Laser Ultrasonics for Non-destructive Evaluation of Metal Matrix Composites

Laser ultrasonics is based on the generation of elastic waves by transient surface heating or ablation with short laser pulses. These waves are traveling through the sample, are reflected and scattered at interfaces and the backside of the sample. To realize an ultrasound measurement device we have used either an interferometer coupled to a continuous wave laser (contact-free) or the sample was coupled to an optical prism for the detection of the elastic waves on the sample surface. In the prism we use the change of optical reflectance (acoustorefractive effect) at the interface between a glass prism and a liquid coupling layer near the critical angle of total internal reflection. The run-time of these waves in various directions of the samples can be measured contactless with a Fabry-Perot interferometer(point-source-point-receiver technique). From these data the 5 independent components of the complete elastic elasticity tensor for the unidirectional fiber reinforced composites can be determined. These results agree with measurements by a resonant beam technique (RBT), acoustic resonance and 4-point bending tests. The laser pulse technique has several advantages: it is contactless, non-destructive and it can be used for online quality control in production processes. Introduction: At low power levels the optical generation of ultrasound is based on the thermoelastic effect, which is the generation of elastic waves by transient surface heating with a laser pulse [1]. At high power level the laser source operates in the ablation regime by vaporizing a small amount of surface material. The radiated field of such a source resembles a monopole radiating strongly in all direction from the point source [2]. Laser ultrasonics uses one laser with a short pulse for the generation of elastic waves and another one, a long pulse or continuous laser, coupled to an optical interferometer for the detection of the ultrasound [3]. We have used for detection either a Fabry-Perot interferometer coupled to a continuous wave laser (contact-free) or the sample was coupled to an optical prism for the detection of the elastic waves on the sample surface (see Fig. 1). In the prism we use the change of optical reflectance (acoustorefractive effect) at the interface between a glass prism and a liquid coupling layer near the critical angle of total internal reflection [4].