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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 cyclic stress–strain constitutive relationships for copper film smooth specimens with different damage degrees before and after laser irradiation treatment were investigated. Compared with the changes of the constitutive relationships under different damage degrees, a healing variable for the repaired copper film by laser irradiation was defined. Based on the analysis of the healing variable, a fatigue life prediction method was proposed. The experimental lives of the damaged notched specimens after laser irradiation treatment were used to verify the proposed life prediction method. The results showed that the predicted lives are in good agreement with the experimental results.

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