Nondestructive Internal Defect Detection Using Photoacoustic and Self-Coupling Effect
A demand for non-contact and nondestructive internal defect detection is increasing. Using a photoacoustic effect, an internal defect could be detected by a low power semiconductor laser and an ultrasonic sensor. But the sensitivity of the ultrasonic sensor is low in detecting a photoacoustic signal. Then, an internal defect detection using a photoacoustic and self-coupling effect of semiconductor laser has been studied. To generate photoacoustic signal, a semiconductor laser with output power of 20mW is focused by a lens and is illuminated at 45 degrees on the sample. To detect photoacoustic signal by the self-coupled effect, another laser with output power of 10mW is illuminated at right angle. Sample defect can be detected by illuminating these laser lights at the same point on the sample. The self-coupling sensor can detect an internal defect with more sensitivity than the ultrasonic sensor does. An edge effect and a frequency characteristic are also observed. As a result, it was able to detect a very small defect of 0.07mm.