Study On Air-coupled Lamb Waves Imaging Detection For Impact Damage Of Composite Laminates

The composite laminates are widely used in aerospace and other important fields because of their advantages such as low density,good strength and strong mechanical properties.While its vertical resistance have poor performance during the service time.It may suffer the striking form the hail,rock,tools and so on.The impact damage defects always can not obviously be seen on the surface of the composite material,but the internal damage may have great damage to the strength and mechanical properties of the material,which poses a serious threat to the safety of the whole structure.For the detection of large-area composite plates,air-coupled ultrasonic Lamb wave detection method has the advantages of non-contact rapid detection and single-side arrangement of sensor,which has outstanding advantages in automatic imaging detection.Firstly,according to the basic theory of thin plate Lamb wave,the calculation process of dispersion curve calculation of multi-layer anisotropic material is introduced.The dispersion relation curves of the corresponding Lamb wave propagation and the displacement distribution of the particles in the thickness direction are studied,and the influence law between the detection frequency and the excitation angle is analyzed.The results show that the A0 modal Lamb wave should be used when using air-coupling Lamb wave.Theoretically,when detection frequency is 0.4MHz,excitation angle of 11.5°could generate A0 mode Lamb wave..Secondly,based on the finite difference numerical calculation method,the leakage lamb wave detection model of carbon fiber composite laminates is established.The influence of the incident angle,defect and material anisotropy on the leakage Lamb wave signal is compared and analyzed by simulation and experiment.The results show that Lamb wave signal could be received effectively when the incident angle is set in the range of 11o to 14o.The Lamb wave signal at the defect has obvious attenuation.The scanning detection method can judge the severity of the defect.The propagation velocity and attenuation law of Lamb wave are basically isotropic.Then,four kinds of damage index DI(Damage Index)were introduced to characterize the impact damage of the composite laminates.According to the characteristics of the analytical method,it is varied as Domain damage index,frequency domain damage index,time-frequency domain damage index,time-reversed damage index.Based on the time-frequency index method and time-reversed index method of the D-scan signals,the feature value DI of the damage is calculated.The results show that the numerical range and the applicable condition of the four damage index algorithms are different,but they can show the severity of the defect.Finally,based on the elliptical imaging theory of probability algorithm,the probabilistic damage scanning imaging algorithm is improved according to the actual situation of air coupling Lamb wave scanning detection.The four damage indexes are used as feature value to reconstructed structure defects in plates.The feature value data of different scanning paths are combined with the probability imaging algorithm to obtain the two-dimensional image of the impact defects of the composite laminates.The description of features such as position and severity could be showed on the image.Compared with the four kinds of damage index,we can see that there are interference signasl in the time domain damage index imaging map,the frequency domain damage algorithm and the time-frequency domain imaging are relatively clear,and the time-reversal damage index imaging contrast is relatively small.