Interfacial shear strength in short-fiber thermoplastic biocomposites: pressure dependence and implications for manufacturing

Abstract Short-fiber thermoplastic biocomposites combine the ease of processing of thermoplastics with the improved end-of-life options provided by bioderived or biodegradable matrices and natural fibers. However, their performance is often limited by low fiber–matrix interfacial shear strength ( $$\:{\tau\:}_{y}$$ ), which is commonly assessed through pull-out tests. The assumptions underlying this method, along with the need to avoid fiber breakage, require samples with very small, embedded fiber lengths, often prepared without applying pressure. Because pressure is inherently involved in thermoplastic (bio)composite processing, this may lead to a discrepancy between measured and processing-relevant $$\:\:{\tau\:}_{y}$$ values, thus limiting the understanding and optimization of composite performance. Here, we investigate the pressure dependence of $$\:{\tau\:}_{y}$$ between poly(lactic acid) and hemp fiber bundles/technical fibers. The combination of pull-out tests on samples prepared at different pressures and micromechanical modeling using the Bowyer–Bader model showed that $$\:{\tau\:}_{y}$$ increases significantly with pressure, rising from ~ 5.50 MPa at ~ 0.5 bar to ~ 15.68 MPa at ~ 43 bar, with Bowyer–Bader estimates at ~ 200 bar remaining in the same order of magnitude. This behavior is attributed to pressure-induced conformity of the matrix to the surface topography of hemp fiber bundles, which increases the effective fiber–matrix contact area not accounted for in standard pull-out analysis. Overall, our findings (i) raise concerns about the reliability of $$\:{\tau\:}_{y}$$ values obtained from pressure-uncontrolled pull-out tests, (ii) highlight the crucial role of processing pressure in the manufacturing of thermoplastic biocomposites, and (iii) show that pressure affects the critical fiber length governing effective reinforcement.