Low-Velocity Impact Resistance and Compression After Impact Strength of Thermoplastic Nanofiber Toughened Carbon/Epoxy Composites with Different Layups

This study investigates the effectiveness of polyether block amide (PEBA) thermoplastic elastomeric nanofibers in reducing low-velocity impact damage across three carbon fiber composite lay-up configurations: a cross-ply [0 degrees/90 degrees]2s (CP) and a quasi-isotropic [0 degrees/45 degrees/90 degrees/-45 degrees]s (QI) lay-up utilizing unidirectional plies, and a stacked woven [(0 degrees,90 degrees)]4s (W) lay-up using twill woven fabric plies. The flexural strength and interlaminar shear strength of the composites remained unaffected by the addition of nanofibers: around 750 MPa and 63 MPa for CP, 550 MPa and 58 MPa for QI, and 650 MPa and 50 MPa for W, respectively. The incorporation of nanofibers in the interlaminar regions resulted in a substantial reduction in projected damage area, ranging from 30% to 50% reduction over an impact energy range of 5-20 J. Microscopic analysis showed that especially the delamination damage decreased in toughened composites, while intralaminar damage remained similar for the cross-ply and quasi-isotropic lay-ups and decreased only in the woven lay-up. This agrees with the broad body of research that shows that interleaved nanofibers result in a higher delamination resistance due to toughening mechanisms related to nanofiber bridging of cracks. Despite their ability to mitigate delamination during impact, nanofibers showed limited positive effects on Compression After Impact (CAI) strength in quasi-isotropic and cross-ply composites. Interestingly, only the woven fabric composites demonstrated improved CAI strength, with a 12% improvement on average over the impact energy range, attributed to a reduction in both interlaminar and intralaminar damage. This study indicates the critical role of fiber integrity over delamination size in determining CAI performance, suggesting that the delaminations are not sufficiently large to induce buckling of sub-layers, thereby minimizing the effect of nanofiber toughening on the CAI strength.