A computer code has been written to calculate realistic ultrasonic fields in solids. The program calculates the field due to a transducer coupled to the solid through either a fluid medium such as water or a solid medium such as a plastic shoe with a thin layer of fluid couplant. In this technique the transducer face is divided up into many small areas, each of which is assumed to be a source of spherical waves. A ray is traced from each source point, through the coupling medium and into the solid to the requested field point. The vector sum of the fields of each of the rays is then calculated to find the total incident field at that point for a particular frequency. The calculations have been compared to the fields predicted by the models of Thompson and Gray [R. B. Thompson and T. A. Gray, “Analytic Diffraction Corrections to Ultrasonic Scattering Measurements,” in Review of Progress in QNDE 2, edited by D. O. Thompson and D. E. Chimenti (Plenum, New York, 1983), pp. 567–586] and Thompson and Lopes [R. B. Thompson and E. F. Lopes, “The Effects of Focusing and Refraction on Gaussian Ultrasonic Beams,” J. Nondestruct. Eval. 4, 107–123 (1984)] and with experimental results. [Work supported by the U. S. Department of Energy, Office of Energy Research, Office of Basic Energy Sciences, under DOE Contract No. DE-AC07-76ID01570.]
An eight-channel, data-acquisition system is used to acquire and analyze acoustic-emission [AE] data from aluminum surface-crack specimens . The system is calibrated using known source locations and laser-generated ultrasound to determine the transducer locations by finding the arrival time of the longitudinal wave and then doing a nonlinear, least-squares fit . From these transducer locations, the origin of AE sources can be determined using a similar procedure. Automated methods for determining source location by finding the first signal above noise on each channel and identifying this signal as the longitudinal wave arrival are developed for processing the vast amount of data generated during a typical experiment. The application of these methods to data acquired during tensile testing is discussed.A goal of this research is to sense acoustic events to enable the prediction of conditions for initiation of crack growth . The predictions will be based on models being developed by Parks and McClintock [1] that calculate the effects of specific crack growth conditions. These models require detailed information about the location of the crack initiation sites and about the types of crack growth for various material properties and geometrie eonditions. The development and verifieation of these models are closely tied to fracture mechanics experiments being performed at the Idaho National Engineering Laboratory (INEL) . The experimental pro gram will point the direction for development of the models and can then be used to confirm the predictions using the source location and source identification methods described in this paper. The source location teIls where to conduct metallographie examinations so the extent of cracking can be identified. Then, the actual conditions for the real materials around the crack border can be compared to predicted and assumed conditions in the model. DIGITAL AE WORKSTATIONThe current AE detection system is an expanded and faster version of the system described in Reference 3. As currently configured, the system can digitally acquire signals on up to eight channels. Once armed, the system constantly digitizes the output of all active channels until the receipt of a "stop" trigger, generated when the signal level in a reference ehannel crosses a predetermined threshold. After receipt of the stop trigger, a specified number of pre-and post-trigger sampIes are 1153
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