Simple practical method for determining the ductile instability of cracked structures

A method for determining the maximum load bearing capacity of a structure during ductile stable crack growth is shown to be equivalent to an R-curve instability analysis. This equivalence is demonstrated using as an example a center cracked plate. The method is applied to two cases pertinent to pressure vessels, namely internally pressurized thick and thin walled cylinders. The results of these examples show that for values of the tearing modulus, T/sub mat/, typical of pressure vessel steels, the instability pressure is approximately determined by the plastic collapse pressure obtained using the instantaneous crack length. This may be considerably below the collapse pressure corresponding to the initial defect size. Practical aspects of instability analyses are discussed, such as the relevance of J/sub R/ (Δa) curves obtained in the laboratory to practical problems and the different treatments required for part and full penetration cracks. Finally the suggested procedures for performing an instability analysis based on a failure assessment diagram are summarized