Study On Nonlinear Ultrasonic Testing Method Of Closed Crack In Metal Plate

The metal plate structure is widely used in various types of equipment,fatigue crack is one of its main form of damage.The occurrence of fatigue cracks seriously endanger the safe operation of the equipment.If the defects are not detected and the measures are not taken,the crack will accelerate the expansion under the load,and eventually lead to catastrophic accidents,which caused by the structural faults.Therefore,the urgent need for the development of fatigue crack early detection and evaluation methods.Aiming at the requirement of structural closed crack detection,this paper studies the nonlinear ultrasonic detection method of structural closed crack.The research contents include:(1)Study on non-collinear body wave mixing detection method for closed cracks in block metal structures.Based on the classical nonlinear theory of the interaction of two columns of waves,the theoretical curves of 10 different types of mixed nonlinear effects are analyzed.Taking the typical noncollinear shear wave mixing as an example,for nonlinear ultrasonic detection which the classical material nonlinear and the contact nonlinear due to the closed crack are numerical simulation.The difference between the contact nonlinearity and the classical nonlinearity of the same and different frequencies is studied,and the influence of closed crack in different directions on the propagation direction of mixed scattering wave is analyzed.Based on the above research,the experimental study on the noncollinear wave(shear wave)mixing test of closed crack in block metal structure is carried out.(2)Study on nonlinear lamb wave detection method of closed crack in metal plate.Based on the nonlinear spring model and the Hertzian contact theory,the second harmonic and mixed nonlinear lamb wave theory of the plate are studied.The relationship between the second harmonic and the mixing nonlinearity and the closed crack size is established.The influence of closed crack length and width on the second harmonic and mixing nonlinearity of Lamb wave is studied by finite element simulation.The results show that the nonlinear effect is changed with the change of the crack size.Based on the above research,the experimental study on the nonlinear Lamb wave detection of closed crack in the plate is carried out.The results show that the method can realize the fatigue crack detection in the plate well.(3)Time-frequency analysis of nonlinear lamb wave signals.Aiming at the problem of weak nonlinear effect in nonlinear Lamb wave detection,four time-frequency analysis techniques,such as short-time five transform,continuous wavelet transform,Wegener’s transformation and nonlinear frequency modulation wavelet transform,are applied to nonlinear Lamb wave signal analysis.The results show that the nonlinear frequency modulation wavelet transform is more suitable for the characterization of weak nonlinear effects in nonlinear Lamb wave signals compared with the other three methods.