Deformation and fracture of lithosphere-inspired polymeric multi-layer composites

• „ Lithomers “ as a newly innovative class of polymeric multi-layer composites • Fracture toughness improvement due to material inhomogeneity • Stiffness improvement due to complex stresses in the soft layer during deformation • Lithomer in the initial wave stage show a 27-time improvement in fracture toughness with a moderate reduction in stiffness Inspired by the diversity of structures and patterns inherent in the earth's lithosphere, this study endeavors to enhance the interplay between stiffness and toughness through the introduction of a new class of polymeric multi-layer composite materials termed by the authors as "lithomers". Structured single-edge notched bending specimens were fabricated using a combination of additive manufacturing and casting, employing two different methacrylate-thiol resins. The outer layers exhibit a stiff and brittle characteristic, while the layer in between is compliant in nature. Three types of lithomers with wave-like structures and one with a rectilinear structure were investigated regarding their stiffness and toughness in a 3-point bending setup. The results were compared with those of a pure stiff matrix material. The findings revealed that fracture toughness increased regardless of the interlayer's shape compared to the pure matrix material. Correspondingly, this enhancement in fracture toughness correlated with a reduction in stiffness. The most balanced results in terms of stiffness and fracture toughness were achieved, with the lithomer having a wave-like structure in its initial stage. It exhibited a roughly 27 times improvement in fracture toughness with a moderate decrease in stiffness of approx. 1/5 compared to the pure matrix material.