Abstract
An investigation has been carried out to find the influence of specimen size (thickness and width) on the blunting line in the J Ic test procedure (where J Ic is the critical value of the elastic-plastic toughness parameter). Compact tension specimens prepared from aircraft quality AISI 4340 steel heat treated to a yield strength level of 1000 MPa were used. The J Ic values were determined by two different methods, i.e. (i) multiple-specimen resistance curve and (ii) by measurement of the critical stretch zone width on the fracture surface. This investigation shows that the stretch zone size ahead of the crack tip during blunting as well as the post-crack-initiation stage is independent of specimen size. The slope of the blunting line was found to be 1.2, compared with 2.0 (eqn. (2)) in the standard J Ic test procedure. J Ic values obtained by measurement of the critical stretch zone size were found to be in good agreement with the J Ic value obtained by the multiple-specimen resistance curve method. J, the crack-opening displacement COD and the stretch zone size ( Δα) sz were found to be related by the expressions J = 0.6 σ 0 (COD) and COD ≈1.86 ( Δα) sz (where σ 0 is the flow stress).
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