In this study, the fatigue damage modes of carbon fiber/epoxy composite laminates with symmetrical architecture were investigated by acoustic emission (AE) technique under fully reversed loading. The principal component analysis and the K‐means cluster analysis using correlation AE characteristic parameters, including the energy, the amplitude, and the duration time, were performed to identify various damage modes during the fatigue test process. The analysis results of the AE signals indicated that the high‐intensity AE signals were generated by the compressive load and the low‐intensity AE signals were generated by the tensile load. The maximum energy and duration time generated by the compression load are approximately 20 and 10 times that of the tensile load, respectively, which was consistent with the force‐controlled static test results under tension and compression loadings. Therefore, the fatigue damage caused by the compressive load is much greater than that of tensile load under fully reversed loading. The results of the multi‐AE parametric clustering analysis combined with scanning electron microscope micromorphology revealed that the damage modes of the laminate specimens were classified into five types, namely matrix cracks, fiber/epoxy interface debonding, shear, delamination, and fiber breakage. In addition, the damage modes at different stages during the fatigue test process were also analyzed and discussed.
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
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