Acoustic emission-based characterization technique for assessing folding strength of thermoformed flax-reinforced polypropylene composite

The main objective of this study is to characterize the folding strength of a thermoplastic polypropylene matrix composite reinforced with unidirectional (UD) flax fibers. Composite plates were produced by thermocompression technique, then cut into specimens and subjected to a thermo-folding shaping process. Mechanical tests instrumented by acoustic emission (AE) were carried out to identify and quantify the damage mechanisms leading to fracture. A multivariate analysis, based on the k-means algorithm, was used to distinguish damage modes. The AE recorded signals from bio-composites are categorized into signal clusters, allowing the attribution of damage mechanisms such as matrix cracking, interfacial decohesion or delamination, and fibre breakage. The chronological sequence of AE events revealed that matrix cracking occurs first, followed by fiber–matrix debonding, with fiber breakage appearing later. Matrix cracking persists until failure, while debonding and fiber breakage dominate in the post-failure phase. The combination of these three mechanisms ultimately leads to specimen rupture. These findings provide new insights into the progressive damage sequence in UD flax/polypropylene composites under bending, highlighting the potential of AE combined with clustering analysis to monitor and understand thermoformed bio-composite failure mechanisms.