Abstract:

Two separate experiments conducted with concrete samples containing cracks illustrate that acoustical methods have promise in damage detection. Samples of concrete were 2.6 mx19 cmx12 cm and 1.7 mx20cmx20 cm for the first and the second experiments, correspondingly. Cracks were located near sample centers and had dimensions of sample sections. In one experiment, high-frequency (5.3-kHz) longitudinal or rotational waves were modulated by low frequency (3 to 200-Hz) flexural vibrations. The relative amplitude of the modulation signal was about -20 dB. When the crack was filled by water, the modulation amplitude decreased in amplitude by approximately 3--4 dB. This result is expected because fluid should diminish the nonlinearity on the crack contact. In an uncracked sample, no modulation should be observed. In the second experiment, small explosive sources (less than 100 kg/cm[sup 2]) were used to study the interaction of a large amplitude, broad-band frequency pulse, with a weak impulse from small explosive source. It was first observed that the weak pulse did not propagate through the crack. The weak signal passed through the crack when a stronger explosion pulse was simulteneously applied to the bulk, transiently closing the crack. The results are two of many experiments that indicate linear and nonlinear wave methods may be applied to characterize damage in concrete. [Work supported by INCAS, Nizhny Novgorod and by RFBR Grant 97-02-17524.]