The fracturing behavior of materials can be nondestructively monitored by the acoustic emission (AE) technique, using sensors that detect the transient elastic waves after any crack propagation event. In addition to the information relatively to the total activity and the location of the cracks, certain waveform features supply detailed information on the type of cracking. The waveform of the emitted AE signal depends on the relative motion of the crack sides and therefore, it carries information on the mode of cracks. Therefore, AE is used for classification of the active cracking mode. This enables characterization of the current fracturing condition within the material and warning before final failure. Tension-related cracks, which in most materials and loading conditions are nucleated first, emit signals with higher frequency content and shorter rising time than shear cracks. However, in most cases wave propagation from the crack to the sensor is attenuative and dispersive. This results in signal distortion which is enhanced by geometry restrictions and material or damage-induced inhomogeneity. This results in strong change of the waveform shape and the calculated AE parameters. This effect is stronger as the propagation distance increases rendering crack classification troublesome for structures where the separation distance between sensors is long. In the present study, fracture experiments were conducted in cementitious specimens in order to investigate the influence of distance on the AE parameters as measured by sensors at different distances from the source. Numerical simulations based on the finite difference method are also used to enlighten the problem and expand to different material conditions. This is one of the first studies of wave dispersion examined not from the classical ultrasonics point of view of phase velocity dependence on frequency but from the AE view, where specific waveform parameters are of interest. Experimental and numerical results show that the influence of the propagation path is crucial and should be taken into consideration for AE characterization of large structures, while it should not be neglected even in small-scale laboratory studies in order to improve crack characterization. (C) 2012 Elsevier Ltd. All rights reserved.
