PSI - Issue 52

Miray Yasar et al. / Procedia Structural Integrity 52 (2024) 165–175 Miray Yasar et. al. / Structural Integrity Procedia 00 (2019) 000 – 000

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integrity. Perforation increased the bending strength and modulus by 23.74 % and 5.63 %, respectively, compared to the NP-PVDF CFRC. Physical examination of the cross section of the non-perforated composites revealed the presence of dry carbon fibres, indicating inadequate fibre-matrix adhesion. Perforating the PVDF layer enhanced resin infusion throughout the thickness of the composites. Thus, P-PVDF CFRC resulted in flexural strength of 646 MPa, and flexural modulus of 76.56 GPa, while the NP-PVDF-CFRC showed lower flexural strength and modulus of 522.62 MPa and 72.48 GPa, respectively, demonstrating the importance of permeability in the piezoelectric layer, as expected. 3.4. Interlaminar Shear Strength The interlaminar shear strength (ILSS) of the CFRC, NP-PVDF CFRC and P-PVDF CFRC composites was analysed following ISO 14130. The significance of the perforation that allows resin infusion through the thickness of the composites was analysed using ILSS and flexural tests. The CFRC sample (reference) exhibited an ILSS value of 41 ± 2 MPa, while the NP-PVDF CFRC showed a value of 35.69 ± 3.84 MPa, and the P-PVDF CFRC composites showed the value of 32.62 ± 1.31 MPa. The PVDF layer therefore resulted in a reduction in shear resistance under these loading conditions. The adhesion between PVDF, resin, and carbon fibre is a significant factor for enhancing interlaminar shear strength of the composites. Additionally, somewhat surprisingly, perforation of the PVDF layer produced slightly lower ILSS compared to NP-PVDF CFRC. Surface modification of the PVDF layer may be explored to potentially improve the ILSS values of the PVDF layer integrated composite. 3.5. Mode I Fracture Toughness The mode I fracture properties of the CFRC and P-PVDF CFRC were analysed and compared to evaluate the effect of the PVDF layer integration. Mode I fracture toughness test for NP-PVDF CFRC samples could not be performed due to poor resin infusion resulting in test failure. Figure 5 shows typical load-displacement and crack length versus displacement curves of both the reference and the perforated PVDF included composites.

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Fig. 5. (a) Force Displacement Curve and (b) R-curves of CFRC, P-PVDF CFRC and NP-PVDF CFRC

The force-displacement curve shows a linear trend and reaches a maximum point, indicating the crack initiation. The force-displacement curve then exhibits a stick-slip behaviour with increasing and decreasing load values as the crack propagates. The mode I fracture toughness (G IC ) of the composites, calculated based on the corrected beam theory (CBT), gave the average value for the CFRC as 393.5 ± 49.5 J/m 2 . For P-PVDF CFRC composites, the average value of G IC was 51 ± 20 J/m 2 . NP-PVDF CFRC composites did not show any delamination, as they failed due to bending, which was considered a weak zone due to improper fibre wetting. The importance of perforation was realized during the mode I fracture toughness sample preparation and testing, as the edges of the composites showed dry fibres