The influences of low-velocity impact loading on the vibration responses of the carbon/glass fiber-reinforced epoxy composites interleaved with various non-woven thermoplastic veils

Time-dependent variable stresses that occur in composites subjected to mechanical and dynamic loads have devastating impacts on the material properties. Since these stresses reduce the service life, it is critical to detect and enhance structural responses before and after dynamic loadings. Therefore, the present study aimed to increase the toughness and delamination resistance of the conventional fiber-reinforced composites by means of thermoplastic veil interleaves, thereby improving the vibration responses both before and after low-velocity impact (LVI). In this context, carbon fiber (CF) and glass fiber (GF) reinforced epoxy composites interleaved with five different thermoplastic veils as Polyamide (PA), Polyetheretherketone (PEEK), Polyetherimide (PEI), Polyimide (PI) and Poly-Phenylene Sulphide (PPS) were manufactured, and machined in accordance with the LVI standard. Composite specimens were subjected to the LVI tests, and then vibration tests were carried out for the non-impacted and impacted specimens to determine dynamic properties. As a result, although thermoplastic veils generally have favorable effects on damping ratios of the GF composites, it has been revealed that these veils other than PPS and PI cause deterioration in CF composites. On the other hand, since vibration reduction depends on inherent damping and structural stiffness, this study also examined the storage-to-loss modulus ratios which denote the loss factors. In this respect, it was discovered that, while PPS, PEEK, PA, and PI thermoplastic veils included among the GF laminates ascend the loss factors of composites, only PI and PPS thermoplastic veils were shown to be positively effective in CF laminates. Moreover, CF and GF reinforced composites interleaved with thermoplastic veils generally exhibited higher natural frequency and lower damping ratio compared to the entirely CF or GF laminated composites. These results show that composite specimens gained bending stiffness due to local deformation hardening, and improved dynamic properties thanks to thermoplastic veil interleaved was attributed to increased toughness and delamination resistance.