Sn/3.5Ag銲錫含頻率效應之疲勞初始壽命預估-含損傷內涵時間黏塑性理論之應用

Abstract

In this paper, the damage-coupled Endochronic viscoplasticity was used to integrate the experimental data-Cyclic Stress-Strain Curves, the Strain-Life Curves and the Load Drop Curves of uniaxial cyclic tests of Sn-3.5Ag solder alloy at 293K under frequency [0.001~1.0Hz] and strain range [0.5%~2%]. Before experiment, specimens had already have surface cracks, these lead to a sudden drop of stress amplitude within several cycles. Based on and , the kernel function was determined as ( ) and the strain rate sensitive function as . Under at and , the cyclic stress - strain curves calculated under the effect of surface cracks, were in good agreement with the experimental data. According to the Load Drop Curves without effects of surface cracks adjust and then obtain the modified cyclic stress – inelastic strain curve . Let represent the cycles affected by the surface cracks ,the critical cyclic damage factor and initiation fatigue life defined in Load Drop Curves, were adjusted into and . From the phenomenon of cyclic damage, the damage degree was proposed to depend positively on and nonpositively on , and then derived modified damage formula. Using the Endochronic viscoplasticity and constant value of , the modified Lee-Coffin-Manson ( ) life prediction formula: can be derived. According to 、 , and under the Load Drop experimental data, the damage exponent and . Based on , the experimental data of the fatigue life( 0.001Hz ~ 1Hz) with , can be potted in vs. paper. Divided at , two straight lines can be drawn in which the slope when and when . These lines were then compared with the straight line( ) of modified Coffin-Manson emipical formula. When , both lines can predict data quite well, but when line predicts lower values of life. These conservative life prediction are more reasonable due to theoretical bases in the derivation of equation.