A B S T R A C T For high-cycle fatigue of metals, the DC electrical resistance is a more sensitive parameter to the initiation of micro-cracks during the irreversible fatigue damage accumulation process. This implies that the electrical resistance is a suitable parameter that can be consistent with the fatigue damage physical mechanism. The relation between the ratio of electrical resistance changes and the cyclic fraction of the fatigue specimen may reasonably represent deterioration in mechanical properties of structural steels during the high-cycle fatigue process. The high-cycle fatigue damage accumulation model based on electrical resistance for structural steels was proposed. The model was verified by some experimental data for three structural steels; normalized 45C steel, 20 Mn steel and 16 Mn steel, and good agreement was obtained. The corresponding fatigue lifetime on the basis of the electrical resistance model was also performed. The results show that the approach to fatigue lifetime prediction and failure based on the electrical resistance is a good non-destructive technique.A,Ã = apparent and effective cross-sectional areas of the damaged specimen A 0 = cross-sectional area of the initial specimen b = fatigue exponent of materials D, D c = damage variable of the damaged specimen, critical value of damage N , N f = elapsed cycles and cycles to failure of the fatigued specimen R, R = electrical resistances of the virgin specimen and the damaged specimen r = ratio of electrical resistance in the damaged state to electrical resistance in the virgin state r c , r (N f −1) = critical ratios of electrical resistance in the damaged specimen α = material parameter at a prescribed stress amplitude r = increment of ratio between electrical resistances in the damaged state and in the virgin state σ a = cyclic stress amplitude σ f = fatigue strength coefficient of materials I N T R O D U C T I O NUnder the action of cyclic loading, many factors, which are generally coupled with each other, may affect fatigue damage evolution in metals. 1-13 Therefore, a satisfactory theory with sound physical reasoning should be estabCorrespondence: Sun Binxiang.

The recrystallization of b-Sn profoundly affects deformation and failure of Sn-Ag-Cu solder joints in thermomechanical fatigue (TMF) testing. The numerous grain boundaries of recrystallized b-Sn enable grain boundary sliding, which is absent in as-solidified solder joints. Fatigue cracks initiate at, and propagate along, recrystallized grain boundaries, eventually leading to intergranular fracture. The recrystallization behavior of Sn-Ag-Cu solder joints was examined in three different TMF conditions for five different ball grid array component designs. Based on the experimental observations, a TMF damage accumulation model is proposed: (1) strain-enhanced coarsening of secondary precipitates of Ag 3 Sn and Cu 6 Sn 5 starts at joint corners, eventually allowing recrystallization of the Sn grain there as well; (2) coarsening and recrystallization continue to develop into the interior of the joints, while fatigue crack growth lags behind; (3) fatigue cracks finally progress through the recrystallized region. Independent of the TMF condition, the recrystallization appeared to be essentially complete after somewhat less than 50% of the characteristic life, while it took another 50% to 75% of the lifetime for a fatigue crack to propagate through the recrystallized region.

The present work addresses the effects of thermomechanical history on the recrystallization behavior of lead free and backward compatible solder joints. 30 mil SAC305 balls were reflowed onto BGA pads using either a SAC305 or a eutectic SnPb paste.

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