Impact Damage Assessment In Orthotropic CFRP Laminates Using Nonlinear Lamb Wave:Experimental And Numerical Investigations

Composite materials are been widely used in aerospace and military engineering because of their excellent mechanical properties.Correspondingly,the composite structure health monitoring technology,especially the nondestructive testing technology,has also been the focus of researchers.In the service process of composite materials,due to the low impact energy,small damage or other reasons,the existence of low-velocity impact damage can't be directly judged visually,which makes it difficult to detect the health of composite materials.Furthermore,the small damage is not easy to detect with traditional linear ultrasound method.However,the nonlinear ultrasonic nondestructive testing technology has its own advantages because of its sensitivity to micro-damage.In this paper,the nonlinear ultrasonic response of composite under different impact energy is studied by both experiment and finite element simulation method,and the low-velocity impact damage areas in composite laminate is quantitatively predicted by wavelet analysis signal processing technology.Firstly,the theory of ultrasonic Lamb wave is introduced,and the propagation characteristics of Lamb wave in composite laminate are described.According to the equation of Lamb wave propagation in composite laminate,the dispersion curve equation is obtained,and then the phenomenon of Lamb wave dispersion in plate structures is explained.Moreover,here the mechanism of ultrasonic nonlinearity is explained and the relative acoustic nonlinear parameter is introduced.According to the dispersion curve of the target composite laminate,the excitation frequency of ultrasonic Lamb wave in experiment and simulation is determined.After that,the nonlinear ultrasonic detection of composite low-velocity impact damage is studied by experimental method.The standard specimens are made to produce low-velocity impact damage in the drop hammer impact test system,and then the ultrasonic nonlinear detection is carried out in the ultrasonic nonlinear detection system.The second harmonics are taken as the characteristic for analyzing the relationship between low-velocity impact damages and nonlinear response of ultrasonic Lamb wave in composite under different impact energy levels.Then,the nonlinear ultrasonic detection of low-velocity impact damages in composite is studied by the finite element simulation method.In the simulation,a integrated finite element model which including three analysis steps of impact-vibration out-ultrasonic detection is introduced,and the reliability of the model is verified after comparing with the experimental results.On this basis,taking the second harmonic as the characteristic,the relationship between low-velocity impact damages and ultrasonic Lamb wave nonlinear response under different impact energy levels in the finite element model is studied,and the same conclusions as the experiment are obtained.Finally,the quantitative prediction method of the damage area in composite laminates under low velocity impact is studied.With the results and data in the finite element model,by using wavelet decomposition,filtering,FFT and data fitting,the method of measuring the characteristic coefficient to predict the low-velocity impact damage areas in composite laminates is realized,and the accuracy is verified.