Stiffness and toughness of soft, liquid reinforced composites

Soft composites with a soft solid as the matrix and incompressible liquid inclusions as reinforcements have been synthesised in recent times. These soft composites can be designed to respond to external stimuli and offer the prospect of providing a targeted combination of stiffness and toughness. In fact, counter-intuitively, soft solids, when reinforced with liquid inclusions, can become stiffer than the matrix material. This effect is attributed to liquid like surface stresses at the liquid–solid boundaries and is therefore, accentuated when the inclusions are very small. We perform computational homogenisation on liquid inclusion reinforced soft solids with a view to understand the effect of surface stresses on their overall stiffness and initiation toughness. Especially, we look at the hitherto unexplored effect of the surface strain dependent part of surface stresses on the fracture behaviour of soft solids reinforced with fluid inclusions. Finite deformation based hyperelasticity is the computational framework used. Results indicate that, when surface stresses are sensitive to surface strains, stress concentrations at the microscopic level are alleviated. Also, in pre-cracked composites, though initiation of microcracks from the main crack occurs quite early in the deformation history, the microcracks are deflected off the symmetry plane by the liquid inclusions, indicating a possible significant increase in the propagation toughness of the material.