3.1. Compression Properties and Stress–Strain Behavior
For the 2D PMC/CMC, room-temperature tensile properties were established from eight tension tests, and compressive properties were established from six compression tests. The tensile and compressive properties of the 2D PMC/CMC at T
right = 329 °C were also established from eight tension and six compression tests. Average tensile and compressive properties of the 2D PMC/CMC at 23 °C and at T
right = 329 °C are summarized in
Table 1 with the corresponding percent coefficients of variation (COV) given in parentheses.
Figure 4 shows typical stress–strain curves of the 2D PMC/CMC in tension and compression at 23 °C and at T
right = 329 °C. Given that the 2D PMC/CMC contains a 2D PMC portion, it is informative to compare and contrast the mechanical behavior and properties of the 2D PMC/CMC and those of a 2D PMC with the same constituents and fiber architecture. Therefore, the results of previous efforts [
9,
19] for a 2D PMC comprising 15 plies of de-sized Cytec T650-35 carbon fiber 8HSW fabric in a P2SI
® NRPE polyimide matrix are included in
Figure 4 and
Table 1 for comparison.
The results in
Figure 4 and
Table 1 reveal that the compressive strength of the 2D PMC/CMC is significantly lower than its tensile strength, while the compressive modulus appears to be close to the tensile modulus. At 23 °C, the compressive strength is only about 25% of the tensile strength. In contrast, the compressive modulus constitutes about 88–90% of the tensile modulus at both test temperatures. Similarly, compressive strength of the 2D PMC is lower than its tensile strength, but the difference is not as dramatic as in the case of the 2D PMC/CMC. The average compressive strength of the 2D PMC is close to 50% of its tensile strength. Compressive modulus of the 2D PMC represents about 90% of its tensile modulus, as was the case with the 2D PMC/CMC. Notably, high-temperature compressive strength and modulus obtained for the 2D PMC/CMC and the 2D PMC are close to the room-temperature values. Evidently, temperature has a minor effect on compressive properties of both composites.
The tensile and compressive moduli of the 2D PMC/CMC are very close to the respective properties of the 2D PMC. In contrast, the tensile and compressive strength of the 2D PMC/CMC are considerably lower than the corresponding strength values of the 2D PMC. Tensile strength of the 2D PMC/CMC is some 20% lower than the tensile strength of the 2D PMC, whereas compressive strength of the 2D PMC/CMC is about 60% below the compressive strength of the 2D PMC. To gain insight into such dramatic decrease in tensile and compressive strengths we examine the stress–strain behavior and aggressive delamination of the 2D PMC/CMC during monotonic tension and compression tests to failure (
Figure 5).
In a tension test, delamination of the test specimen becomes noticeable as the tensile stress reaches and goes above 300 MPa. Delamination progresses and becomes extreme with increasing tensile stress. The CMC and PMC plies curve outwards on the sides of the specimen (
Figure 5a). It is noteworthy that while all PMC plies arch out, the CMC ply located at the co-cure plane stays reasonably flat. It is possible that the co-cure process raised the stiffness of this CMC ply. We also note that the PMC part of the specimen is noticeably thicker than the CMC part. Thus, the co-cure plane does not coincide with the mid-plane of the 2D PMC/CMC. As a result, the outward arching of the PMC and CMC plies is not symmetric relative to the mid-plane of the specimen. Such non-uniform deformation of the specimen is bound to add bending stresses to the applied tensile stress. Ply delamination together with non-uniform deformation spread well past the specimen gage section. We attribute the lower tensile strength of the 2D PMC/CMC to extensive delamination and non-uniform deformation seen in tension test.
Damage and failure of composite materials under compression have been studied by many researchers [
25,
26,
27,
28,
29,
30,
31,
32,
33,
34,
35]. By and large, compressive failure of a 2D composite is associated with fiber micro-buckling, fiber kinking and subsequent fiber fracture.
Figure 6 shows a typical failure of a 2D PMC specimen in a compression test.
