MEASUREMENT OF STRETCH ZONE HEIGHT AND ITS RELATIONSHIP TO CRACK TIP OPENING DISPLACEMENT AND INITIATION J‐VALUE IN AN AISI 316 STAINLESS STEEL

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Abstract
—: An accurate method using SEM for the measurement of stretch zone height (SZH) on fracture surfaces has been established. This method does not make any assumption regarding crack blunting angle θ, and for 316 stainless steel in the present investigation θ was in the range of 50 to 67°, contrary to the common assumption of 45°. The semi‐empirical equations, SZH(T) = 2.5 SZW (stretch zone width) and SZW = 89(J/E), reported in the literature were found to give very conservative predictions of initiation toughness Ji for an AISI 316 stainless steel. Lowerbound values of the coefficient m in the equation, J = mσyδ, relating J to CTOD, δ, is found to be 1.5–1.7 for the 316 SS used in this investigation; the upperbound value for m is found to be 2.6. The plastic CTOD values (δmax) corresponding to the maximum load‐point on the instrumented impact test traces are not sensitive to the aging conditions and as they incorporate significant crack growth effects they cannot be used for predicting Ji, by a procedure similar to that discussed above. The ratio t1/tT, where ti is the time to crack initiation and ti is the time to total fracture, in precracked Charpy tests of unaged and aged 316 SS used in this investigation (a/W=0.55 to 0.8) corresponds to 0.15 to 0.17.