An improved failure criterion for biological and engineered staggered composites

Abstract

High-performance biological materials such as nacre, spider silk or bone have evolved a staggered microstructure consisting of stiff and strong elongated inclusions aligned with the direction of loading. This structure leads to useful combinations of stiffness, strength and toughness, and it is therefore increasingly mimicked in bio-inspired composites. The performance of staggered composites can be tuned; for example, their mechanical properties increase when the overlap between the inclusions is increased. However, larger overlaps may lead to excessive tensile stress and fracture of the inclusions themselves, a highly detrimental failure mode. Fracture of the inclusions has so far only been predicted using highly simplified models, which hinder our ability to properly design and optimize engineered staggered composites. In this work, we develop a new failure criterion that takes into account the complex stress field within the inclusions as well as initial defects. The model leads to an 'optimum criterion' for cases where the shear tractions on the inclusions is uniform, and a 'conservative' criterion for which the tractions are modelled as point forces at the ends of the overlap regions. The criterion can therefore be applied for a wide array of material behaviour at the interface, even if the details of the shear load transfer is not known. The new criterion is validated with experiments on staggered structures made of millimetre-thick alumina tablets, and by comparison with data on nacre. Formulated in a non-dimensional form, our new criterion can be applied on a wide variety of engineered staggered composites at any length scale. It also reveals new design guidelines, for example high aspect ratio inclusions with weak interfaces are preferable over inclusions with low aspect ratio and stronger interfaces. Together with existing models, this new criterion will lead to optimal designs that harness the full potential of bio-inspired staggered composites.

Overcoming the brittleness of glass through bio-inspiration and micro-architecture

“…For comparison, the tensile strength of nacre is about 10 times lower, in the order of 100 MPa. The strength of nacre is controlled by shearing of the soft interfaces between the tablets, and the apparent strength of the tablets is much lower than 1 GPa, because of stress concentrations arising from the staggered arrangement 53 . The decrease in strength in nacre compared with pure mineral is therefore in the same order as the decrease in strength in our material compared with intact glass.…”

Optimization design of strong and tough nacreous nanocomposites through tuning characteristic lengths

“…The effective strength and toughness of nacreous nanocomposites are closely related to the unique stress-transfer mechanism of the BM structure in the tension-shear-chain (TSC) model (Jager and Fratzl, 2000;Kotha et al, 2001; Gao et al, (Barthelat et al, 2013) reproduced with permission, Copyright 2013, Royal Society Publications, (b) profile of the effective interface constitutive relation, (c) schematics of the brick-and-mortar structure, (d) sketch of a TSC model under uniaxial tension (e) the right side of the RVE, (f) the left side of the RVE. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)…”

Role of the polymer phase in the mechanics of nacre-like composites

“…Besides simulating natural nacre, a number of different models have been developed to identify the optimum design of artificial bioinspired brick-and-mortar composites (Barthelat, 2014;Barthelat et al, 2013;Begley et al, 2012;Pimenta and Robinson, 2014). These models often indicate that the failure mechanism is determined by the aspect ratio of the reinforcing tablets and the strength of the compliant interfaces.…”