Fatigue of insect cuticle

Abstract

SummaryMany parts of the insect exoskeleton experience repeated cyclic loading. Although the cuticle of insects and other arthropods is the second most common natural composite material in the world, so far nothing is known about its fatigue properties, despite the fact that fatigue undoubtedly limits the durability of body parts in vivo. For the first time, we here present experimental fatigue data of insect cuticle. Using force-controlled cyclic loading, we determined the number of cycles to failure for hind legs (tibiae) and hind wings of the locust Schistocerca gregaria, as a function of the applied cyclic stress. Our results show that, although both made from cuticle, these two body parts behaved very differently. Wing samples failed after 100,000 cycles when we applied 46% of the stress needed for instantaneous failure (the UTS). Legs, in contrast, were able to sustain a stress of 76% of UTS for the same number of cycles to failure. This can be explained by the difference in the composition and structure of the material and related to the well-known behaviour of engineering composites. Final failure of the tibiae occurred via one of two different failure modes - crack propagation in tension or buckling in compression - indicating that the tibia is evolutionary optimized to resist both failure modes equally. These results are further discussed in relation to the evolution and normal use of these two body parts.