Local Approach Studies of the Effect of Load History on Ductile Fracture

Abstract

Local Approach methods provide alternative routes for carrying out fracture mechanics assessments. These methods use a finite element analysis of the cracked component that incorporates a micro-mechanical model of material behaviour in the region of the crack tip. The development of damage, and eventual failure of the material for particular loading conditions, is calculated directly by the micro-mechanical model. A potential advantage of such models is that they can automatically incorporate the effects of loss of constraint and load history. This paper describes studies that compare the predictions of the Gurson-Tvergaard-Needleman (GTN) local approach model of ductile fracture with traditional fracture mechanics parameters. The model is used to consider the effect of load history on ductile fracture initiation in a typical fracture mechanics Compact Tension (CT) specimen. The aim is to investigate the benefits of applying such models to situations where events in the plant loading history can have a significant effect on crack driving force for typical defects that subsequently develop during service. It is anticipated that the results will be used to make improvements to the accuracy of the traditional assessment procedures. The results show that in certain circumstances, for instance where residual stresses are present in the vicinity of a defect, load excursions on the specimen can have a beneficial effect on global load carrying capacity and that load history effects can be captured by adopting the local approach for the assessment of ductile fracture. It is also noted that the trends observed in other local parameters, e.g. J and CTOD, must be treated with caution, probably due to near crack-tip softening associated with the implementation of the GTN model in a region of high stress concentration.