Which weld joint is most vulnerable to fatigue failure?

A fillet weld is considered the most vulnerable to fatigue failure due to its geometric configuration. In a fillet weld, the weld bead creates a transition between two pieces of metal that are typically joined at right angles. This design introduces stress concentration points where the geometry changes from one thickness to another.

When a cyclic load is applied to a structure with a fillet weld, the sharp corners and uneven distribution of stress can lead to crack initiation. Under repeated loading conditions, these cracks can grow over time, resulting in fatigue failure. The lack of reinforcement in a fillet weld compared to other joint types can further exacerbate this problem, as it does not provide the same level of support against the stresses induced by loading cycles.

While other joint types like butt, corner, and lap welds each have their own vulnerabilities, they generally distribute stress more evenly and have more robust configurations to handle loading applications. This distinction is what makes fillet welds particularly susceptible to fatigue-related issues.