Crack Depth Effects Measured by Dynamic Fracture Toughness Tests

Abstract

Dynamic fracture toughness tests with an instrumented inverse geometry impact hammer have been made on small, side-grooved, prefatigued Charpy-type specimens (10 by 10 by 55 mm) with varying crack depth. The a/W was approximately 0.05, 0.1, 0.3, and 0.5. Prefatiguing was performed with wider specimens after which the specimens were machined to achieve identical sharp cracks of specified length. Impact tests were performed at a temperature corresponding to the transition region of the material's ductile to brittle transition curve. Load and energy were measured as a function of specimen displacement. Fracture toughness, Jc, at cleavage crack initiation was calculated as a result of the test. The test results were used for determining the applicability of parameters like T stress, Q parameter, and small-scale yielding correction (SSYC) for in-plane constraint correction. It was concluded that the small-scale yielding procedure corrects the effect of constraint effectively. T stress and Q as indexing parameters were not suitable for a quantitative constraint correction. The SSYC was also applied to some results from the literature. The effect of SSYC improved as the width of the specimen decreased from 200 to 50 mm. The results imply that although SSYC is highly promising, more FEM-analysis is needed before it can be used as a standard procedure.