Research On The Key Technology Of Laser-generated Ultrasonic Nondestructive Testing System

Laser-generated ultrasonic nondestructive testing technology is developed in recent years, which is a new type of non-contact ultrasonic nondestructive testing technology and has become a kind of important means and technologies in nondestructive testing field. This thesis lucubrates the key technologies of the laser-generated ultrasonic nondestructive testing system, and the main contents include the following aspects:Firstly, we introduced the thermoelastical and ablation mechanism of laser ultrasound, and discussed the thermoelastical mechanism in details. Simplified the pulse laser as trapezoidal waveform, established the mathematical model of the thermoelastical ultrasonic, analyzed the temperature field, stress field and displacement field of the laser ultrasound, and obtained the ultrasonic stress pulse expression of samples with tightening surface and free surface. The results showed that the ultrasonic stress pulse had dual polarity and single polarity when the specimen with tightening surface and free surface, respectively. Then, we summarized the laser ultrasound optical detection method, including the non-interference detection and interference detection.Secondly, aiming at the de-nosing in laser ultrasonic nondestructive testing, we put forward wavelet transform to analyze and process the ultrasound information. We deduced the mathematical model of wavelet transform, established the wavelet de-noising algorithm model, analyzed the wavelet thresholding de-noising method and discussed the selection principle of wavelet function. Then the ultrasonic testing apparatus was constructed, we de-noised the collected ultrasonic signals by using the wavelet transform method. The results showed that the wavelet threshold de-noising could control ultrasonic signal noise. Combining the dbN wavelet function and the rigrsure wavelet soft threshold method to the ultrasonic de-noising, we could obtain better effect.Thirdly, in order to overcome the shortage of the traditional ultrasonic testing system, we developed and designed the laser ultrasound testing system software based on virtual instrument. We compiled the testing system interface under the LabVIEW software platform and build the sub VI to complete data acquisition, analysis, processing and storage. Especially in data processing module, it had powerful function which could realize digital filtering, wavelet de-noising, power spectrum analysis and peak extraction of the ultrasonic signal. The whole software interface is friendly, simple operation, easy to achieve and to expand function, which can satisfy the demand of material defect detection and localization.Finally, we carried out the experiments of the laser ultrasound testing system. First, the adaptive laser interferometer based on the photorefractive effects was introduced. Next, we studied the influence of different sample thickness to the ultrasonic longitudinal wave, the comparison the longitudinal wave and surface wave of the same sample, the influence of the defect location to the ultrasound and completed defect localization. The results showed that the time of the first ultrasonic longitudinal wave would prolong with the increase of the sample thickness; the longitudinal wave and surface wave generated from the same sample had different polarities; the defect location would affect the time of the reflection wave, moreover, the relative error of the defect localization was less than 3%, which could satisfy the actual detection requirement.