| Laser ultrasonic nondestructive testing(LASD)is one of the most popular technologies in the field of defect testing,which has the characteristics of high resolution,non-contact and non-damage.Due to the influence of stress,the surface crack of material usually has an oblique Angle,so the oblique crack on the surface of metal plate is studied and analyzed in this paper.In order to study the influence of the defect depth and inclination Angle of the inclined crack on the propagation path and time-frequency domain characteristics of the ultrasonic detection,this paper analyzes the ultrasonic detection from the aspects of numerical simulation and experiment.The specific content is as follows:1.The simulation model and laser line source model of oblique crack were established by ABAQUS software,and the mesh size and time step were determined.A laser ultrasonic detection system was built,and ultrasonic waves were received and detected by ultrasonic probe and interferometer respectively,which provided data support for later research.2.In order to study the influence of oblique crack on ultrasonic detection,numerical study was carried out,and it was found that the model boundary,the position of excitation point and receiving point,the depth of oblique crack and the Angle of oblique crack all had influence on the detection signal.The results show that,in addition to the direct reflected wave R100 and transmitted waveTR01 0,there are also longitudinal waves P101,reflected waveR02 1,R101 and transmitted waveTR011,which are affected by the model boundary.The position of the excitation point and the receiving point has little effect on the waveform characteristics.When the inclined Angle of the oblique crack is 90°and the depth of the oblique crack is around 0.1mm to 0.4mm,the peak values or peak-to-peak values of the longitudinal waves P10 1 and the reflected waveR00 1,R02 1andR101 of the plane sweep change obviously,while the peak values of the transmitted waveTR010 andTR011 can better represent the change of the defect depth from 0.1mm to 1.0mm.3.In order to verify the correctness of the simulation results,a probe and an interferometer were used to verify the variation of the peak and cutoff frequency of the oblique crack reflection wave.The experimental results show that the signal-to-noise ratio of the experimental waveforms collected by the probe is larger,and the reflection coefficient and cutoff frequency of the direct reflection of the two waveforms increase with the increase of the vertical crack depth,but the reflection coefficient of the experimental waveforms collected by the interferometer changes better when the depth is from 0.1mm to0.4mm.The experimental data show that the reflection coefficient of the direct reflection wave with acute Angle is obviously larger than that with obtuse Angle,which is basically consistent with the simulation results. |
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