Abstract :
Automatic, production-type, sonic inspection based on the measurement of the fundamental vibrating frequency and/or decrement of a mechanically shocked casting has been in operation in our foundries for several years. The method has been suitable for the detection of cracks, voids, porosity, mottle, shrink, etc., on many of the parts having simple geometries. However, the more complex castings can vibrate in many basic modes and these modes upon shock excitation may interact indistinguishably. Swept-frequency, continuous-wave, forced vibration obviates mode interaction and permits casting evaluation by comparison of the sonic energy absorption spectra. Center frequencies, bandwidths, and amplitudes of certain of the absorption peaks usually establish overall quality. The multichannel Swept Sonic Tester evaluates a given casting by determining the presence of a response within each of several preselected bands. These channels have very sharp limits which are established by active, tunable, twin-T networks. Each channel is self-contained permitting the system to be extended to accommodate any degree of casting complexity by adding channels in building block fashion. The spectrum sweep, having an adjustable sweep width, sweep rate, and starting frequency, is accomplished by heterodyning a crystal oscillator with a reactance-controlled frequency-swept oscillator and approximately follows the derived ideal frequency vs. time characteristic for conserving inspection time.
