Calculations of notch stress factor of a thin-walled spreader bracket fillet weld with the use of a local stress approach

摘要

Presence of geometric notches in welded joints causes concentration of strains and stresses, therefore reducing fatigue strength of such joints. This article presents an analysis of stress concentrations in a fillet weld of a spreader mounting bracket on a small sailing yacht. The aim of this article is to direct the attention of designers, manufacturers and regulatory bodies to issues of fatigue cracks that form in brackets fastening spreaders to masts. The issue concerns mainly yachts with overall length of a hull below 10 m. Despite frequent defects the solution is still being chosen by designers, mainly for technological reasons. Current design regulations of classification societies and other regulatory bodies for small recreational yachts do not specify a technical solution for spreader fastening. With the use of finite element analysis it was shown that for short term design loads structural strength of a bracket is sufficient. Then, with the use of the notch stress approach, it was proven that the spreader bracket welded of thin plates is a structural solution that runs a risk of fatigue cracks. The below presented calculations of the geometric concentration factor of weld section in the locations of fatigue cracks were performed for different geometric cases of joints. The analysis results indicate the need for greater attention that should be paid to the problem of fatigue cracks that form in welds of brackets fastening spreaders to masts. The determined values of effective stress concentration coefficients show the need for conducting fatigue calculations of brackets welded of thin plates, or choosing an alternative solution for fastening spreaders to masts. In both cases it seems appropriate to conduct fatigue calculations for a structural element which is "a weak link" of the main drive on a sailing yacht.