Evaluation of the effect of water content on the stress optical coefficient in PMMA

A series of experiments was completed to study the effect of water content on the stress optical coefficient of PMMA samples under uniaxial tension. The stress optical coefficient is an inherent material property that is utilized in interferometric techniques, classical photoelasticity and the method of caustics to obtain deformation measurements and fracture values. The stress optical coefficient depends on the variation of the mechanical properties and the refractive index of the polymer, both of which are affected by water uptake. The main objective of this study was to quantify the variation of the stress optical coefficient with water uptake, so that it can be utilized in the analysis of visualization techniques and applied to deformation and fracture studies in humid or liquid environments. Experiments were performed at two different strain rates, 5 × 10−4 - 5 × 10−3 s−1, on both dry and pre-conditioned samples. The preconditioned samples were submerged in distilled water for either 8 or 40 days, allowing samples to reach water contents of 0.8 wt % and 1.4 wt %, respectively. Measurements were completed utilizing a Fizeau interferometer and high-speed photography. Results showed a variation on the stress optical coefficient of up to 11% for a water content of 0.8 wt % and a strain rate of 5 × 10−4 s−1. This variation was no longer observed when the strain rate was increased by an order of magnitude.