Laser dynamic photoelasticity and digital image analysis for nondestructive evaluation of surface and near surface flaws through ultrasonic spectroscopy

摘要

The interaction of Rayleigh surface waves with artificial surface-breaking and near surface flaws in a half-space have been investigated. The work reported in this research substantiates the validity of ultrasonic spectroscopy as an effective tool in nondestructive evaluation of the depth of such flaws;Dynamic photoelasticity was used to obtain full-field visualization of ultrasonic waves propagating in birefringent materials and their interaction with flaws of interest. PSM-1, which is a tough polymer of very low stress fringe value, was chosen as the birefringent material for this research. Sufficient short exposure times were achieved with a high-power pulsed YAG laser which has a pulse duration of about 15 nsec. The output was coupled into an optical fiber which then illuminated the dynamic model. The results are photoelastic pictures with excellent fringe resolution and negligible smearing effect. The maximum fringe orders counted in the transmitted and reflected wave-trains for each surface and sub-surface slit were used to calculate their corresponding transmission and reflection coefficients. The obtained results were compared and agreed well with the available results in the literature;The Fast Fourier Transform of the constructed spatial waveforms was performed to yield the frequency spectrums of the wave-trains. It was observed that some of the variations in the spatial frequency spectrums, particularly the shift in the location of the peaks in these spectrums, were related to the changes in the size of defects. Characteristic plots which demonstrated this relationship, and related the peaks in the spatial frequency spectrums to defect sizes, were constructed. Both the transmitted and reflected wave-trains from the interaction of Rayleigh waves with the defects proved to be effective in the estimation of surface slit depths or the ligament sizes of the near surface slits;The optimal ranges for the detection of the depth of surface slits and the ligament sizes for near surface slits were determined with respect to the Rayleigh wavelength in different materials. In both cases, an estimation was made as to the maximum depth of a surface slit or a ligament size which could be detected and characterized using ultrasonic spectroscopy.