Oblique quasi-static indentation and damage characterization of hybrid fibre composites using acoustic emission monitoring

This study investigates the effect of oblique quasi-static indentation (QSI) on the damage mechanisms and residual flexural strength of glass/epoxy, carbon–glass/epoxy, and Kevlar–glass/epoxy hybrid laminates. The laminates were fabricated using a hand lay-up process with 16 bidirectional plies arranged in a symmetric cross-ply configuration. Indentation tests were conducted at loading angles of 0°, 10°, 15°, and 20°, followed by three-point bending tests to evaluate post-damage flexural behaviour. Real-time acoustic emission (AE) monitoring was employed to identify the sequence and nature of damage events, including matrix cracking, delamination, fibre pull-out, and fibre fracture. The results indicate that carbon–glass hybrid laminates exhibit the highest peak indentation load, stiffness, and residual flexural strength across all loading angles, whereas Kevlar–glass hybrids demonstrate superior energy absorption and damage tolerance. An increase in indentation angle leads to a reduction in peak load and AE signal frequency, indicating a transition from localized to more distributed damage. AE analysis shows that high-frequency signals (above 250 kHz) are associated with fibre fracture, while low-frequency signals (below 150 kHz) correspond to matrix cracking and interlaminar delamination. The residual flexural strength increases slightly with increasing indentation angle, suggesting less severe central damage under oblique loading conditions. Overall, the combined mechanical and AE results confirm that hybridization with carbon fibres enhances indentation resistance, whereas Kevlar hybridization improves toughness and energy dissipation. This study provides new insights into the multi-axial indentation response and AE-based damage evolution of hybrid composites, supporting their application in structures subjected to complex loading conditions.