Nondestructive testing of subsurface defects measurement by photothermal tomographic imaging

Abstract

ABSTRACT Photothermal tomographic imaging is investigated in this paper. A new reconstruction model is proposedaccording to the thermal wave physics and our experiment setup. It was observed that the position and shapeof the subsurface defects are more accurately reproduced in this method of reconstruction. Defects overlappedin depth direction is also resolved. 1. INTRODUCTION Thermal wave image has emerged as a useful technique for nondestructive evalution of subsuface fea- tures in opaque solids. This method has advantages ,especiaily in near-surface defects detection within variousdepth range ,compared with conventional techniques such as ultrasonic and X-ray inspection. In thermal waveimaging a focused laser beam of energy , continuously-modulated or pulsed at a certain frequency scans acrossthe sample which is served as a pump. The resulting periodic heat flow in the material is a diffusive process, producing periodic temperature distribution which is called as thermal wave. These waves will be reflectedand scattered by the features beneath the surface that have different thermal characteristics from their sur-roundings. The signal detected due to these processes caused by the subsurface boundaries is a function ofsuch parameters as thermal conductivity ,thermal diffusivity,surface optical obsorption and the distance be-tween the pump point and detecting point as well as the modulation frequency. Proper theory are applied toextract information about the patameters mentioned above. Several methods have been used to measure the