Specimen Size and Constraint Effects on J-R Curves of SA333 Gr.6 Steel

Abstract

An attempt is made to study the effect of constraint and of specimen size on the J-R behaviour of SA333 Gr.6 steel. The SA333 Gr.6 steel is used for structural fabrication of PHT piping of Indian PHWRs, and therefore has to be fully qualified with intensive LBB analysis. The present study has been based only on the L-C crack orientation of fracture specimens. Three specimen geometries, viz. TPB, CT and CCT, have been chosen to study the effect of specimen geometry on the J-R behaviour. In each geometry, three specimen sizes (two in the case of CCT), represented by variation of specimen width W or specimen thickness B or both, have been selected for investigating specimen size effects. In each specimen size of a given geometry, J-R curves have been obtained for a range of a/W in order to understand the effect of constraint produced by crack length variation. PHT pipes of two sizes — of 8" and 16" diameter, were used to fabricate the specimens. The primary aim of this study, carried out at CSIR-NML, Jamshedpur, is to present the J-R curves for the various constraint conditions in a variety of specimen geometries and to portray the trends observed in some of the critical fracture parameters. Techniques for analyses of raw experimental data had to be standardized so that the results are minimally affected by human subjectivity. Such techniques and procedures are documented in this report. Fractographic studies on stretch zone formation during the process of ductile fracture in the SA333 Gr.6 material has also been included in this report. The main conclusions of the study carried out can be summarized as follows: 1) The SA333 Gr.6 material exhibits a loss of crack tip constraint, even for deeply cracked configurations. This is particularly due to the high toughness of the material necessitating high levels of loading, and the relatively low strength of the material that ensures early plastic flow. 2) For both TPB and CT specimens, characteristic toughness parameters obtained from J-R curves exhibited an exponential decrease with increase in crack length. However, the effect saturates early, and is followed by abrupt rise in the values of toughness parameters. This is thought to be due to plasticity engulfing the remaining ligament totally. 3) Ductile fracture surface of specimens consisted of a series of parallel ridges with the valleys between the ridges exhibiting stretch zone features. The repeated formation of stretch zones was thought to be due to the loss of constraint in the material. 4) The first expanse of stretch zone of the ductile fractures was highly non-uniform. Critical fracture toughness obtained from the stretch zone width underpredicted the initiation toughness estimates.