Three-Dimensional Finite-Element Investigation on Caustics for Measuring Impact Fracture Toughness.

Abstract

The caustics method as applied to the measurement of the impact fracture toughness KId is studied by using a three-dimensional finite-element method (3D-FEM). A semi-infinite crack in an elastic plate with a finite thickness is considered, and a time-dependent (ramp) uniform pressure is assumed to be applied to the crack surfaces. The dynamic J-integral, J, is calculated for various points on the crack front and is converted to the dynamic stress intensity factor KI(t). The crack is assumed to begin to propagate when the value of KI(t) at the mid-thickness of the plate reaches a critical value Kcrit. The caustics in reflection are generated by using the reflection law of light. The measured KI(t) is obtained from the diameter of the caustics, and the measured KId is determined from the peak of the time history of the measured KI(t). It is found that the delay time to detect the crack initiation is larger than that predicted by 2D-FEM, and the measured KId depends on the shape of a propagating crack front.