TWI Industrial Member Report Summary 147/1981
By M S Kamath
Background
Whereas for primary membrane loading The Welding Institute's CTOD design curve and the Central Electricity Generating Board's (CEGB) R6 approach tend to provide similar estimates of the allowable flaw size, in the presence of stress gradients the former approach is more conservative in the yielding fracture mechanics regime. This arises because the CTOD design curve is based on a single tensile dislocation model and the peak strains in the cracked region (but calculated for the uncracked body) are assumed to apply throughout. In the present report a simple extension to the CTOD design curve is proposed which permits the effects of peak stresses and non-uniform primary and secondary stresses to be taken into consideration whilst also enabling a failure locus to be constructed. Further, for bending fields, linear elastic part wall flaw correction factors from ASME XI have been adapted for use in the design curve procedure.
Application of the proposals is illustrated for a number of practical examples involving defects subject to primary membrane and bending, peak and secondary (residual, thermal) stresses. The special case of a nozzle corner crack was also assessed and the failure locus method gave predictions which agreed broadly with those of R6.