PSI - Issue 82

Sana Ullah et al. / Procedia Structural Integrity 82 (2026) 138–145 S. Ullah et al. / Structural Integrity Procedia 00 (2026) 000–000

143

6

In Fig. 4, in [T45°] specimens, failure and breaking layers are mainly due to in-plane induced shear stress, and yarn bundles were also broken along with cracking and pulling off the phenolic resin from the surface and inside the CFRP. Along with the thickness direction, in comparison with [T0°] specimens, delamination occurred due to shearing, but appears to be more regular, consistent with a smaller crack zone, and the surface layer mainly shows a pull-off after the failure of the specimen. 3.2. Compression test results for CFRP and C/C pyrolyzed Compression tests on six specimens on each layup [C0°] and [C45°] were performed, representing a qualitative repeatability in the stress-strain curves as shown in Fig. 5 (a, b). Axial strain data were calculated using an extensometer having a gauge length of 25 mm placed inside the marked gauge length area of the specimen. For [C0°] specimens, curves are smoother, showing a linearity between stress and strain up to nearly 250 MPa for CFRP and 45 MPa for C/C samples due to compression loading aligned in the fibre direction. After that, due to matrix cracking at the failure strain, sudden brittle failure was observed. For [C45°] samples, curves are non-linear from the start as loading is in the shear direction of the laminate, and buckling occurs mainly due to in-plane shear deformation, and hence progressive failure is due to multiple factors involving matrix cracking, fibre-matrix debonding, shear yielding, fibre rotation, and reorientation.

Fig 5. (a) CFRP compression stress-strain; (b) Pyrolyzed C/C compression stress-strain.

The constitutive properties are reported in Table 3. Young’s modulus is calculated by taking the tangent to the linear part of the curve, and it is quite close and hence comparable to that calculated in the monotonic tensile tests being reported in Table 1, showing the qualitative performance of both experiments.

Table 3. Compressive properties of CFRP and C/C samples with different layups. Specimens Elastic modulus (GPa) Failure strength (MPa)

Failure strain (%) 0.47 ± 0.06 1.46 ± 0.24 0.11 ± 0.03 0.23 ± 0.07

CFRP [C0°] CFRP [C45°] C/C [C0°] C/C [C45°]

61.6 ± 2.7 13.2 ± 0.4 64.6 ± 0.05 8.8 ± 0.09

267.9 ± 9 138.5 ± 2 51.4 ± 2 14.0 ± 0.5