Tensile and flexural behavior of bio-inspired mechanical interlocking functionally graded biocomposites manufactured by material extrusion

Purpose This study aims to investigate the mechanical performance of functionally graded material structures produced by material extrusion using biodegradable and conventional polymers, with a focus on evaluating newly developed bio-inspired interlock designs alongside conventional geometries for joining dissimilar materials. Design/methodology/approach Three materials (polylactic acid [PLA], PLA–wood biocomposite and polyethylene terephthalate glycol [PETG]) were combined using three interlock geometries: a conventional shackle and two bio-inspired designs (tooth and spine). As part of a full factorial design, nine experimental groups were formed by combining three materials and three interlocking designs. Mechanical performance was evaluated through tensile and three-point bending tests. The results were evaluated using analysis of variance, and the contribution ratios of material, joint type and material × joint type interaction were discussed. Findings The highest tensile strength (25.57 MPa) and flexural strength (43.38 MPa) were achieved in the PLA/PLA–wood tooth joint. These results show strength values that are nearly equal to those of a single-piece PLA–wood composite. Test findings indicate that material compatibility is a determining factor in joint performance, since PLA and PLA–wood share a similar PLA-based matrix structure, which promotes improved interfacial adhesion and more effective load transfer. Under tensile loading, the strength ranking was tooth, shackle and spine. Under flexural loading, the optimal joint type depended on the material combination; however, the PLA/PLA–wood pair clearly provided the highest overall performance. Bio-inspired designs generally outperformed the conventional interlock. Originality/value This study offers two main original contributions. First, it demonstrates the high-speed fabrication of functionally graded biocomposite structures at a selected value of 250 mm/s using PLA–wood biocomposite in combination with PLA and PETG. Second, it comparatively evaluates novel bio-inspired interlocking geometries, such as spine and tooth designs, under tensile and flexural loading. These findings highlight the potential of such geometries for precise and complex joining interfaces, even at high printing speeds.