A design strategy of bio-inspired defensive structures with stiffness programmability for reusable impact-resistance protection

As a natural defensive structure, insect cuticle exhibits fascinating impact resistance properties in their body protection due to their spatial hierarchical architecture and the stiffness distribution along their thickness designed by nature. In this study, inspired by the pattern of insect cuticle, we present a novel design strategy of graded elastic ring systems with programmable stiffness gradients to enhance the protective properties. By studying the influence of the elastic modulus, radius and thickness of rings on the impact response of the bioinspired graded ring systems, the optimal solution of programing the stiffness gradient can be obtained. The mechanical analyses of the single-column ring systems and the bio-inspired ring array architectures with different stiffness gradients under impact loading show that both the distributions of stiffness gradients along the impact direction and the concavity and convexity of the stiffness profiles have a great influence on preventing stress wave propagation and improving impact-tolerance. Compared with convex stiffness gradients, the concave stiffness gradients significantly improve the protective properties as well as reduce the peak value of collision force applied to inner structures. In concave stiffness gradients, the results show that the bio-inspired ring array architectures with exponential (EXP) stiffness gradient have been identified to result in the minimum values of stress, collision force and impulse under impact loading. The strategy proposed in this paper greatly improves the impact resistance property and defensive effect compared with that of the case without protection. The knowledge gained from this work will inspire the designs of novel advanced reusable and lightweight protective structures with improved impact resistance capability by programming reasonable stiffness distributions.