| Carbon fiber composite specimens for impact test,static strength test and fatigue test are designed with reference to the national standard.The impact test is carried out,and a nonlinear damage finite element model is established to simulate the impact process.Static strength test,fatigue test,and post-fatigue static strength test are carried out for the post-impact specimens.Fatigue and residual strength tests are carried out for non-destructive specimens.The macroscopic state and mesoscopic appearance of the specimen fracture sections are analyzed.The main conclusions are as follows:(1)The results of impact test and numerical analysis show that:the peak value of the contact force of the impact head first increases and then decreases with the increase of impact energy.The maximum displacement and energy loss rate of the impact head show a monotonically increasing trend with the increase of impact energy.The impacted area of the specimen mainly bears compressive stress,and other areas mainly bear tensile stress.The matrix tensile fracture occurs first after impact,and it extends along the fiber direction from the impact area.The matrix tensile damage area of the specimens in the low-energy group increases linearly with the increase of the impact energy,while the high-energy group exhibits a nonlinear increase trend.Delamination damage is mainly concentrated in the impact area,and the adhesive layer is intact in areas far from the impact point.The delamination area increases linearly with the impact energy.(2)The static tensile test after impact shows that:the mean value of the maximum load shows a nonlinear decreasing trend with the increase of the impact energy.The average elongation of the specimen at failure shows a monotonically decreasing trend with the increase of the impact energy.The fracture sections of the non-destructive specimens are all located near the symmetry plane.The cross-sections of C-10,C-15,C-20,C-25 groups of specimens all extend from the impact area along the width direction of the specimens,and most of the specimens have a relatively flat cross-sectional shape.(3)The tensile fatigue test after impact shows that:low-energy impact significantly reduces the fatigue life of composite laminates.The fatigue life of C-0-P specimens at three stress levels is greater than 1 million load cycles.The average residual fatigue life of the three groups of specimens impacted by different energies show a monotonic nonlinear decreasing trend with the increase of impact energy.The logarithmic fatigue life of C-10-P and C-15-P specimens shows a monotonic nonlinear increase trend with the decrease of stress level,and the logarithmic fatigue life of C-20-P specimens increases linearly with the decrease of stress level.Low-energy impact damage and changes of fatigue load affect the rate of damage propagation in the laminates and thus fatigue life.The external appearance of the non-destructive specimens is basically unchanged after 1 million load cycles.The fatigue fracture sections of the post-impact specimens are all located in the impacted area,and the macroscopic appearance is rough,in which discontinuous mesoscopic appearance is observed.(4)The residual static strength test of fatigue specimens with initial impact damage shows that:the average residual strength of C-0-S group specimens after fatigue test at 55%,50%and 45%stress levels increases linearly with the decrease of stress level,but the difference is small compared with the static tensile strength of the C-0 group specimens.The average residual strength of C-10-S group specimens after fatigue test at 55%,50%and 45%stress levels increases nonlinearly with the decrease of stress level,which increases by 2.98%,8.61%,and 9.19%compared with the C-10 group.(5)The fatigue and residual strength tests of non-destructive specimens show that the average post-fatigue residual static strength of non-destructive specimens increases first and then decreases with the increase of fatigue loading cycles. |
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