90)2 }. The HTCL panels were made using two surface plies of titanium foil and eight plies of Graphite/PIXA-M with symmetric stacking sequence: {Ti, PIXA-M (0, 90, 0, 0, 0, 0, 90, 0), Ti}. 2.2. Processing Materials were consolidated by autoclave and induction heating methods shown schematically in Fig. 1 [4]. Autoclaving (Fig. 1(a)) is a combination of vacuum and pressure bag molding for fabrication with prepreg tapes. Typically, to cure and consolidate a composite panel using the autoclave process takes 27–32 h. The overall design of a typical induction processing system consists of the ceramic die contained in a phenolic box. A schematic diagram of the induction heating equipment and its elements are depicted in Fig. 1(b). The phenolic boards serve as the casting containment walls and pressure plates for the subsequent reinforcement procedure. Induction coils are cast into the die and provide electromagnetic energy as well as contain coolant to assist in the rapid cooling rates. To provide a post stresses compressive state to the ceramic die and to enable improved duability to the system, reinforcement rods are fixed to the interior of the phenolic box before the ceramic is cast and later tightened to the press. Forming, consolidation, and curing of the part take place in the part cavity. Within this part cavity, the electromagnetic energy is converted into thermal energy. During process pressurization, a strong metallic back is used as a stiff plate to keep the tools dimensionally accurate, and a mechanical constraint is used to keep the die halves together. The induction processing system also includes flexible coil connections to provide the ability to open and close the dies while the coils are connected and a power and coolant inlet/outlet in which the coils are connected to the power supply and the cooling tower system [4]. With the induction heating press, the process is complete in 1.5 h and there is no vacuum bag required. An abrasive water jet (AWJ) cutting method was used in trimming the composite tensile test specimens from the processed composite panels. All the specimens were fabricated by using: garnet abrasives, mesh size 80 with an abrasive flow rate of 10 g/s, pressure 240 MPa, traverse rate 2.5 mm/s and standoff distance 2.5 mm. Figs. 2(a) and 3(a) show photomicrographs of the sectioned PEEK composite material processed by the autoclave
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Fig. 1. Schematic of autoclave (a) and induction heating processes (b).
Fig. 2. Photomicrographs of autoclaved panels: (a) PEEK, (b) PIXA-M and (c) HTCL.
Fig. 3. Photomicrographs of induction-heated panels: (a) PEEK, (b) PIXA-M and (c) HTCL.
and induction heating methods, respectively. Note that the integrity of the materials is extremely high regardless of processing methods. Figs. 2(b) and 3(b) show the photomicrographs of the sectioned PIXA-M composite material processed by the autoclave and induction
heating methods, respectively. The material is of high quality in the case of the autoclave-processed material, however the induction-heated panel shows some voids and edge delamination. Figs. 2(c) and 3(c) show the photomicrographs of the sectioned HTCL composite
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material processed by the autoclave and induction heating methods. The induction-heated sample shows non-uniform fiber distribution and the presence of voids. 2.3. Testing The specimens were tested in accordance with ASTM D3039 [11] and ASTM D790 [12]. Three specimens of each material type and processing method were tested. For tensile testing a 500 kN test machine was used with a load cell capacity 250 kN. The test specimens were mounted in the grips of the test machine and monotonically loaded in tension while recording load. A constant cross-head speed displacement rate of 2 mm/ min was used. The applied force was measured by the load cell and the strain was determined from an extensometer. For flexural testing, a commercial servo hydraulic materials test system was used. The flexural test fixture has a support span of 40 mm, loading span of 20 mm and a deflection rate of 1 mm/min.
