Evaluation Of Microcrack Damage In A Plate Using Nonlinear Lamb Waves

Solid plates are widely used in aerospace,railway industries and many other engineering equipment.Due to the damage produced during the process of early fabrication or the effects of various adverse factors,such as the influence from the external environment and fatigue loading,microcracks will be produced within the plate structure.Without effective method in detecting and controlling the micro-cracks,they will accumulate gradually and finally grow into macrocracks.This will bring negative effect on the performance of normal functions of structures,leading to structural failure or even serious accidents and disasters.In the early on-service stage of the components,microcracks are very small,which cannot be effectively detected by traditional ultrasonic nondestructive testing technology,particularly for the closed microcracks.Nonlinear ultrasound is very sensitive to the early damage of the structure,and can detect the microcracks and closed cracks with dimensions less than the wavelength of ultrasound.Lamb wave has the advantages of fast propagation speed,long propagation distance,small attenuation and high detection efficiency,which makes it an ideal way in detecting damages in plate.Based on the theory of contact acoustic nonlinearity,the nonlinear ultrasonic nondestructive detection and evaluation of microcrack damage in a plate structure using nonlinear Lamb wave is investigated theoretically and numerically in this paper.Firstly,the dispersion equations of an aluminum plate and piezoelectric plate are derived theoretically;and their dispersion curves are thus plotted.According to the conditions of cumulative propagation of the second harmonic Lamb waves,the S1-s2 mode pair,which satisfies the phase velocity matching,and the A0-s0 mode pair,whose phase velocity matches approximately,are thus chosen to be examined in this research.The propagation process of S1-s2 and A0-s0 mode pairs of the nonlinear Lamb waves in an aluminum plate with randomly distribution microcracks and a piezoelectric plate with a single microcrack is simulated numerically.The short-time Fourier transform(STFT)method is employed to process the signals of the nonlinear Lamb waves collecting in time domain.The relations between the nonlinearity parameter and length of the micorocrack zone,crack density and the proportion of surface cracks are thus obtained.It shows that:(1)For aluminum plate with randomly distributed micro-cracks,the nonlinearparameters of S1-s2 and A0-s0 mode pairs increase in an approximate linear way with the length of microcrack zone,crack density and distribution ratio of the microcracks.For the piezoelectric plate with a single microcrack,the nonlinear parameters of S1-s2 and A0-s0 mode pairs increase in an approximate linear way with the crack depths.(2)For aluminum plate with randomly distributed micro-cracks,the nonlinear parameters of S1-s2 mode pair are more sensitive than that of A0-s0 mode pair when the length of microcrack zone is taken into consideration.The nonlinear parameters of A0-s0 mode pair change more sensitively with the crack density and the proportion of surface cracks than that of S1-s2 mode pair.(3)For the piezoelectric plate with a single microcrack,the nonlinear parameters of S1-s2 mode pair change more sensitively with the microcrack depth than that of A0-s0 mode pair.This study demonstrates that the nonlinear Lamb waves can be used effectively in detecting and evaluating the microcrack damage within a plate structure.The results obtained in this paper can provide a very important reference for the application of nonlinear Lamb waves in health monitoring of plate structures.