Extensive ply delamination is evident. Delamination triggers formation of fiber kink bands across the entire specimen cross-section, followed by fiber fracture. In the case of the 2D PMC/CMC (
Figure 5b), compressive damage also includes severe delamination. The fiber kink bands probably formed but did not encompass the entire specimen cross-sections. Fiber fracture is present but is not sufficient to cause the separation of the specimen into two distinct parts as in the case of the 2D PMC. In contrast, the non-uniform deformation noted in a tension test is also observed during a compression test. As in a tension test, the PMC plies and the CMC plies curve outwards under compressive loading. The arching of the plies is not symmetric relative to the specimen mid-plane. However, under compressive loading, none of the composite plies remain planar. Recall that under tensile loading, the CMC ply next to the co-cure plane stayed nearly planar. The non-uniform deformation of the 2D PMC/CMC observed under monotonic compression probably adds bending stresses to the applied compressive stress, resulting in a dramatically decreased compressive strength compared to that of the 2D PMC.
Compressive stress–strain behaviors of the 2D PMC/CMC and the 2D PMC are linear at first, but turn markedly nonlinear as the failure approaches. The 2D PMC fails in a catastrophic manner as compressive strain nearing 1%. In contrast, the 2D PMC/CMC fails at compressive strains not quite reaching 0.5%. Compressive failure of the 2D PMC/CMC does not have a catastrophic character, in the sense that the specimen does abruptly separate into two distinct parts. Under compression, the 2D PMC/CMC suffers massive delamination coupled with the non-uniform deformation. Compressive failure is signaled by the loss of load-carrying capacity.
3.2. Tension–Compression Cyclic Fatigue at Elevated Temperature
Fully reversed tension–compression fatigue tests were carried out at T
right = 329 °C with zero mean stress (i.e., a ratio of minimum stress to maximum stress R = −1). Experimental results are presented in
Figure 7 as fatigue (S–N) curves, where S is the maximum stress and N is the number of cycles to failure. Results of the tension–tension fatigue tests from an earlier effort [
9] are plotted in
Figure 7 for comparison. It is noteworthy that in tension–compression fatigue tests, all specimens failed during the compression segment of the fatigue cycle by a sharp drop in sustained load. Conversely, specimens tested in tension–tension fatigue failed by separation into two parts.
The results in
Figure 7 demonstrate that tension–compression fatigue is significantly more destructive than tension–tension fatigue. In tension–compression fatigue tests, the 2D PMC/CMC achieved fatigue runout at 86.4 MPa, signifying that the fatigue limit lies between 86.4 and 90.3 MPa. Note that the fatigue runout stress of 86.4 MPa represents 55% of the composite’s compressive strength but only 13% of its tensile strength. In contrast, the runout stress obtained in tension–tension fatigue was 472 MPa (about 71% of the composite’s tensile strength), which is more than five times the runout stress of 86.4 MPa achieved in tension–compression fatigue. For any given cyclic life, the maximum stress in tension–tension fatigue is typically over five times that in tension–compression fatigue. Including compressive loading in a fatigue cycle drastically decreases the cyclic life and reduces the fatigue limit of the 2D PMC/CMC.
Examination of the S-N curves in
Figure 7 reveals that the 2D PMC exhibits a much better tension–tension fatigue performance than the 2D PMC/CMC, achieving higher fatigue limit and longer fatigue lives for a given maximum stress. Notably, the tension–tension S-N curves obtained for the 2D PMC/CMC do not parallel those obtained for the 2D PMC. Hence, the reduced tension–tension fatigue performance of the 2D PMC/CMC is not exclusively due to its lower tensile strength caused by microstructural defects. The addition of the co-cured CMC plies influences the fatigue mechanism and accelerates the deterioration of fatigue strength with cycles. The 2D PMC also outperforms the 2D PMC/CMC in tension–compression fatigue, as is demonstrated by its higher fatigue limit. The 2D PMC achieves tension–compression fatigue runout at 243 MPa, which represents nearly 30% of the composite’s tensile strength and about 60% of its compressive strength. Tension–compression fatigue runout stress obtained for the 2D PMC also is about three times that obtained for the 2D PMC/CMC. Once again, the tension–compression S-N curves for the two materials are not parallel. Thus, the degradation of tension–compression fatigue performance cannot be attributed exclusively to the lower strength of the 2D PMC/CMC. Once again, the co-cure of two dissimilar composites results in non-uniform deformation under tension–compression cycling, affecting the fatigue performance and accelerating fatigue failure.
