Study on the accuracy of influence factor method when applied to surface cracks in a welded joint of a ship structure

Abstract

It is necessary to calculate the Stress Intensity Factor (SIF) of surface cracks in welded joint in order to carry out fatigue assessment based on fracture mechanics approach. The SIF of a surface crack in weld can be calculated by using Influence Factor Method (IFM) developed by Shiratori et al. (1986). Influence Factor (IF) database is needed to be prepared in order to carry out IFM calculations. Only the database for cracks in flat plates had been prepared. The use of this IF database in the SIF calculation of cracks in welds leads to calculation error because the size and shape of a structure with a weld differs from that of a flat plate. The degree of this error can be examined by comparing IF values of cracks in a weld and that of cracks in a flat plate. However, reports of such comparative study have apparently not been published yet.In this study, a full-automated IF calculation software system is developed. The validity of the developed system is examined by comparing SIFs of a crack in a flat plate derived from the IF database constructed by the developed system with that assessed by Newman-Raju's formula. The IF values for flat plates and welds are compared by using the developed IF calculation system. As results, the followings are found:(1) The developed system reduces dramatically the man-hours needed to construct an IF database.(2) The SIF of a crack in a flat plate derived from the IF database constructed by the developed system agrees very well with the Newman-Raju's solution. The SIF of a crack in a fillet weld derived from the IF database constructed by the developed system agrees well with the direct FE analysis result. These show the validity of the developed system.(3) IFs have significant values only in the vicinity of SIF calculation point while those at far-off nodes are negligible.(4) IFs in the vicinity of the crack mouth for a fillet weld become up to 15% smaller than those for a flat plate. However, the difference in IFs in the vicinity of the crack bottom of cracks in flat plates and welds is negligible.(5) The SIF of a crack at the crack mouth in a fillet weld is overestimated when the IF database for flat plates is used in IFM calculation while the estimation error is negligible for the SIF at the crack bottom.