Functional significance of graded properties of insect cuticle supported by an evolutionary analysis.

The exoskeleton of nearly all insects consists of a flexible core and a stiff shell. The transition between these two is often characterized by a gradual change in the stiffness. However, the functional significance of this stiffness gradient is unknown. Here by combining finite-element analysis and multi-objective optimization, we simulated the mechanical response of about 3000 unique gradients of the elastic modulus to normal contacts. We showed that materials with exponential gradients of the elastic modulus could achieve an optimal balance between the load-bearing capacity and resilience. This is very similar to the elastic modulus gradient observed in insect cuticle and, therefore, suggests cuticle adaptations to applied mechanical stresses; this is likely to facilitate the function of insect cuticle as a protective barrier. Our results further indicate that the relative thickness of compositionally different regions in insect cuticle is similar to the optimal estimation. We expect our findings to inform the design of engineered materials with improved mechanical performance.

Wing cross veins: an efficient biomechanical strategy to mitigate fatigue failure of insect cuticle.

Locust wings are able to sustain millions of cycles of mechanical loading during the lifetime of the insect. Previous studies have shown that cross veins play an important role in delaying crack propagation in the wings. Do cross veins thus also influence the fatigue behaviour of the wings? Since many important fatigue parameters are not experimentally accessible in a small biological sample, here we use the finite element (FE) method to address this question numerically. Our FE model combines a linear elastic material model, a direct cyclic approach and the Paris law and shows results which are in very good agreement with previously reported experimental data. The obtained results of our study show that cross veins indeed enhance the durability of the wings by temporarily stopping cracks. The cross veins further distribute the stress over a larger area and therefore minimize stress concentrations. In addition, our work indicates that locust hind wings have an endurance limit of about 40% of the ultimate tensile strength of the wing material, which is comparable to many engineering materials. The comparison of the results of the computational study with predictions of two most commonly used fatigue failure criteria further indicates that the Goodman criterion can be used to roughly predict the failure of the insect wing. The methodological framework presented in our study could provide a basis for future research on fatigue of insect cuticle and other biological composite structures.

Experimental and numerical investigations of Otala lactea's shell-I. Quasi-static analysis.

At the first glance, mollusk shells may seem complex spatial structures with interesting shapes, forms and colors. However, from an engineering point of view, they are mechanical barriers which provide remarkable protection against environmental factors. These biological composites which exhibits an attractive combination of stiffness, strength and toughness, may be mimicked in bio-inspired materials. In the present work, a mathematical method is used to develop comprehensive three-dimensional (3D) numerical models of mollusk shells. The models are employed to study the mechanical behavior of the shells under static loading conditions. Numerical analyses are conducted using ANSYS finite element (FE) codes. A combination of indentation testing and scanning electron microscopy (SEM) is utilized to confirm the validity of the models and the solving procedures. A good agreement is observed between the shape, size and location of the failure obtained from experimental tests and numerical predictions. The results indicate that the columella increases the ability of mollusk shells to withstand applied mechanical forces without failure. Further, it can be concluded that the coiling geometry of the shells adequately modifies the stress distribution and reduces the stress concentration.