Fatigue life of 63Sn–37Pb solder related to load drop under uniaxial and torsional loading

The low cycle fatigue behavior of 63Sn–37Pb solder was investigated under strain controlled uniaxial and torsional loading. A continuous load drop was observed during the test. It was found that the load drop percentage had little effect on the elastic strain–life curve but a strong effect on the plastic strain–life curve. The fatigue strength coefficient, fatigue strength exponent and fatigue ductility exponent had no great changes with the amount of load drop. However, the fatigue ductility coefficient showed a great difference and was linearly varying with load drop. A fatigue criterion of Coffin–Manson type was proposed in relation to load drop. The descending curve of the peak stress with cycle was observed to consist of transient, steady state and tertiary regions. The percentage of load drop corresponding to the turning point from the steady state to the tertiary region was about 25% for all strain ranges of uniaxial fatigue and 20–30% for all strain ranges of torsional fatigue. The von Mises stress–strain data under uniaxial and torsional cyclic loading can be fitted by the Ramberg–Osgood relationship. The uniaxial and torsional fatigue lives were correlated with von Mises equivalent strain amplitudes well.