Modeling of Fatigue Behavior in Fillet Welded Connections in Lightweight Ship Structures.

Abstract

Fillet welded connections are most commonly for construction of marine structures that are often subjected to various forms of cyclic loading in service. Due to its complexity in stress concentration, today’s fatigue design rules are empirical and mostly based on test data from 1970’s, often resulting in significantly oversized welds or overwelding. As structural lightweighting becomes increasingly important, there is an urgent need for more quantitative fatigue-based fillet weld sizing criterion. In this work, by taking advantage of recent developments in mesh-insensitive method for dealing with stress singularity at weld root and weld toe, a closed-form weld throat traction stress solution has then been developed for analytically determining weld throat stress state as a function of weld throat plane angle. This closed form solution enables a formulation of an equivalent traction stress parameter that can be used to determine the critical fillet weld size that separates weld root failure mode from weld toe failure mode. In dealing with fatigue test specimens produced in representative shipyard production environment, it is found that both weld penetration and joint misalignments must be taking into account in order to effectively interpret fatigue test data generated as a part of this study. Along this line, the analytical traction stress method developed is further expanded to take into account of weld penetration and joint misalignments. In the latter, a series of new analytical solutions for calculating stress concentration factors caused by joint misalignments are developed. With these new developments, analytically determined fatigue failure mode transition in terms of relative weld fillet size with respect to base plate thickness is proposed and validated by fatigue test data. Then, a quantitative weld sizing is proposed based on a detailed data analysis through a logistic regression method. Aluminum fillet-welded cruciform specimens are also considered for confirming the applicability of the failure mode transition criteria developed based on steel weldments. Non-load carrying fillet-welded cruciform specimens are also tested and analyzed using the traction stress method for developing baseline information for supporting the development of fatigue design curve for titanium hull structures.