How does the profile and toe radius of a fillet weld significantly impact the fatigue performance of a welded joint by influencing stress concentration?

A fillet weld is a type of weld that joins two pieces of metal at an angle, creating a triangular cross-section. The weld profile refers to the shape of the weld's exposed surface; common profiles include convex (bulging outward), flat (straight from toe to toe), and concave (curving inward). The toe radius is the curvature at the transition point where the weld face meets the base metal. Stress concentration is the localized amplification of stress at geometric discontinuities, meaning that stress becomes much higher in a small area than in the surrounding material. Fatigue performance describes a material's resistance to crack initiation and propagation under repeated or cyclic loading.

The profile of a fillet weld significantly influences its toe radius, which in turn dictates the level of stress concentration and subsequent fatigue performance. A convex weld profile, which bulges outward, inherently creates a sharper angle and a smaller, tighter toe radius where the weld metal transitions into the base plate. This small, sharp radius acts as a severe geometric discontinuity. Applied forces are unable to flow smoothly around this sharp corner, causing stress lines to crowd together intensively at the weld toe. This crowding leads to a high degree of stress concentration, meaning the actual local stress at the weld toe is many times greater than the nominal (average) stress applied to the joint.

Conversely, a concave weld profile, which curves inward, or a properly executed flat profile typically results in a larger, smoother toe radius. This larger radius provides a more gradual and gentle transition between the weld and the base metal. The gradual change in geometry allows stress lines to distribute more evenly across the area, reducing the crowding effect and significantly lowering the level of stress concentration at the weld toe.

Fatigue failure almost universally initiates at points of maximum stress concentration. When a welded joint is subjected to cyclic loading (repeated application and removal of force), the highly concentrated stress at a sharp weld toe (resulting from a convex profile or small toe radius) accelerates the formation of microscopic cracks. These cracks nucleate at lower overall stress levels and after fewer load cycles compared to a joint with a smoother toe. Once initiated, the high local stress further drives the propagation of these cracks, leading to premature fatigue failure of the joint. Therefore, a small toe radius and the associated high stress concentration drastically reduce the fatigue performance, while a larger, smoother toe radius, achieved through a favorable weld profile, reduces stress concentration and significantly enhances the fatigue life of the welded joint by delaying crack initiation and propagation.