Fracture Studies Combining Photoelasticity and Coherent Gradient Sensing for Stress Determination

An experimental study of in-plane tensorial stress determination from full-field phase-shifting photoelasticity and transmission Coherent Gradient Sensing (CGS) for stress intensity factor estimation is presented. Phase-shifting photoelasticity determines principal stress directions and the difference of principal stresses, while transmission CGS measures the x and y first derivatives of the sum of principal stresses. Combining these two methods for the same field of view allows for principal stress separation and, using the principal stress directions, the full in-plane stress tensor. The present study, which is the first experimental study for full-field tensorial stress determination around sharp cracks in a photoelastic material, applies this hybrid method for millimeter-scale fields of view including Mode I-dominant cracks in Homalite-100, a linear elastic brittle bulk polymer. The cases presented here range in Mode I stress intensity factor from about one-quarter of the fracture toughness to just below the fracture toughness. This experimental method can detect and determine mode-mixity ratios as small as 0:0043 with a range of –0:010 to 0:020. The experimental stress fields have excellent global agreement with the full-field 2D asymptotic crack solution using the experimentally calculated Mode I and Mode II stress intensity factors.