Microstructural evolution and failure in short fiber soft composites: Experiments and modeling

In this work, we systematically study the mechanics of short glass fiber composites with a soft, hyperelastic matrix (polydimethylsiloxane). The fiber orientation can be controlled by fabricating the materials with direct ink writing, an extrusion-based 3D printing method. We specifically characterize the damage evolution in these materials when subject to tensile and cyclic loading by developing a micromechanical model, a continuum model, and by performing experiments. The stress-stretch behavior and damage evolution are highly-dependent on both volume fraction and fiber orientation. We focus primarily on loading that is parallel with the fiber alignment, which shows rich complexities as a function of volume fraction, including variable softening and non-monotonic increase of the yield strength. Unlike most prior work, we use in situ optical measurements and digital image correlation during mechanical loading to quantify the highly-nonhomogeneous stretch field at macroscopic and microscopic length scales, and to simultaneously visualize the mechanisms underlying the damage evolution and complexities in the stress-stretch response, namely fiber-matrix debonding. The micromechanical model is based on modifications to the classic shear lag model, which include incorporation of fiber length distribution as a function of volume fraction and use of Weibull distributions to account for the probability of fiber-matrix debonding. The continuum model is based on hyperelastic strain energy with damage parameters for both matrix and fibers. The combination of the two models is able to capture the stress-stretch behavior and damage evolution well, and also the Mullins-like, history-dependent cyclic loading behavior we observe in experiments. (C) 2020 Elsevier Ltd. All rights reserved.