Effect of Load Waveform on Creep-Fatigue Crack Propagation in Lead-Free Solder

Abstract

Crack propagation tests of lead-free solder were conducted using center-notched plates under cyclic tension-compression of three waveforms : pp waveform having fast loading and unloading rates, cp-h waveform having a hold time under tension, and cc-h waveform having a hold time under tension and compression. In fatigue loading at fast loading-unloading rates, i.e. pp waveform, the path of crack propagation was macroscopically straight, perpendicular to the maximum principal stress direction. The introduction of creep components by tension and compression holds in cc-h waveform promoted shear-mode crack propagation even under tension-compression loading. For fatigue loading of pp waveform, the crack propagation rate was expressed as a power function of the fatigue J-integral and the relation was identical for load-controlled and displacement-controlled conditions. The creep component due to the hold time greatly accelerates the crack propagation rate when compared at the same values of the fatigue J integral or the total J integral (the sum of fatigue J and creep J integrals). The creep crack propagation rate was expressed as a power function of the creep J integral for each case of cp-h and cc-h waveforms. The crack propagation rate for cp-h waveform is higher than that for cc-h waveform. The predominant feature of fracture surfaces was striations for pp waveform and grain boundary fracture for cp-h waveform. Grain fragmentation was evident on the fracture surface made by cc-h waveform.