3. Results 3.1. Tensile tests As shown in Figs. 4(a) and 5(a), the tensile stress– strain curves for both autoclaved and induction-heated PEEK are linear up to failure, indicating brittle fracture. This type of sudden failure is typical for polymer matrix composites. Comparable stresses, strains, and moduli were observed. Regardless of processing methods, the tensile stress–strain curves shown in Figs. 4(b) and 5(b) for both autoclaved and induction-heated PIXA-M are linear until failure, again indicating brittle fracture. The panels showed a high degree of consistency between the test specimens. Comparable stresses, strains, and moduli were observed. In the case of HTCL, as shown in Figs. 4(c) and 5(c), there is a knee in the tensile stress–strain curves due to yielding of the titanium. Table 1 shows the mean and standard deviation for ultimate tensile strength, percent strain at failure, and tensile modulus of elasticity of the three PEEK, PIXAM, and HTCL specimens tested for each processing condition. Note that material properties for autoclaved and induction heated PEEK, PIXA-M and HTCL are reasonable consistent. A classical laminated plate theory analysis confirms this conclusion. Results of this analysis are given in Table 2. 3.2. Flexure tests Figs. 6 and 7 show the results from the flexural testing. As shown in Figs. 6(a) and 7(a), the force–displacement curves are very similar, which indicates the
Fig. 4. Tensile stress–strain response: autoclaved specimens. (a) PEEK, (b) PIXA-M and (c) HTCL.
first severe failure of the PEEK test specimen occurred close to 0.7 kN of force and 2 mm of displacement for both autoclaved and the induction-heated test specimens. In Fig. 6(b), the force–displacement curves show that the major failure happened close to 1.8 kN of load and 2 mm of displacement in the autoclaved case of PIXA-M, while the curves show in Fig. 7(b) indicates that the severe failure occurs at a force of 1.7 kN and a displacement of 1.5–2 mm in the induction-heated PIXA-M. The force–displacement curves shown in Figs. 6(c) and 7(c) for both autoclaved and induction-heated HTCL are linear until failure at about 0.6 kN of force and 4-4.8 mm of displacement. Table 3 summarizes the mean maximum force, deflection at maximum force, maximum stress, strain at maximum stress, and flexural modulus of elasticity for the PEEK, PIXA-M, and HTCL flexural test specimens. Again, three test specimens of each type were tested to calculate the statistics for each processing condition.
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In the tensile testing, photomicrographs of failed autoclave cured and induction-heated test specimens are shown in Figs. 8 and 9, respectively. Delamination, fiber pullout, and fiber fracture are the predominant failure modes regardless of manufacturing method for the PEEK specimens. Note from the photomicrographs that both autoclaved and induction-heated PIXA-M test specimens showed severe delamination, fiber pull out, and fiber fracture. In the case of autoclave cured and induction heated HTCL test specimens, delamination of the titanium plies and fiber breakage are the primary failure mechanisms. However, the induction-heated test specimens show severe matrix and titanium layer delamination, whereas the autoclaved cured test specimens show more local delamination damage. For flexural test results, photomicrographs of as tested autoclaved and induction-heated test specimens are shown, respectively, in Figs. 10 and 11. Fiber fracture and localized delamination are the major failure mechanisms of the PEEK regardless of the manufacturing method. The failure of the PIXA-M is caused by fiber breakage and delamination both in autoclaved and induction-heated test specimens. In the case of the HTCL specimens, the delamination between the titanium and the PIXA-M is the predominant test failure mechanisms in HTCL regardless of the manufacturing method.
4. Discussion
Fig. 5. Tensile stress–strain response: induction-heated specimens. (a) PEEK, (b) PIXA-M and (c) HTCL.
Based on the results from the tensile tests, the following general observations may be made. The failure modes in the PEEK and PIXA-M test specimens were indicative of brittle fracture. No yielding of the material
Table 1 Summary of average PEEK, PIXA-M and HTCL tensile properties Material type
Mean maximum failure force (kN)/SD
Mean ultimate tensile strength (MPa)/SD
Mean percentage strain at failure (%)/SD
Mean tensile modulus (GPa)/SD
PEEK (A) PEEK (I) PIXA-M (A) PIXA-M (I) HTCL (A) HTCL (I)
21.95 0.41 23.01 1.36 34.30 1.68 33.24 1.61 24.80 1.02 26.59 1.10
857.96 19.36 884.34 60.79 862.79 32.86 836.27 38.73 1409.10 76.82 1697.9 69.55
1.49 0.03 1.60 0.11 1.50 0.04 1.52 0.02 1.43 0.12 1.42 0.08
57.20 1.58 55.96 1.40 57.36 1.41 55.18 2.78 109.12 3.61 121.8 7.21
Cure method: (A) autoclaved; (I) induction heated. Three samples per condition.
