An interpretation of Twyman-Green interferograms from static and dynamic fracture experiments

Abstract

The application of a Twyman—Green interferometer to measurements in static and dynamic fracture mechanics is described. Twyman—Green interferometry shows the absence of a dominant singular field in the out-of-plane deformation in the vicinity of a crack. For relatively brittle fracture and except near the crack flanks, the spatial gradient of the out-of-plane displacement for a quasi-statically propagating crack is well predicted by elastostatic simulations. In addition, this method has been employed to elucidate certain transient features involving cracks propagating at large fractions of the material wave speeds. The free surface out-of-plane deformation surrounding a constant velocity dynamically propagating crack in a plate is compared with the equivalent elastostatic case. It appears that the dynamic deformation may not be obtained from the static deformation simply by a Prandtl—Glauert mapping as might be suggested by the solution for the equivalent two-dimensional generalized plane stress problem. Alternatively, the general character of the three-dimensional dynamic surface deformation may be approximately fitted by a Doppler mapping of the static deformation when scaled by the crack to plate wave speed ratio.