Experimental Analysis of Folding Strength in Thermoformed Flax Fiber-Reinforced Polypropylene Composite Using Acoustic Emission

To face recent ecological challenges, researchers and engineers are looking for lighter and greener materials for numerous applications. Thermoplastic composites reinforced with flax fibers offer a promising eco-friendly solution for the partial or total substitution of conventional reinforcements in structural and semi-structural applications. Furthermore, the thermoforming process constitutes a major advantage, allowing the fabrication of composite parts with complex geometries without the need for complex and costly molds or autoclave equipment's. The deployment of thermoformed bio-based composites in high-performance structural applications is significantly hindered by the complex damage mechanisms inherent to their manufacturing process, which can progress during their service life. This study aims to characterize the folding strength of a thermoplastic polypropylene matrix composite reinforced with unidirectional (UD) flax fibers using acoustic emission (AE) technique. The stacking sequence adopted was [0/-45/-45/0/45/90/90/45]. Composite plates of 350 × 350 mm² were manufactured by thermocompression molding, then cut into specimens of 100 × 15 × h 3.4 mm3 and subjected to a thermo-folding shaping process into an 'L' shape. To minimize the effect of processing variability, all specimens were extracted from the same composite plate and manufactured under identical thermoforming conditions. Prior to mechanical testing, specimens were screened by X-ray tomography to ensure the absence of significant visible damage in the inner folding zone. Mechanical tests instrumented with two piezoelectric sensors were subsequently carried out to evaluate folding strength and to identify and quantify the damage mechanisms level leading to fracture. The potential of AE technique to characterize damage in bio-based composites is further explored through b-value statistical analysis of signal amplitudes. The mechanical response of the specimens is assessed through mechanical indicators.