Creep-Fatigue Crack Propagation in Lead-Free Solder under Various Strain Waveforms

Abstract

Crack propagation tests of lead-free solder were conducted using center-notched plates under strain-controlled conditions of fast-fast (pp), slow-slow (cc), slow-fast (cp), and fast-slow (pc) strain waveforms. A method to estimate creep J-integral and fatigue J-integral from load-displacement relations was proposed, and those integrals were used to correlate the crack propagation rate. For the case of pp waveform, the crack propagation rate was expressed as a power function of the fatigue J-integral. For the other cases, the creep component greatly accelerates the crack propagation rate when compared at the same values of the fatigue J-integral. The creep crack propagation rate was expressed as a power function of the creep J-integral for each case of cp, pc and cc waveforms. The crack propagation rate for cp and pc waveform is higher than that for cc waveform. In fatigue loading under pp waveform, the path of crack propagation was macroscopically straight, perpendicular to the maximum principal stress direction. The introduction of creep components by slow strain rates promoted shear-mode crack propagation. The predominant feature of fracture surfaces observed by scanning electron microscopy was striations for pp waveform, and grain boundary fracture for cp and pc waveforms. Grain fragmentation was evident on the fracture surface made under cc waveform.