Failure assessment diagrams and J estimates: A comparison for ferritic and austenitic steels

Abstract

Comparisons are made between the R6 assessment procedures and a J estimation procedure for a number of specimen geometries made of A533B steel and an austenitic steel. The two procedures differ in three important respects: 1. (i) The R6 procedures make explicit use of the plastic limit load of a structure while, for the J estimate, this can only be inferred from the load at which J increases rapidly. 2. (ii) The R6 procedures use a flow stress, generally equal to the arithmetic mean of the yield and ultimate tensile stress, to define this plastic limit load while the J estimate attempts to allow for strain hardening using the Ramberg-Osgood law. 3. (iii) The R6 procedures interpolate between the linear elastic and fully plastic limits using a modification of the strip yielding model while the J estimates use a first order plasticity correction. Despite these differences, it is shown that for most structures built of A533B steel the R6 procedures can be expected to give results comparable to the J estimates. However, for structures built of austenitic steel there is a need to make an explicit allowance for strain hardening. The accuracy of the J estimates depends upon how closely the Ramberg-Osgood law fits the material's true stress-true strain curve. This, in turn, depends upon the strain hardening capacity of the material. The fit is only satisfactory when the strain hardening capacity is low. Thus the J estimates, although of proven reliability for some geometries of A533B, can still be seriously in error for austenitic steels. Even for A535B steels the nature of the Ramberg-Osgood law produces a tendency to overestimate plastic limit loads. A strain hardening form of the R6 procedures is developed which avoids these problems. It is based upon the ratio of the ultimate to yield stresses obtained from the engineering stress-strain curve. It is demonstrated that although this form of the R6 procedures is in good agreement with the results of test data from compact specimens of austenitic steels, the modification will have an insignificant effect on the results obtained for ferritic steels. It can also be presented in a diagrammatic form which is particularly suitable for code applications.