The distribution of multiple materials within a single structure is a strategy that various biological systems rely on to achieve outstanding mechanical performances. These biological examples illustrate the effective utilization of hard rigid and soft flexible materials in particular. The proper composition of such hard-soft materials exceeds the structural limitations found in their individual material counterparts. The manufacturing of hard-soft material structures is especially relevant today due to recent developments in additive manufacturing that certify the technology with local material-specific functionalities and enlarged design spaces. However, to unravel the next generation of unprecedented structural performance, today’s state of engineering and research has yet to overcome the challenges encountered in multi-material design. The dissimilar material junctions within multi-material structures are prone to load transmissions, so they carry a crucial structural responsibility. Therefore, the interface design process must be subjected to representative interface characteristics which are often overlooked in the literature. In this work, we present a method to characterize and model multi-material structures to provide an optimal interface design in terms of the multi-material’s joining strength.

We consider the joining strength of 3D printed hard Verocyan and soft Agilus30 by its fundamental joining principles of material bonding and mechanical interlocking. Material bonding is characterized by the extent of allowable traction between the two materials. We experimentally quantify the loading-dependent critical stress at which interface debonding initiates through mapping of digital image correlation deformations on a finite element model. We numerically define the extent of mechanical interlocking by the force required to achieve an unlocked multi-material state. The finite element models contain experimentally calibrated elastoplastic and hyperelastic material models to represent the hard and soft material behaviors, respectively. Subsequently, a structural optimization based on a genetic algorithm iteratively updates a constrained parametrized interface design according to material bonding and mechanical interlocking objectives.

The numerical evaluations of calibrated hard and soft material characteristics show good agreement in structural response with their real-world equivalents. The digital image correlation deformation method successfully acquires the loading-dependent critical stresses at which the two materials debond from one another. The finite element analyses of individual joining principles adequately determine a design’s material bonding and mechanical interlocking performances. The optimization’s objective function value evolution suggests a trade-off in joining contributions where mechanical interlocking maximizes performance in more shallow, wider interface designs, whereas material bonding performs better in narrow, deeper ones. Validation experiments illustrate the dominating contribution of material bonding in A30-VC structures. Optimizing for two distinct hypotheses of interface failure equations shows no significant difference in physical joining strength. However, they do support the concept that the interface characteristics affect the optimal joining shape. Despite adequate estimation of the individual joining principle performances, a more accurate approximation of the multi-material physical joining strength necessitates the consideration of the effects induced by the interaction of material bonding and mechanical interlocking. Nonetheless, this work underlines the emphasis regarding interface characteristics in the promising structures of multi-material.