Influence of thickness and temperature on stretched zone size in J 1C tests

The nature of the microscopic processes occurring at the crack tip during crack initiation and the early stages of propagation were investigated by detailed characterization of the stretched zone in J integral specimens of two thicknesses tested at two different temperatures. The J integral tests were conducted on HT–9, a martensitic stainless steel, using the single specimen compliance method. Tests were conducted on modified 0·5 in thick compact tension (CT) specimens at two thicknesses, 12·7 and 2·5 mm, fatigue precracked before the test and at temperatures of 25 and 200°C. The topography of the crack tip region was mapped by detailed stereographic measurements on scanning electron micrographs of the region containing the stretched zone. The measured J _1C values and the stretched zone dimensions for each test condition were then compared using the relationship of Knott: J _1C = mσ_fδ_c, where m is a constraint factor, σ_f is the flow stress, and δ_c is the crack opening displacement taken as twice the stretched zone height. Theory predicts values of m of about 2 for specimens under plane strain conditions and lower values of m for thin specimens under plane stress conditions. The current experiments showed good agreement with theory for the thicker specimens but larger values of m, near 6·0, for the thin specimens. A rationale for this lack of agreement and an analysis of the crack tip deformation processes are presented.