Multiscale modelling of sutures in a high-performing biological protective structure: The turtle shell

Many natural protective structures, such as alligator armour, turtle shells, and the skulls of many animals including humans, contain networks of sutures; those are, soft tissue that bonds adjacent stiff plates typically made of bone. Such protective structures ought to withstand large loads associated with predator attacks. Considering the ubiquity of suture networks in natural protective structures and the optimization process of evolution, it is reasonable to hypothesize that sutures improve the mechanical behaviour of protective structures during predator attacks. However, the effect of sutures in such loading scenarios is not well understood. This is adressed by using computational models of turtle shells, where special attention is paid to the influence of the network of sutures. Additionally, the structure-function relationship is elucidated using parametric studies varying the suture geometry. Computational experiments are carried out at the suture scale to investigate its mechanical behaviour and at the shell scale to elucidate the effect that sutures have on the shell. Among other insights, it is shown that: the compliance of the shell during small deformations can be increased by increasing the height of the interlocking bone protrusions and suture thickness; the bone plates interlock for sufficiently large deformations of sutures with sufficiently long protrusions; suture geometry can be used to tailor stress-wave propagation; and the presence of sutures can reduce the maximum strain energy density, a key indicator for a material failure, during a predator attack by 31 times. The work presented paves the way for the inclusion of sutures in biomimetic protective structures such as helmets and body armour. Computational solid mechanics aspects include multiscale modelling, model order reduction, and finite strain constitutive modelling aspects, such as viscoelasticity, hyperelasticity, and anisotropy.