Muhammad Akhsin Muflikhun, Bodo Fiedler
Carbon Fiber Reinforced Polymer (CFRP) laminates are widely used in aerospace structures. However, understanding their damage mechanisms under Mode II loading and establishing reliable non-destructive testing (NDT) techniques remain critical challenges for structural health monitoring. This study investigates the fracture behavior and damage evolution of thin CFRP laminates with two stacking sequences, [0°/+45°/−45°] 2S and [+45°/−45°/0°] 2S , using End-Notched Flexure (ENF) testing combined with Acoustic Emission (AE) and Ultrasonic Longitudinal Wave (ULW) evaluation. The results show that the [0°/+45°/−45°] 2S laminates exhibit higher flexural stiffness and delayed crack initiation with sudden catastrophic crack propagation, whereas the [+45°/−45°/0°] 2S laminates demonstrate earlier crack initiation and more progressive crack growth during loading. The average Mode II interlaminar fracture toughness values were approximately 1.70 kJ/m² for [0°/+45°/−45°] 2S , and 1.78 kJ/m² for [+45°/−45°/0°] 2S , indicating that stacking sequence significantly influences delamination resistance and energy dissipation. AE signal parameters, including absolute energy, duration, and average frequency, reveal distinct damage evolution stages, while ultrasonic scanning confirms different damage distributions where [0°/+45°/−45°] 2S laminates show broader delamination zones and [+45°/−45°/0°] 2S laminates exhibit more localized defects. This study provides valuable insights into the structural health monitoring of aerospace composite structures and demonstrates the potential of integrated NDT systems for developing damage-detection methods for advanced lightweight composite components.