Damage shielding mechanisms in hierarchical composites in nature with potentials in design of tougher structural materials

Load-carrying materials in nature, such as wood and bone, consist of relatively simple building blocks assembled into a hierarchical structure, ranging from the molecular scale up to the macroscopic level. This results in composites with a combination of high strength and high toughness, showing very large fracture surfaces indicating energy dissipation by cracking on multiple length scales. Manmade composites instead consists typically of fibres embedded in a uniform matrix, and frequently shows brittle failure through the growth of critical clusters of broken fibres. In this paper, a hierarchical structure inspired by wood is presented. It is designed to incapacitate cluster growth, with the aim of retaining high strength. This is done by introducing new structural levels of successively weaker interfaces with the purpose to reduce the stress concentrations if large clusters would appear. To test this hypothesis, a probability density field of further damage growth has been calculated...