13th International Conference on Fracture June 16–21, 2013, Beijing, China -8- Assume the decline of fatigue performance of FSW joint is only affected by residual stress. We can estimate the fatigue life of FSW joint by base metal S-N curve according to the maximum local stress of FSW joint. Goodman formula is used to establish the relationship between the maximum local stress and the base metal S-N curve. Goodman formula is a constant life formula, which is expressed as, 1[1 ]. m a b S S S (5) where, aS : The stress amplitude of load spectrum, mS : The mean stress of load spectrum, 1 S : Stress amplitude when the stress ratio is -1, b: The ultimate strength of material. Through calculation, the fatigue life of FSW joint is equal to that of base material specimen under the constant amplitude load spectrum which maximum stress is 247.6MPa and stress ratio is 0.06. According to Fig.4 in reference [9], the fatigue life of base material specimen which considering the influence of residual stress is 252674 cycles, with an error of 12.5%. This result is basically identical with the experimental one, means that the residual stress is the main factor affecting the fatigue life of FSW joint. 4.2 Fatigue life estimate of lap joint The fatigue life of lap joint is estimated according to stress-strain results and local stress-strain method. Calculation formulas of local stress-strain method are shown as, max min ( )/2, a (6) max min ( )/2, m (7) ' ' (2 ) (2 ) . f m b c a f N N E (8) where, max , min : The maximum and minimum strain, ' f , ' f , E, b, c: Material parameter, max , min : The maximum and minimum stress, N: Fatigue life. According to the simulation results of FEA, the fatigue life of lap joint is 90400 cycles under the constant amplitude load spectrum which maximum stress is 120MPa and stress ratio is 0.06. The
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