Comparison of fracture toughness measurements from ferritic steel compact and surface-flawed specimens

Comparisons are made between predicted values of stress for initiation of crack growth in surface-flawed specimens and experimentally determined values. The comparisons are made for test temperatures corresponding to the lower shelf and the lower transition region which have elastic and elastic-plastic conditions, respectively. Predictions of stress levels for initiation of crack growth are based on Appendix A, Section XI of the ASME Boiler and Pressure Vessel Code and on an approach developed by Newman-Raju. The experimental results are based on acoustic emission techniques used to detect loads corresponding to crack initiation; their use in the comparison involved both the measured elastic-plastic stress value and an “equivalent” elastic stress. Evaluation of the analytical approaches showed that neither method was consistently more conservative over the full range of input dimensions. For tests conducted at lower-shelf temperatures (where very little plastic deformation occurs), the predicted stresses were lower than experimental values. At higher temperatures (where increasing amounts of plastic deformation occurs), some nonconservative predictions were made. A correlation is noted between the predicted-to-experimental stress ratio and a parameter used to quantify the severity of the surface flaw. This correlation may provide a basis for an empirical method for predicting the stress required to initiate crack growth.