56. Two Modified Digital Gradient Sensing with Higher Measurement Sensitivity for Evaluating Stress Gradients in Transparent Solids

Two modified full-field Digital Gradient Sensing (DGS) methods with higher measurement sensitivity are presented for quantifying small angular deflections of light rays caused by a non-uniform state-of-stress in a transparent solid. These methods are devised by combining or altering previously proposed methods, reflection-mode DGS (r-DGS) (Periasamy and Tippur, Meas Sci Technol 24:025202, 2013) and transmission-mode DGS (t-DGS) (Periasamy and Tippur, Appl Opt 51:2088–2097, 2012). In this presentation, the working principles of r-DGS and t-DGS are introduced first. Then, the so-called t2-DGS method is proposed with the aid of a separate reflective planar surface located behind the transparent solid. The sensitivity of t2-DGS is shown to be twice that of t-DGS. Next, an even higher sensitivity method called the transmission-reflection DGS or simply tr-DGS is developed by making the back surface of a transparent planar solid specularly reflective. The governing equations of tr-DGS are proposed followed by a comparative demonstration of t2-DGS and tr-DGS methods by measuring stress gradients in the crack tip region during a dynamic fracture experiment. The tr-DGS is ∼1.5 times more sensitive than t2-DGS, and at least three times more sensitive than t-DGS approach.