Numerical Study Of Laser Ultrasonic Detection Of Surface Cracks Of Cylindrical Parts

As an important load-bearing component in the industry,cylindrical parts are prone to surface fatigue cracks under the action of alternating stress loads.The timely and effective detection of surface cracks of cylindrical parts is particularly important for ensuring industrial safety production.Laser ultrasound,as a new type of non-destructive testing technology,can achieve high-efficiency and rapid testing by virtue of its non-contact excitation and reception of ultrasound.Based on the advantages of laser ultrasonic inspection technology,this paper applies it to the surface crack detection of cylindrical parts,and studies the detection methods suitable for uncoated and coated cylindrical surface cracks.In this paper,the finite element method is used to establish laser ultrasonic multi-physics model of the uncoated and nickel-coated aluminum cylindrical.Simulating the process of pulsed laser excitation of surface acoustic waves on the cylindrical surface and analyzing the law of propagation of surface acoustic waves on the uncoated and coated cylindrical surfaces.Studies have shown that: the surface acoustic wave will have geometric dispersion on the surface of the uncoated cylinder and the phase of the surface acoustic wave at different circumferential surface detection points will gradually change.When the surface acoustic wave propagates on the surface of the coated cylinder,coating dispersion and geometric dispersion will occur at the same time.The dispersion phenomenon in the time domain waveform is obvious and the time domain waveform has complex characteristics.The energy amplitude of the surface acoustic wave decays faster in the propagation process.In order to realize the crack detection of uncoated cylindrical surface and reduce the influence of cylindrical geometric dispersion,this paper adopts the method of scanning detection point.The mutation in amplitude of surface acoustic waves before and after scanning the surface cracks is used to confirm the location of the uncoated cylindrical surface cracks.It explores the reflection,transmission and mode conversion processes of surface acoustic waves near cracks,analyzes the propagation path of surface acoustic waves near surface cracks.Combining the surface acoustic wave time-domain waveform characteristics of the front and rear edges of surface cracks,a quantitative formula to characterize the depth and width of uncoated cylindrical surface cracks is proposed.The dispersion of coating makes the surface acoustic wave time-domain waveform characteristics more complex.In order to detect the surface crack of coating cylinder and reduce the impact of the coating dispersion,in this paper,wavelet time-frequency analysis is used to extract the wavelet coefficient time-domain amplitude map of the frequency corresponding to the maximum value to determine the approximate area of the surface crack of the cylinder,and the specific location of the surface crack is further detected by the method of scanning detection point.On this basis,it performs wavelet packet analysis on the extracted front and back edge time domain data of surface cracks.Studies shows that: the surface acoustic wave maximum amplitude difference in the time-domain waveforms at the detection points of the front and back edges of the surface crack has a certain linear relationship when the surface crack depth varies between 0 and 1.0mm.On the detection point of the rear crack,the energy in frequency band of 0-2.6MHz of the surface acoustic wave is more sensitive to the change of crack depth.With the increase of the surface crack depth,the energy proportion in frequency band of 0-1.3MHz of the surface acoustic wave increases,while the energy proportion in frequency band of 1.3-2.6MHz decreases.When the surface crack depth is greater than 0.6mm,the energy proportion of frequency band of 0-2.6MHz is gradually stable.