Instrument for the automated ultrasonic time-of-flight measurement - a tool for materials characterization

The propagation velocities of ultrasonic waves are functions of the density, the elastic constants and the strain-state of the material under test. Techniques are developed to use ultrasonic velocity or time-of-flight measurements in order to determine elastic properties of materials and hence to characterize materials behavior. A prototype is built which enables the automated time-of-flight measurement. In order to minimize the influences of amplitude variations, the system determines the zero crossings of the RF-signals and generates the start and stop signals for the counter. The programmable high performance counter has a temperature stabilized 500 MHz time base and thus a single shot resolution of Plus,Minus 2 ns. Usual repetition rates of ultrasonic pulsers enable a time-of-flight resolution of some hundred picoseconds by simple averaging procedures. The measured times-of-flight are read into the personal computer by an IEEE 488 bus with a transfer rate of 30 measured data per se cond. All operational functions are completely controlled by the computer system. Besides the time-of-flight, the coordinates of the measuring point and the temperature are also taken. The software, so far implemented in the prototype enables the evaluation of surface and bulk stress states. In order to evaluate the stress state, the particular evaluation equations are taken from the storage; the processor proves whether the appropriate data are measured and performs the evaluations. If necessary, the influence of temperature on the measuring quantities is corrected, using stored temperature coefficients. The measured data, and also the results can be displayed on the graphic-screen and on the printer, and they are stored on disks. The system is housed in two dustproff, portable boxes