Table 2 Comparison of predicted and actual mean tensile moduli Material
Predicted tensile modulus (GPa)
Actual mean tensile modulus (GPa)
Predicted vs. experimental (%)
PEEK (A) PIXA-M (A) HTCL (A) HTCL (I)
58.2 58.2 115 115
57.2 57.4 109 122
1.7 1.4 5.5 6.1
Cure method: (A) autoclaved; (I) induction heated.
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Fig. 6. Force–displacement response: autoclaved specimens: (a) PEEK, (b) PIXA-M and (c) HTCL.
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Fig. 7. Force–displacement response: induction-heated specimens: (a) PEEK, (b) PIXA-M and (c) HTCL.
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Table 3 Summary of mean flexural PEEK, PIXA-M and HTCL properties Cure method
Mean maximum failure load (kN)
Mean deflection at maximum load (mm)
Mean maximum stress (GN/m2 )
Mean strain at maximum stress (mm/mm)
Mean flexural modulus (kN/m2 )
PEEK (A) PEEK (I) PIXA-M (A) PIXA-M (I) HTCL (A) HTCL (I)
0.80 0.03 0.78 0.03 1.94 0.06 1.69 0.03 0.52 0.02 0.55 0.02
2.67 0.05 2.80 0.05 2.20 0.09 1.89 0.35 4.22 0.30 4.28 0.32
1.01 0.10 1.04 0.10 1.06 0.05 0.94 0.03 1.28 0.13 1.71 0.25
0.021 0.003 0.022 0.001 0.027 0.002 0.023 0.002 0.024 0.001 0.021 0.001
53.4 1.0 55.5 4.5 44.7 3.5 46.2 2.5 92.2 3.2 90.3 1.0
Cure method: (A) autoclaved; (I) induction heated. Three samples per condition.
Fig. 8. Photomicrographs of fractured edge surface of autoclaved (tensile test): (a) PEEK, (b) PIXA-M, (c) HTCL and (d) HTCL.
Fig. 9. Photomicrographs of fractured edge surface of induction heated (tensile test): (a) PEEK, (b) PIXA-M, (c) HTCL and (d) HTCL.
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Fig. 10. Photomicrographs of fractured edge surface of autoclaved (flexural test): (a) PEEK, (b) PIXA-M, (c) HTCL and (d) HTCL.
Fig. 11. Photomicrographs of fractured edge surface of induction heated (flexural test): (a) PEEK, (b) PIXA-M, (c) HTCL and (d) HTCL.
was observed. Polymer matrix composite laminates typically do not yield because of the brittle nature of the carbon fibers. However, the HTCL material showed evidence of yielding as indicated by the knee in the stress–stain curve caused by yielding of the titanium plies. This observation is consistent with other researchers [3]. The nature of failure of the inductionheated test specimens is similar to that of the autoclaved test specimens, in the cases of PEEK, PIXA-M, and HTCL. Comparable stresses, strains and moduli were observed. The experimentally determined results for both processing methods of test specimens are consistent with that of predictions.
