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

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3. Results and Discussion 3.1. Monotonic tensile test results for CFRP and C/C pyrolyzed

Monotonic tensile tests were conducted on six specimens [T0°] and six specimens [T45°] for CFRP and C/C stage. A close agreement is found in the obtained responses of the specimens in both sets of layups. The stress strain relationship is presented in Fig. 2 (a,b), in which the set of curves represents the [T0°] and [T45°] layup specimens for CFRP and C/C stage. Shear responses obtained using tensile testing on [T45°] specimens were presented in Fig. 3 (a,b). Strain data were obtained using two extensometers applied to the specimens during testing. For [T0°], a linear relationship was observed between stress and strain for all samples up to 400 MPa for CFRP and 80 MPa for C/C pyrolysis samples, and non-linearity initiated as load increases due to elastic-plastic deformation and reduction of elastic modulus. On the contrary, the linear part was very small for [T45°], and after a very short time non-linearity between shear stress and strain started. This non-linearity is mainly due to the gradual degradation of the matrix because of the crack growth in the matrix. The elastic modulus is calculated using the linear part of the curve and evaluating the slope of the tangent on that linear part according to the standard. Poisson ratio is evaluated using axial and transversal strain measurements with the help of two extensometers placed on specimens during tests. The constitutive and shear properties calculated after elaboration are reported in Table 1 and 2.

Table 1. Tensile properties of CFRP and C/C samples with different layups. Specimens Elastic modulus (GPa) Failure strength (MPa)

Failure strain (%) 0.85 ± 0.05 1.55 ± 0.22 0.85 ± 0.05 0.23 ± 0.03

Poisson’s Ratio

CFRP [T0°] CFRP [T45°] C/C [T0°] C/C [T45°]

64.8 ± 1.8 14.0 ± 0.9 67.5 ± 2.2

527.4 ± 24 109.9 ± 3

0.27 ± 0.17 0.35 ± 0.06 0.17 ± 0.07 0.36 ± 0.05

94.5 ± 4 20.5 ± 1

10.1 ± 1

Table 2. Shear properties of CFRP and C/C samples for [T45°] orientation. Specimens Shear modulus (GPa) Failure shear strength (MPa)

Failure shear strain (%) 2.11 ± 0.23 0.37 ± 0.09

CFRP [T45°] C/C [T45°]

4.4 ± 0.3

55.0 ± 1.39 10.2 ± 0.56

3.06 ± 0.06

The different layup has a significant effect on the failure modes of the samples. In [T0°], applied force is along the 0° laminate, and thus initial damage occurs due to matrix cracking at 90 ° laminates. Crack initiation zone is transverse to the loading direction, and the final failure occurred suddenly due to delamination and fibre breakage between layers towards the outward from both sides in the crack zone. In [T45°], monotonic loading is applied in the shear direction of the laminates, and hence initial damage occurred due to shear-induced matrix cracking. Crack initiation zone is along the ± 45° plane, and final failure occurred gradually due to shear out and delamination. The difference of stiffness between the [T0°] and [T45°] is mainly due to the different damage mechanism and crack behaviour that involve the two layups. However, stiffness is also affected by the fibre-matrix interaction.