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This paper presents data and methods for predicting fatigue lives of spot welds subjected to variable amplitude random-sequence load fluctuations. An equivalence between the fatigue behavior of spot welds under constant amplitude and under variable-amplitude cyclic-load fluctuations is also established by using an effective damage-parameter range approach. The analyses presented show that the damage parameter for fatigue of spot welds is where ΔP and ΔθN are the fluctuations in the applied stress and in the nugget rotation respectively. The analyses also show that the effective damage-parameter range derived by using the Miner cumulative damage law, EMiner', resulted in an excellent correlation of fatigue lives for spot welds under variable-amplitude and constant-amplitude cyclic loading. The root-mean-square, RMS, damage-parameter range, resulted in a conservative correlation.

Tensile-shear spot-weld specimens were tested to determine the effect of base-metal tensile properties on fatigue life. Four sheet thicknesses (t) were investigated. Base-metal yield strength ranged between 186 and 757 MPa (27 and 110 ksi). Results showed that fatigue life was independent of base-metal strength for lives greater than 104 cycles. At shorter lives, fatigue performance improved with increasing base-metal strength. For a given cyclic load range, ΔP, fatigue life increased at all lives with increasing sheet thickness (t). Fatigue results from both this and a previous study were used to develop a spot-weld fatigue design curve relating spot-weld tensile-shear fatigue life to the correlation parameter, ΔE, equal to ΔPΔΘN1/2/t. Weld-nugget rotation, ΔΘN, is a design-dependent variable related to joint stiffness.