Experimental investigation of interfacial behavior of multi-material additively manufactured structures

The structural integrity of multi-material components fabricated via Material Extrusion (MEX) is often limited by interfacial adhesion challenges. This study investigates the interfacial fracture toughness between rigid Polylactic Acid (PLA) and ductile Tough PLA thermoplastics. Single Leg Bending (SLB) tests were conducted under Mixed Mode (I/II) loading to evaluate adhesion performance. The research focuses on structure-property relationships by analyzing the effects of printing orientation (0°, ±45°, 0°/90°), layer configuration (symmetric 3-3 and asymmetric 4-2), and crack length using both experimental deformation energy GUc and analytical stiffness-based GSLB methods. Results demonstrated that layer configuration is a critical determinant of both interfacial and structural integrity. Symmetric configurations significantly outperformed asymmetric ones by preserving the neutral axis position. Specifically, for short crack specimens with a 0° orientation, symmetric samples achieved a GUc of 1408.0 J/m2, whereas asymmetric specimens exhibited a substantial reduction to 577.3 J/m2. Analytical evaluations for long cracks corroborated this trend, yielding higher GSLB values for symmetric designs (895.7 J/m2) compared to asymmetric ones (143.0 J/m2). Crucially, post-failure analysis revealed that while symmetric specimens achieved complete interfacial separation, asymmetric long crack specimens suffered from catastrophic adherend fracture due to excessive bending moments. Furthermore, the discrepancy between GUc and GSLB values highlights the extensive plastic deformation zone developed within the ductile Tough PLA layers. Consequently, this study establishes that symmetric layer stacking and axial fiber alignment are essential design strategies for maximizing interfacial fracture resistance in additive manufacturing.