Static and fatigue strength of laser welded lap joints

Static and fatigue tensile shear strength of laser welded lap joints were investigated.

Penetrated welds were made on low carbon steel sheets using CO₂ laser output of 3kW with traveling speed of 3–5m/min. Thickness of the steel sheets were 0.7 to 1.6mm and tensile strength were 280 to 1200N/mm².

Tensile strength of the lap joints increased as the thickness of the steel sheets increased. However, the strength was saturated when the thickness was over 1.2mm.

Fatigue cracks initiated at the edge of weld metal in faying surface, and propagated along the fusion line to the free surface. We employed fracture mechanics to evaluate the fatigue strength of the lap joint with different thickness. Stress intensity factor of the lap joint was calculated. This result showed that all specimens were under a mixed mode, where K₁ was dominant, however, K_{1 1} was not negligible. Crack under ΔΚ₁ and ΔΚ_{1 1} propagate in a direction configurated with θ₀, given by:

ΔΚ₁ sinθ₀ + ΔK₁₁ (3cosθ₀ - 1) = 0

ΔK_{θ max} in the direction is given by:

ΔKθ_max = cos(θ₀/2)[Δk₁ cos²(θ₀/2) - 1.5ΔK_{1 1} sinθ₀]

By this criterion, the fatigue strength was well characterized. This approach, which was originally developed to evaluate fatigue strength of spot welded lap joint, was also effective for fatigue strength of laser welded lap joints.