TWI Industrial Member Report Summary 743/2002
M J Cheaitani and T D Swankie
There are three possible modes of loading which can be applied to a cracked component; mode I, II and III. These are illustrated in Fig.1. In practice, mode I can occur separately or in combination with mode II and mode III. Mode II and III rarely occur separately. A typical example of mixed mode I/II loading is that of a flaw due to lack of side wall fusion in a pipe girth weld, where the weld-parent metal interface is inclined with respect to the pipe axis and the pipe is subjected to axial or bending loads. An example of mixed mode I/III loading is that of a through thickness flaw at the weld toe in the chord member of a tubular joint, where the brace member is subjected to axial loading.
Significant research work has been dedicated to studying mixed mode fracture, especially within the context of critical components associated with pressure-containing equipment in nuclear and aerospace applications. Although it appears that no failures have been directly attributed to mixed mode loading, the possibility of such failures cannot be ruled out. In practice, mode I loading is considered to have the most practical importance, since most materials are more susceptible to fracture by opening tensile stresses than by shear stresses. Consequently, existing fracture assessment procedures address primarily mode I loading, and are relatively far better established for the assessment of mode I than for mixed mode fracture.
Existing procedures for the assessment of a crack subjected to mixed mode loading, consist of applying simplifying assumptions to enable the crack to be evaluated using mode I assessment procedures and mode I fracture toughness. Whilst these approaches appear to work well in most practical cases, they suffer from significant limitations in comparison with mode I assessment procedures. reflecting a limited understanding of the mechanics of mixed mode fracture.
Perform a review of analytical and experimental studies on mixed mode crack loading, including data and assessment procedures.