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 are made from cuticle, these two body parts behave very differently. Wing samples showed a large fatigue range, failing after 100,000 cycles when we applied 46% of the stress needed for instantaneous failure [the ultimate tensile strength (UTS)]. Legs, in contrast, were able to sustain a stress of 76% of the UTS for the same number of cycles to failure. This can be explained by the difference in the composition and structure of the material, two factors that, amongst others, also affect 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 ʻoptimizedʼ by evolution to resist both failure modes equally. These results are further discussed in relation to the evolution and normal use of these two body parts.

Take-off speed in jumping mantises depends on body size and a power limited mechanism

“…The maximum bending moment on a leg is independent of its length, but the compressive forces are inversely proportional to length (Bennet-Clark, 1990) and the tendency to buckle is proportional to the square of the length (Popov, 1990). Thus, despite lower compressive stresses and similar bending stresses, longer legs will have to be more reinforced against buckling (Dirks et al, 2013). The tibiae of some bush crickets with hind legs three times the length of the body will sometimes buckle under the stresses of take-off (M.B., personal observations) and the tibiae of locusts have an inbuilt shock absorber to lessen damage to joints should a hind leg slip at take-off (Bayley et al, 2012).…”

Damage, repair and regeneration in insect cuticle: The story so far, and possibilities for the future

“…This requires more loading energy to propagate the crack through the material to cause a failure than with a uniformly isotropic material. The ability of the insect to withstand repeated cyclic loads was investigated by Dirks et al (2013). It was found that cuticle could be induced to fail by fatigue.…”

Fatigue-resistant components: What can we learn from nature?

“…Current design approaches tend to incorporate a high degree of conservatism because the design rules used are empirical and not well validated by experimental data. Consider the problem of a circular tube of length L, radius r and wall thickness t, loaded at its ends with a combination of axial 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 Increasing r/t whilst keeping the area of the cross section constant gives a thin-walled tube which will be more resistant to these failure modes, but increasingly likely to fail by one of two other buckling mechanisms: local buckling (driven by compressive stress in thin, wide sheet) causes local out-of-plane instabilities, whilst ovalisation buckling results from an overall change in cross section (from circular to elliptical) which reduces the second moment of area I to the point where elastic instability is possible The present author and colleagues have investigated the tubes which form the legs of insects, crabs and humans (9)(10)(11). This work included the first ever fatigue tests of the materials from which insects and crabs are made: cuticle (11).…”