Investigation of Dynamic Fracture Using Strain Gages and Photoelastic Coatings

Abstract

This study is divided into two parts namely, the use of strain gages and photoelastic coatings to investigate dynamic fracture. In the first part use of electrical resistance strain gages to determine instantaneous stress intensity factors for running crack is presented. Dynamic strain field equations are developed using the Westergaard's stress function approach. These equations are then analyzed to study the effect of various parameters like crack velocity, distance from the crack tip, etc. on the dynamic strain profile. Finally, a series of experiments was conducted with two different geometries on brittle birefringent polyester, Homalite 100 to determine dynamic stress intensity factors with strain gages. The K-values thus determined were compared with similar results obtained from the method of photoelasticity in transmission mode. In the second part of this thesis, mirror like reflective photoelastic coatings were used to study dynamic fracture in brittle structural metals. Dynamic fracture experiments are conducted on heat-treated 4340 Steel and 7075-T6 Aluminum with face grooved SEN specimen geometry. The mirror like reflective coating used in conjunction with focused light is the key to obtaining clear isochromatic fringe patterns. Instantaneous stress intensity factors, K1d, as a function of crack length are obtained independently from the method of photoelasticity and the method of strain gages and compared.