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.

Crack propagation tests of lead-free solder were conducted using center-notched plates under cyclic tensioncompression 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.

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Torsional fatigue tests were conducted for circumferentially notched bars of austenitic stainless steel (JIS SUS316L) and carbon steel (JIS SGV410) under completely reversed cyclic torsion without static tension (Case A) and with static tension (Case B). The propagation behavior of cracks formed at the notch root was examined from a viewpoint of fracture mechanics. The length of crack was evaluated by the electrical potential method under the assumption of concentrical cracks propagating inward on the minimum cross section of specimens. For Case A of both steels, the crack propagation rate decreased with crack length, and turned to increase after taking the minimum rate point. Sharper notches gave rise to a slower rate for SUS 316L, while to a faster rate for SGV410. At the same stress amplitude, the crack propagation rate in SUS316L is lower than in SGV410. For Case B, the crack propagation rate was monotonically increased with crack length. Similar tendency was observed when the crack propagation rate was correlated to the stress intensity range. The J integral range was estimated from the hysteresis loop between torque and twist angle. For Case B, the crack propagation rate was expressed as a power function of the J integral range. For Case A, the crack propagation was lower than that for Case B, because of retardation due to the sliding contact between crack faces. The amount of retardation was larger for sharper notches at lower stress amplitudes in SUS316L, while blunter notches gave larger retardation in SGV410. The amount of retardation in Case A was closely related to the roughness or the acuteness of the factory-roof fracture surface.