Experimental investigation of indentation damage in flax/epoxy omega-stiffeners using acoustic emission

This study examines the identification and chronology of damage mechanisms in a quasi-isotropic quasi-homogeneous (QIQH) flax/epoxy omega-shaped stiffener subjected to quasi-static indentation, using acoustic emission (AE) monitoring combined with unsupervised clustering. Indentation tests were performed at two loading rates (5 and 50 mm·min −1 ) to investigate rate effects on damage evolution in a curved, structurally representative composite geometry. A multivariable k-means clustering applied to AE descriptors consistently identified three physically meaningful clusters. The first one, associated with matrix micro-cracking and fiber–matrix debonding, is characterized by low-amplitude signals (≈40–86 dB at 5 mm·min −1 and 40–63 dB at 50 mm·min −1 ) and dominates the early loading stage. The second one, corresponding to delamination and interlaminar damage, exhibits intermediate amplitudes (≈63–92 dB and 50–72 dB) and emerges at the knee point, marking the onset of energetically significant interlaminar damage. The third one, defined by high-amplitude signals (≈63–100 dB), is associated with fiber-dominated failure and governs final collapse. Increasing loading rate accelerates damage development and shifts low- and intermediate-energy clusters toward lower amplitudes. These results demonstrate that AE clustering provides a robust framework for resolving damage mechanisms and their chronology in bio-based composite stiffeners