Stiffness distribution in natural insect cuticle reveals an impact resistance strategy

In nature, many insects have evolved hard cuticles to shelter their soft body, which is thought to be the "body-armour" for insects to protect against predators' sharp teeth or dynamic load damage caused by harsh environments. In recent years, researchers have found that the "body-armour" is composed of multi-layer materials with different elastic modulus from inside to outside. The gradient change rule of the formed material modulus reflects the evolutionary history of insect cuticle's adaptation to external changes. In this article, the mechanical properties of spatial hierarchical architecture of insect cuticle, especially for the shield-like beetle elytra under impact loading, was investigated to reveal the impact resistance strategy and in-depth mechanisms of crashworthy protection. The results show that both discontinuity at the cuticle layers interface and the distributions of stiffness gradients through layers' thickness have a great influence on preventing stress wave propagation and improving impact-tolerance. Insect cuticle such as beetle elytra with discontinuous exponential stiffness gradient (DC-EXP) along the thickness has been identified to result in the minimum values of stress and interaction force under impact loading, which leads to the best impact resistance property and defensive effect. Furthermore, we compared and discussed the protective properties of insect elytra with different sclerotized endocuticle under quasi-static compression and impact loading, respectively. The knowledge gained from this work reveals the advantages of nature's choice of the stiffness distribution and may serve to inspire further research of developing advanced multifunctional structures with improved impact resistance capability by programming reasonable stiffness distribution.