Measurement of Residual Stress in PMMA Plate Specimen by Means of Ray Tracing Method

Abstract

A ray tracing method is applied to measure residual stress in a transparent PMMA specimen. A laser beam enters into the PMMA specimen and propagates in it. The laser beam changes its propagation direction slightly, caused by the change of refractive index of PMMA due to the residual stress. Measuring the laser beam trajectory gives the distribution of residual stress in the specimen by means of ray tracing equation. The measurement results say that the residual stress near specimen surface is compressive stress and the maximum value is about 11MPa. This value is much smaller than the fracture strength of 79MPa of PMMA, however, may not be negligible in accurate experiments to verify a theory of fracture mechanics.