Damage shielding mechanisms in hierarchical composites in nature with potential for 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. Man-made composites instead consist typically of fibres embedded in a uniform matrix, and frequently show 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 of reducing the stress concentrations if large clusters appear. To test this hypothesis, a probability density field of further damage growth has been calculated for different microstructures and initial crack sizes. The results indicate that the hierarchical structure should maintain its strength by localization of damage, yet rendering large clusters less harmful by weakening the resulting stress concentration to its surroundings, which would lead to an increase in strain to failure. In this context, the potential of using the biomimetic hierarchical structure in design of composite materials is discussed.