Effect of hold time on creep-fatigue crack propagation in lead-free solder

Abstract

Crack propagation tests of lead-free solder were conducted using center-notched plates under load-and displacement-controlled conditions of cyclic tension-compression. Four waveforms were adopted for tests: pp waveform having fast loading and unloading, cc-h waveform having a hold time under tension and compression，cp-h waveform having a hold time under tension, and pc-h waveform having a hold time under compression. The crack propagation rate was correlated to fatigue and creep components of the J-integral range determined from load-displacement curves. For pp waveforms, the crack propagation rate was expressed as a power function of fatigue J integral range and the relation was identical for load-controlled and displacement-controlled conditions. The creep component due to the hold time greatly accelerated the crack propagation rate when compared at the same values of the fatigue J integral, especially under load-controlled conditions of cc-h and cp-h waveforms. Not much acceleration of crack propagation was observed for displacement-controlled conditions. The creep crack propagation rate was expressed as a power function of the creep J integral range for each case of cp-h and cc-h waveforms. The crack propagation rate for cp-h waveforms is higher than that for cc-h waveforms. As a simple estimate, the apparent J-integral range was evaluated from the area between the loading curve and the straight line connecting maximum and minimum loads in load-displacement relations. The relation between crack propagation rate and apparent J-integral range for symmetrical loading waveforms such as cc-h was only slightly higher than that obtained for pp waveforms, and the relation for unsymmetrical loading waveforms with tensile hold such as cp-h was much higher.