Detection And Location Of Damage In FRP Plates By Lamb Wave Considering Material Anisotropy

Laminated composite materials(or composite laminates,hereinafter referred to as FRP plate),are more and more widely used in different engineering and industrial fields especially in civil engineering in recent years because of its high strength,easy molding,chemical corrosion resistance and other advantages.However,FRP plates may have delamination,porosity and penetrating injury during manufacturing and application.There are many kinds of damage detection methods for FRP plates.In comparison with other non-destructive testing technologies,ultrasonic Lamb wave detection technology has many advantage,such as simple operation,wide detection range and so it is widely used.But the anisotropy of the FRP material itself has great impact on the accuracy of Lamb wave damage localization,and the research found that the deviation between predicted and true damage position can reach more than 30 mm for a plate with 170×170mm2 area if the effect of anisotropy is neglected.In order to solve this problem,this paper proposes time probability density method which will consider the material anisotropy to locate the damage position,and verify the feasibility and accuracy of this method to anisotropic FRP plate damage location using ultrasonic Lamb waves through numerical analysis and experiment.The Lamb wave localization method considering material anisotropy for FRP plate damage is presented in this paper.The various modes and dispersion characteristics of Lamb waves propagating in anisotropic composite plates are analyzed.The influence of propagation directions of Lamb waves on the velocities of AO and S0 modes is studied.Based on the ellipse method,the travel time of the reflected wave with damage at any point on FRP plate is calculated considering the group velocity variation of A0 wave mode with the propagation direction and is expressed in the form of the brightness graph.The flight time map can be obtained accordingly.Then,the time probability density map of the existence probability of damage can be obtained by mapping the flight time and the actual damage time.In this graph,the greater the brightness,the greater the probability density value,indicating that the greater the probability of damage in the region.The numerical simulation is carried out by ABAQUS finite element analysis software.Anisotropic healthy and damaged FRP plate models are established,and five cycle narrowband excitation signal is modulated by Hanning window to excitate Lamb wave.The time travel information of reflected Lamb wave generated by damage in FRP plate can be obtained through the time-frequency analysis of Hilbert.Based on this,the damage location is judged by ellipse method and time probability density method respectively.It is found that the time probability density method can reduce the positioning error by 76.7%compared with the traditional ellipse method,which preliminarily verifies the validity of the time probability density method to improve the damage location accuracy of anisotropic FRP plates.Then feasibility of this method for different damages locations and the influence of ply orientation,material anisotropy parameter and damage degree on location accuracy are investigated.Experiment for damage detection of anisotropic FRP plate by using NI data acquisition equipment are carried out.FRP laminates specimen is made of carbon fiber/epoxy resin laminate,and the damage was preformed with artificial holes with radius of 4 mm.The frequency of 150 kHz is selected by frequency sweep method.The sinusoidal signal of 150 kHz is modulated by Hanning window to generate Lamb wave.The signal collected by PZT is filtered and the difference between the signals from healthy and damaged plate is obtained.The travel time information of damage reflection wave can be observed accordingly.It is found that the time probability density method can reduce the location error by 80%compared with the traditional ellipse method,which further demonstrate the advantages and feasibility of the proposed method for accurate damage location of anisotropic FRP plates.