The effect of experimental conditions on acousto-ultrasonic reproducibility

Acousto-ultrasonics (AU) is a nondestructive evaluation (NDE) technique that utilizes two ultrasonic transducers to interrogate the condition of a test specimen. The sending transducer introduces an ultrasonic pulse at a point on the surface of the specimen while a receiving transducer detects the signal after it has passed through the material. The aim of the method is to correlate certain parameters of the detected waveform to characteristics of the material between the two transducers. The waveform parameter of interest is the attenuation due to internal damping for which information is being garnered from the frequency domain. The parameters utilized to indirectly quantify the attenuation are the ultrasonic decay rate as well as various moments of the frequency power spectrum. Here, the sensitivity for each of the parameters was assessed in respect to changing boundary conditions during the experiments. Three conditions were controlled during the ultrasonic characterization of the specimens. First, issues concerning the contact force of the transducer were studied. Second, the support structure of the specimen was addressed. Lastly, the damage state of the material itself was considered. After analyzing the various AU parameters, the overall sensitivity of the AU technique to material change or damage were quantified and compared to changes in the AU values resulting from the experimental boundary conditions. This investigation showed that certain AU parameters could be utilized to gauge damage in composites, although, the experimental boundary conditions may slightly influence the results. The results of this study are important due to the fact that at this point AU is an empirical method.