A B S T R A C T A healing method for fatigue damage was studied by laser shock peening (LSP) with excimer laser for polycrystalline copper film. It is found that work hardening due to LSP could be responsible for the improvement of residual fatigue lives for the damaged and undamaged specimens by LSP, and the hardening degree for the damaged specimen by LSP is obviously higher than that for the undamaged specimen by LSP. In this paper, two basic mechanisms were identified. One is the dissipated energy enhancement mechanism, which improves the fatigue life caused by laser shock stress, and the other is the healing mechanism, which leads to a further improvement. Based on the two mechanisms, a residual fatigue life prediction method is proposed by the view of energy consumption before and after LSP. The predicted lives by the proposed method agree well with the experimental results.Keywords laser shock peening; work hardening; dissipated energy enhancement mechanism; healing mechanism; life prediction.coefficient for the original specimen by LSP k D ¼ hardening coefficient for the damaged specimen by LSP m ¼ modified exponent of hardening degree N 0 ¼ fatigue life for original specimen N D ¼ consumed life of the damaged specimen N DEEM ¼ improved fatigue life caused by the extra plastic strain energy for the original specimen by LSP N D DEEM ¼ improved fatigue life caused by the extra plastic strain energy for the damaged specimen by LSP N LSP D¼0 ¼ fatigue life for the original specimen by LSP N HM ¼ pure healing life caused by HM N r ¼ residual life of the damaged specimen by LSP n′ ¼ cyclic hardening exponent of the material ΔW p0 ¼ plastic strain energy for the original specimen without LSP ΔW LSP;D p ¼ plastic strain energy for the damaged specimen with LSP ΔW LSP;D¼0 p ¼ plastic strain energy for the original specimen with LSP Δσ ¼ stress range Δε p ¼ plastic strain range Δε p0 ¼ plastic strain range of specimen without LSP Δε LSP p ¼ plastic strain range of specimen with LSP Δε LSP;D¼0 p ¼ plastic strain range of the original specimen with LSP Δε LSP;D p ¼ plastic strain range for the damaged specimen by LSP

A B S T R A C T The mechanism of fatigue life improvement for damaged and undamaged copper film specimens with thickness of 25 um was investigated by laser surface irradiation under optimal parameters of laser irradiation at different loading levels. The results showed that the degree of improvement in fatigue life for the damaged specimens is more evident when the applied nominal stress was larger. The hardening induced by laser irradiation and a smooth surface feature can be obtained after the laser irradiation treatment, which results in fatigue life to be extended. A fatigue life prediction method was proposed by the view of equivalent stress. The predicted lives by the proposed prediction method were in good agreement with the experimental results.Keywords healing fatigue damage; laser surface irradiation; copper film; fatigue life prediction. N O M E N C L A T U R ED = damage amount e = nominal strain of notched specimens. h D = hardening coefficient K′ = cyclic strength coefficient K 0 ′ = cyclic strength coefficient for original specimens K 1 ′ = cyclic strength coefficient for the undamaged specimens (D = 0) after LSI K 2 ′ = cyclic strength coefficient for the damaged specimens (D = 0.5) after LSI K σ = effective stress concentration factor LSI = laser surface irradiation n′ = cyclic strain hardening exponent n 0 ′ = cyclic strain hardening exponent for original specimens n 1 ′ = cyclic strain hardening exponent for the undamaged specimens (D = 0) after LSI n 2 ′ = cyclic strain hardening exponent for the damaged specimens (D = 0.5) after LSI N c = fatigue life of specimens corresponding to the conversion stress N r = residual life of specimens healed by laser irradiation treatment R a = arithmetical average roughness R q = root-mean-squared roughness R t = peak-to-valley difference s = nominal stress of notched specimens α σ = theoretical stress concentration factor Δe = nominal strain range of notched specimens Δe 0 = nominal strain range of specimens before laser irradiation treatment Δe t = nominal strain range of specimens after laser irradiation treatment Δs = nominal stress range of notched specimens Δε = strain range Δε e = elastic strain range Δε p = plastic strain range

The healing variable and enhancement variable were first defined by the fatigue ductility, and then based on the relationship of the damage variable, the healing variable and the enhancement variable, a nonlinear fatigue damage‐healing model was proposed for predicting the fatigue life of the healed copper film by laser shock peening (LSP). The nonlinear fatigue damage cumulative process was considered in the model for the original specimen without LSP under constant and variable amplitude loadings. The results showed that the proposed nonlinear fatigue damage‐healing model can predict the residual fatigue life for the damaged copper film specimen well.