After comparing the photomicrographs of the autoclaved panel and induction-heated PEEK and PIXA-M panels of Figs. 8 and 9, it can be observed that there are very few differences. Severe delamination accompanied by fiber pullout and fracture seem to be the predominant failure mechanism in both cases. The severe delamination and fiber pullout indicate existence of out-of-plane or interlaminar stresses and a combination of mode II and mode III failure. The fracture mechanism seems to progress with interlaminar stresses affecting the integrity of the test specimens and then finally, fiber fracture (mode I failure) causing the ultimate failure of the material. For the HTCL material the fracture surface of the
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induction-heated specimens showed severe matrix and titanium layer delamination damage, whereas the autoclaved cured specimens showed more local delamination. The autoclaved cured specimens exhibited better bonding of the titanium layer with the matrix material and overall, better consolidation. The results of the tensile and flexural testing show only slight differences in all cases including the PEEK, PIXA-M and HTCL test specimens. The properties of the test specimens are consistent in each case regardless of the manufacturing methods. Also, from the photomicrographs of the tested PEEK, PIXA-M and HTCL test specimens shown in Figs. 10 and 11, the PEEK and PIXA-M fracture mechanisms are fiber fracture and local delamination, while the primary failure mechanism of the HTCL is the delamination between the titanium and the PIXA-M. Composite materials are by definition heterogeneous. The individual laminas that constitute a continuous fiber reinforced composite laminate are anisotropic. Small changes in stacking sequence or processing parameters may alter the mechanical properties. The current industry ‘‘standard’’ is to use autoclave curing for consolidation of polymer composites. However, fabrication cost is a primary concern for the utilization of these developmental composites. Using induction heating to process these composite materials may lead to significant cost savings. These savings are achieved due to the induction heaterÕs rapid heating and cool down rates.
5. Conclusions In the case of PEEK and PIXA-M, fracture mechanisms are found to be fiber fracture and local delamination, while the primary failure mechanism of the HTCL was the delamination between the titanium and the PIXA-M. This may be attributed to material variations between the panels and temperatures variations in the manufacturing processes. The differences in tensile and flexural properties between the autoclaved and induction-heated panels, in all materials, were less than 10%. Based on this preliminary evaluation the induction-heating process has potential as a cost-effective alternative to autoclave curing process for consolidation
of these materials with only limited effect on mechanical properties.
Acknowledgements The authors would like to acknowledge The Boeing Company for providing the experimental test panels and for supporting graduate education at the University of Washington.
References [1] Veazie DR, Grover RO, Bryant GI. Titanium layer influence of the strength of a hybrid titanium composite laminate. In: Proceedings of the First NASA NANURC Conference, Greensboro, NC. March 31–April 2, 1996. [2] Johnson WS, Li E, Miller JL. High temperature hybrid titanium composite laminates: an early analytical assessment. Appl Comp Mater 1996;3:379–90. [3] Li E, Johnson WS. An investigation into fatigue of a hybrid composite laminate. J Comp Technol Res 1998;20(1):3–12. [4] McDevitt SN. Effects of processing methods on the flexural properties of high temperature composites. Masters Thesis, University of Washington, Department of Mechanical Engineering; 1999. [5] Li E. Residual tension strength study of fatigued open-hole titanium–graphite hybrid composite laminates. American Institute of Aeronautics and Astronautics, AIAA-98-1960; 1998. p. 2098– 105. [6] Fanning JC. Properties and potential non-aerospace applications of TIMETALâ15-3 hybrid titanium composite laminates. The Minerals, Metals and Materials Society; 1998. p. 353–60. [7] Veazie DR, Badir AM, Grover RO. Titanium ply effects on the behavior of a hybrid thermoplastic composite laminate. J Thermoplastic Comp Mater 1998;11(5):443–54. [8] Burianek DA, Spearing SM. Fatigue damage in titanium–graphite hybrid laminates. American Institute of Aeronautics and Astronautics, AIAA 98-1959; 1998. p. 2088–97. [9] Li E, Johnson WS, Lowthere SE, St. Clair TL. An evaluation of two fabrication methods for hybrid titanium composite laminates. In: Hooper SJ, editor. Composite materials: testing and design, vol. 13, ASTM STP 1242, American society for Testing and Materials; 1997. p. 202–14. [10] Rhymer DW, Johnson WS. Fatigue damage mechanisms in advanced hybrid titanium composite laminates. Int J Fatigue 2002;24(9):995–1001. [11] ASTM D3039/D3039M. Standard test method for tensile properties of polymer matrix composite materials; 1995. [12] ASTM D790-86. Standard test method for flexural properties of unreinforced and reinforced plastics and electrical insulating materials; 1998.