A Fatigue Failure Mode Transition Criterion for Sizing Load Carrying Fillet Welded Connections

In load-carrying fillet-welded connections, two distinct fatigue failure modes are possible depending upon fillet weld leg size and loading conditions. One is weld toe cracking through base plate thickness and the other is through weld metal, often referred to as weld root cracking. Weld root cracking mode has always been a concern in design and analysis of load-carrying fillet welded connections. In the past, it has been noticed that weld root cracking mode could be avoided by enlarging the fillet weld size or weld penetration, and numerous theoretical studies had been performed to construct such a design reference to prevent weld root cracking. However, the theoretically developed design reference cannot include shop floor practices, which may result in unfavorable fillet weld size design. In this study, experimentally based analysis on fatigue failure mode transition behavior is performed based on a recent comprehensive fatigue testing program in support of construction of lightweight ship structures reflecting typical shop floor practices and through introducing a newly developed analytical weld throat stress model and misalignment-induced stress concentration factors. As a result, a critical weld size reflecting typical shop floor practice is obtained. It is determined that the actual critical weld size obtained from test data is much lower than the theoretical one due to the combination effect of weld penetration and misalignments. Finally, a statistical procedure is proposed to construct a mathematical model that is able to provide a confidence level to avoid weld root cracking mode for designed fillet weld size.