Use of ultrasonics to follow crack growth

Cubes made of steel fiber reinforced concrete (SFRC) were progressively sawed, while recording the ultrasonic parameters between the saw-cuts. The waves were analyzed in terms of ultrasonic wave velocity, peak amplitude and wave energy. Different relative positions between transducers and notch were considered. It was found that a relation between the ultrasonic parameters and the notch length could be drawn according to the layout of the transducers: the energy and amplitude drop once the portion of material between both transducers is diminished by sawing, whilst the velocity only changes once this whole portion of material has been cut out. The findings were then used to follow the crack growth of a SFRC beam during a bending test. The propagation of ultrasonic compression waves through the samples was recorded at regular time intervals during the tests. The interpretation of the signals was in this case complicated due to the facts that micro-cracking occurred, that the macro-crack was much thinner than a saw-cut and that it was moreover bridged by fibres. It appeared that in this case both peak amplitude and energy dropped before the onset of a visible macro-crack, because of extensive micro-cracking, while the velocity only changed after considerable crack growth. (C) 1998 Elsevier Science B.V