PREDICTING FATIGUE INITIATION LIFE OF SN/3.8Ag/0.7Cu SOLDER USING ENDOCHRONIC CYCLIC DAMAGE-COUPLED VISCOPLASTIC THEORY

This paper studied cyclic behavior Sn/3.8Ag/0.7Cu solder with dendritic microstructure. A cyclic damage factor D under constant strain amplitude fatigue tests, was defined by using the reducing rate of maximum cyclic tensile stress sigma(a). The critical cyclic damage D(C) and it's fatigue initiation life N(1) were determined very consistently by using separately the experimental sigma(a) vs. N curves and the percolation theory. The endochronic cyclic damage-coupled viscoplastic theory proposed by the 1 st author was used to simulate cyclic stress-strain hysteresis loops with damage under strain amplitude (epsilon(a)) 0.8% at 298K. The results were in very good agreement with data. Combining the evolution equation of intrinsic damage and the computed cyclic stress-inelastic strain relation, a modified Coffin-Manson relationship was derived. By setting D(C) = 0.3, it predicted very effectively the N(1) data under epsilon(a) from 0.2% to 1.0%.