How to Improve Fatigue Strength in Welded Joints

1. Reduce Stress Concentration

Fatigue cracks originate at stress concentrations in welded joints and structures. Any measure that removes or reduces those concentrations can increase fatigue strength.

Use a Sound Structural Form

• Prefer butt joints and minimize lap joints. Where possible, replace T-joints or corner joints in critical structures with butt joints and keep welds away from corners. If a T-joint or corner joint is unavoidable, use full penetration.
• Avoid eccentric loading. Internal force should flow smoothly and distribute uniformly without creating secondary stress.
• Provide gradual transitions where plates of substantially different thickness or width meet. Use generous radii at sharp corners and bends.
• Avoid three-weld spatial intersections. Keep welds away from stress-concentration zones and avoid transverse welds on principal tensile members. When unavoidable, ensure high internal and external quality and minimize weld-toe concentration.
• Do not leave permanent backing behind a single-sided butt weld in a critical structure. Avoid intermittent welds because each segment has strong concentrations at its start and end.

Structures under dynamic loading or low-temperature service require more careful detail design than structures carrying static load at room temperature.

Control Weld Shape and Quality

Keep butt-weld reinforcement as small as possible and preferably plane or grind the weld flush. Use concave rather than convex fillet welds for T-joints. Provide a smooth transition at the weld toe, grinding or TIG-remelting it when necessary.

Every weld defect creates some stress concentration. Planar defects—cracks, incomplete penetration, lack of fusion, and undercut—have the greatest effect on fatigue strength. Design every seam for easy access and sound execution, and remove all defects that exceed acceptance limits.

2. Adjust the Residual-Stress Field

Residual compressive stress increases fatigue strength, whereas residual tensile stress reduces it. A favorable compressive field at the surface or a stress concentration can be created by changing the welding sequence or applying localized heating.

In an I-beam butt joint, butt weld 1 carries the greatest bending stress. Leave a short length of fillet weld 3 unwelded at each end, complete weld 1, and then complete web butt weld 2. Shrinkage of weld 2 places weld 1 in residual compression. Finally weld the reserved portions of weld 3; their shrinkage puts both welds 1 and 2 into residual compression. Tests showed that this sequence increased fatigue strength by 30% compared with welding seam 2 before seam 1. For a gusset plate joined by longitudinal welds, properly positioned spot heating at the notch near the weld end can create beneficial residual compression at the concentration.

Surface-deformation strengthening by rolling, hammering, or shot peening can also plastically deform and harden the joint surface, leaving residual compression that improves fatigue strength.

3. Improve Joint Microstructure and Properties

Improving the intrinsic fatigue resistance of base and weld metal requires better metallurgical quality in both the material and wire and fewer inclusions. Where practical, joint heat treatment can create a favorable microstructure, raising strength together with ductility, toughness, and fatigue resistance.

Strength, ductility, and toughness must be balanced. Strength resists fracture, but high-strength materials are notch-sensitive. Ductility absorbs deformation energy, lowers stress peaks, redistributes high stress, and blunts notch and crack tips so growth slows or stops. Because ductility allows strength to be used effectively, increasing ductility and toughness can markedly improve the fatigue resistance of high-strength aluminum-alloy joints.