The effect of fibers under compression on the simple shear response of a soft unidirectional fiber-reinforced composite

The aim of this work is to investigate the mechanical behavior of a soft matrix reinforced by a single family of parallel fibers subjected to simple shear. Specimens with fibers oriented at - 45 degrees with respect to the applied shear load were tested. The shear and normal stress components as well as the amount of shear were simultaneously measured. Due to the initial orientation, fibers were subjected to compressive loads, resulting in buckling. The positive Poynting effect was observed when soft materials were sheared. The off-axis stiffness theory and the Murphy model at small deformations were used to predict the mechanical behavior of the soft composites. In general, there is a reasonable correlation between model predictions and measured data by adjusting only one empirical parameter. However, the model based on the off-axis stiffness shows better prediction than the Murphy model for relatively higher fiber volume fractions. From universal relations for simple shear, the results indicate that the studied composites are of a coaxial type in a weak sense.