On the use of side-grooves in estimating J_IC fracture toughness with charpy-size specimens

The problem of obtaining a meaningful value of toughness from small Charpy-size surveillance specimens, tested at temperature corresponding to the upper shelf where ductile fracture predominates, is investigated. Following the procedures of Green and Knott for measurement of crack opening displacements at initiation of ductile fracture, a test procedure is adopted in which small precracked Charpy-size bend specimens are side-grooved to increasing depths and tested to failure under both quasi-static and dynamic loading rates. Values of the J-contour integral at maximum load (J_max) for specimens side-grooved in excess of 30 percent are found to agree, within acceptable limits, with “valid” initiation J_IC fracture toughness values determined independently using multi-specimen resistance-curve techniques. Three nuclear pressure vessel materials (two base metals, SA533B-1 and SA302B, and a submerged arc weld metal) were evaluated at temperatures between 71 and 177°C, which correspond to upper shelf temperatures. The test procedure described offers a simple, inexpensive, small specimen compromise for estimating the fracture toughness at the onset of ductile fracture from a single Charpy-size bend test piece for both quasi-static and dynamic loading rates. This approach could be readily adopted in nuclear surveillance programs for toughness evaluation of unirradiated and neutron irradiated pressure vessel steels.