Laser Photoacoustic Effect For Nondestructive Evaluation Using Surface Acoustic Wave

Since been discovered, surface acoustic wave has been studied extensively byscientists in both theory and application. All kinds of methods have been discovered forsurface acoustic wave generation and detection. This work is focus on the theory ofsurface acoustic wave generation and the accomplishment of surface acoustic waveexperiment.The physical mechanism of laser acoustic has been explained theoretically. Thesubstance is that the temperature of the sample would rise due to the interaction of thecarriers and the electromagnetic irradiation. The temperature rise can cause strain andstress, which ultimately leads to the production of acoustic wave. A model has been givenfor laser point source and line source of different laser power range using Green’s functionand superposition methods.The experiment was carried on aluminum and brass by using Q-switched Nd:YAGlaser, which was more convenient and flexible than traditional piezoelectric transducer.The frequency and frequency band of surface acoustic wave can be controlled byregulating the parameters of the pulse laser. The direction of the wave propagation can becontrolled by adjusting the location of laser line source. However, the traditionalpiezoelectric transducer cannot meet those demands while it requires contact with thesample. The non-contact optical fiber interferometer used in this experiment has manyadvantages such as simple, sensitive, high accuracy and high spatial resolution. It can beconsidered as the real point to point detection because its typical detection area is almostequal to cross section of single model fiber.This technique also has been applied on human molar and enamel successfully. It isfound that the surface acoustic wave phase velocities have been increased by irradiation of high power pulse laser. This can be explained by the effects of Nd:YAG laser of removingdental caries and melting and resolidifying the surface of human molar. The resultssuggest that it has potential applications in dentistry. Another experiment was carried onfive pieces of enamel from three different producing areas. The experiment showed thatthe differences of dispersion curves and average phase velocities of different producingareas are much greater than that of the same area. As a result, a discriminate can be madebetween them, which indicate that this technique has potential applications in provenanceidentification for enamel. However, further studies are needed.