The impact resistance of soft and hard layered structural materials designed with thickness gradient

Multilayered structural materials inspired by biomaterials exhibit significant impact resistance due to their heterogeneous structures and construction methods. The influence of the microstructural characteristics within a single layer and the stacking mode of soft and hard multilayers on the impact resistance have been widely studied. However, one fact is that the volume fraction of both the hard and soft materials is usually fixed in practical engineering. Is it possible to adjust the impact performance solely through the thickness gradient design of the soft and hard layers, while maintaining a constant volume fraction of the hard and soft layers? The impact response and mechanical mechanisms of multilayered structural materials designed with different thickness gradients were systematically studied through finite element simulation and typical impact experiments in this work. Two scenarios were considered, including the individual design and coupling design of soft and hard layer thicknesses. It shows that the thickness gradient would tune the impact resistance significantly, mainly due to the redistribution of stress and strain energy absorption. Further investigation revealed that the optimal coupling design is highly consistent with the superposition of the respective optimal design scheme for the soft and hard layers, realizing the collaborative performance improvement with the better load-bearing and energy absorption capabilities. The results indicate that a coordinated thickness gradient design can achieve better strength-toughness enhancement without changing the volume fraction of materials, providing an effective design strategy for impact resistant multilayered structural materials.