Figure 7 presents the fatigue S-N curves for the two cycle types on the basis of the maximum stress, S
max, in the cycle. It is seen that S
max does not correlate in the results obtained for the 2D PMC/CMC composite well; the S-N curves obtained for the two different cycle types have different slopes. The same observation can be made regarding the fatigue S-N curves obtained for the 2D PMC.
Figure 8 compares the S-N curves for the two cycle types with the stress amplitude (or alternating stress), S
a, as the correlating parameter. In the case of the 2D PMC, the results obtained for two cycle types now form a more compact group, but a noticeable difference in slopes persists. In the case of the 2D PMC/CMC, the two cycle types still yield somewhat different slopes and do not form a compact group. These results are not entirely surprising. Several researchers [
16,
36,
37,
38] have noted that results obtained for the woven 2D composites in tension–tension fatigue differed considerably from those obtained in tension–compression fatigue. The S-N curves exhibit different slopes. The failure modes are also different.
Figure 9 and
Figure 10 present typical evolution of the stress–strain hysteresis response under tension–compression fatigue for the 2D PMC/CMC and the 2D PMC, respectively. In general, changing shape of the hysteresis behavior with cycles reflects changes in mechanical properties of the material. The area enclosed by the hysteresis loop represents a measure of energy dissipated in a cycle and can be associated with linear and/or nonlinear deformation. Results in Figs. 9a and 10a demonstrate that during fatigue tests with lower stress levels, both composites exhibit minimal strain ratchetting and nearly no change in stiffness. The hysteresis loops produced in these tests are very narrow; the composites’ behavior is practically linear elastic. However, the hysteresis stress–strain loops produced by both composites are not entirely symmetric. In any given cycle, the largest compressive strain exceeds the largest tensile strain to some extent. This tendency is amplified with increasing fatigue stress, especially in the case of the 2D PMC/CMC (
Figure 9b). Early in the test, the slopes of tension-going and compression-going parts of the fatigue cycle are approximately the same. As fatigue cycling progresses, the slope of the compression-going part diminishes much faster than that of the tension-going part. The largest compressive strain becomes nearly two times the largest tensile strain in the cycle. Furthermore, the hysteresis loops produced later in test are markedly curved (see for example a stress–strain loop for cycle 6000 in
Figure 9b). These phenomena signal a rapid damage buildup under compressive loading.
Similar trends are observed for the 2D PMC (
Figure 10b). The hysteresis loops become progressively less and less symmetric, with the slope of the compression-going part decreasing faster than that of the tension-going part. The trend, however, is not as pronounced as in the case of the 2D PMC/CMC. We attribute the rapid damage buildup in the 2D PMC/CMC to the non-uniform deformation of the test specimen. As mentioned earlier, the 2D PMC/CMC, which comprises two different composites co-cured together, produced non-uniform deformation in monotonic tension and monotonic compression tests. The PMC and CMC plies curved outwards. The CMC ply next to the co-cure plane remained virtually planar in a tension test, but curved outwards in a compression test. The 2D PMC/CMC produces similar non-uniform deformation under tension–compression fatigue. The CMC and the PMC plies curve outwards during tension-going part of the cycle, regain their original shape as the load returns to zero, and then curve outwards during the compression part of the cycle, and once again regain their original shape at zero load (
Figure 11). As the tension–compression fatigue tests continue, sizable tensile and especially compressive strains build up, and stiffness diminishes.
