Use of COD in KIC determination

Abstract

It has been theoretically postulated that the crack opening displacement (COD) technique is applicable in characterising fracture of material in the generally yielded as well as linear elastic regimes. Hence, it is possible to determine the plane strain fracture toughness, K IC , of a material using the COD method if the onset of crack propagation and the location of the centre of rotation can be evaluated. In the present investigation, COD values at crack initiation, δ i , for Rochling Moulrex A steel were obtained with 3-point bend specimens at six different tempering temperatures using the multiple specimens load-unload extrapolation technique. It is found that by using the BS 5762:1979 analysis, K IC values were grossly over-estimated. The use of a constant rotational factor is observed to be inadequate. Under small scale yielding conditions, the centre of rotation is postulated from the present results to be located at approximately the end of the stretched zone. This supposition was applied to estimate the K IC values of standard compact tension specimens made from Rochling Moulrex A, Assab 25X and Comsteel En25 steels. Where the material was linear elastic, the estimation was good with discrepancy of less than 7%. Overestimation was seen at high tempering temperatures due to the increasing amount of plasticity in the material that shifted the location of the centre of rotation from the end of the stretched zone to a position closer to that suggested in the British Standard.