Prediction of ductile tearing of compact fracture specimens using the R-6 failure assessment diagram

Abstract

The author has used recent work by Milne 1 of the CEGB on the reinterpretation of the R-6 failure assessment diagram to predict the ductile tearing behaviour of compact fracture test specimens of three different materials: 1. (1) HY130 steel. 2. (2) SA533B steel. 3. (3) 7075-T651 aluminium. The R-6 failure assessment diagram is based on the original CEGB Dowling and Townley two-criteria approach. 2 The load versus slow crack growth (ductile tearing) was predicted for seven steel compact specimens. Maximum loads were predicted to within 7 per cent for all seven specimens, with a standard error of 1·65 per cent. Various combinations of material flow stress and limit load expressions were studied. The conclusion, based on initial predictions, was that the most important input parameter is the expression for limit load. Concern with the drop-off of load after maximum load suggests that alternative failure assessment diagrams are needed to take into account the strain hardening behaviour of the materials in order to accurately predict the complete load versus stable crack growth of ductile materials. In addition, closed form expressions were developed which calculate load directly from the R-6 diagram and thereby eliminate time-consuming graphical procedures. Lastly, maximum loads for 14 compact 7075-T651 aluminium specimens were predicted, using the R-6 diagram, to within a standard error of 1 per cent. These specimens were tested as part of a recent ASTM E24.06.02 committee's predictive ‘round robin’ on fracture. 13