Figure 12 presents the evolution of tensile and compressive strains with cycles for the 2D PMC/CMC and the 2D PMC. There is little accumulation of tensile or compressive strains in tension–compression tests conducted with lower fatigue stresses (S
max = 86.4 MPa for the 2D PMC/CMC and S
max = 243 MPa for the 2D PMC). Recall that these tests achieved runout completing 200,000 cycles without failure. Conversely, considerable accumulation of tensile and compressive strains is seen in tests conducted with higher fatigue stresses (S
max = 102 MPa for the 2D PMC/CMC and S
max = 344 MPa for the 2D PMC). For the 2D PMC/CMC (
Figure 12a), compressive strain builds up faster than tensile strain. As seen in
Figure 12a, tensile and compressive strains in cycle 1 are 0.21% and −0.26%, respectively. Tensile and compressive strains increase to 0.22% and −0.29% in cycle 1500, and to 0.26% and −0.44% in cycle 6000. Interestingly, tensile strains accumulated in tension–tension fatigue tests in prior work [
9] are close to those accumulated in tension–compression tests. Of course, we note that the fatigue stresses (472–580 MPa) used in tension–tension fatigue tests [
9] greatly exceed those used in tension–compression fatigue tests (63–110 MPa). For the 2D PMC (
Figure 12b), compressive strain also grows faster with cycles than the tensile strain. For example, in the tension–compression fatigue test performed with S
max = 344 MPa, tensile and compressive strains in cycle 1 were 0.49% and −0.60%. Tensile strain grew to 0.69%, whereas compressive strain reached −0.89% in cycle 10,000. The 2D PMC accumulated noticeably higher tensile stains in tension–compression fatigue compared to tension–tension fatigue. For instance, tensile and compressive strains accumulated in tension–compression fatigue tests conducted with S
max = 370 MPa were 0.80% and −0.98%. Contrastingly, tensile strains accumulated in tension–tension fatigue tests with a higher S
max = 503 MPa in an earlier effort [
9] did not exceed 0.13%. These observations are hardly surprising. Specimens of the 2D PMC with 0/90 fiber orientation are highly resistant to tension–tension fatigue, hence the low strain accumulations. On the other hand, the 2D PMC/CMC exhibits non-uniform deformation with CMC and PMC plies delaminating and curving outwards under both tension–tension [
9,
23] and tension–compression cycling. As a result, similar tensile strains are accrued in tension–tension and in tension–compression fatigue tests.
It is instructive to examine the loss of stiffness with cycles (hysteresis modulus calculated using the maximum and minimum stress–strain data points in a load cycle), which reflects the damage development during fatigue. The change in normalized modulus (i.e., modulus normalized by the modulus measured in the first cycle) with fatigue cycles for the 2D PMC/CMC and the 2D PMC is presented in
Figure 13.
In tension–tension fatigue, the normalized modulus vs. cycles curves for both composites showed a classic pattern of gradual decrease followed by a rapid drop as the test specimen neared failure [
9]. In the case of the 2D PMC/CMC the normalized modulus vs. cycles behavior adheres to the same pattern under tension–compression fatigue. In contrast, the normalized modulus of the 2D PMC diminishes continually with tension–compression cycles but does not drop sharply before failure. Predictably, for both composites, the normalized modulus decreases more rapidly in tests performed with higher fatigue stress. In runout tests where 200,000 cycles were completed without failure, the loss of normalized modulus was about 11% for the 2D PMC/CMC and about 15% for the 2D PMC. Compared to tension–tension fatigue, a greater drop in the normalized modulus is seen under tension–compression fatigue for both composites. Under tension–compression fatigue, the modulus loss for the 2D PMC/CMC and the 2D PMC extends to 50%. In contrast, under tension–tension fatigue, the modulus loss did not exceed 40% for both composites [
9].
All test specimens that achieved a fatigue runout of 200,000 cycles were tested in tension to failure to measure the retained tensile modulus and strength. The results in
Figure 14 reveal that tension–compression fatigue is more damaging than tension–tension fatigue.
The 2D PMC/CMC retained 89% of its tensile strength following 200,000 tension–tension cycles with Smax = 472 MPa, but only 81% of its tensile strength after tension–compression cycles with a much lower Smax = 86.4 MPa. At first glance, the stiffness retention appears to show an opposite trend. The 2D PMC/CMC retains 68% of its tensile stiffness following tension–tension fatigue and 89% of its tensile stiffness after tension–compression fatigue. However, we note that the fatigue stress in tension–tension fatigue (472 MPa) was about five times that in tension–compression fatigue (86.4 MPa). It is reasonable to assume that stiffness retention would approach 100% in a tension–tension fatigue test performed with a low Smax = 86.4 MPa. The 2D PMC retains about 84% of its tensile strength following tension–tension fatigue with Smax = 585 MPa, but only about 75% of its tensile strength after tension–compression cycles with a much lower Smax = 243 MPa. Modulus retention is about 85% following either tension–tension or tension–compression fatigue. Once again, we note that the maximum stress was much higher in the case of the tension–tension fatigue than in the case of tension–compression fatigue. The addition of the co-cured CMC plies appears to be slightly beneficial—the 2D PMC/CMC exhibits better retention of its tensile strength after both tension–tension and tension–compression fatigue.