Abstract
A diffuse acoustic field has been increasingly used to infer temporal changes in the structures, such as early dislocations and microcracking. This study explores three different techniques to characterise acoustic field by using a single ultrasonic phased array. The first two techniques are proposed to measure spatial uniformity of wave field by examining differences in the integral of energy and the maximum energy respectively at multiple inspection locations. The third one is developed to evaluate the degree of phase coherence between propagating waves transmitted sequentially by two neighbouring array elements. The efficacy of these techniques are investigated by examining their metrics on simulations and well-known samples. The results suggest that two selected metrics can be used to quantitatively estimate the diffuse field start time as well as the field size by comparing their value with the idealised diffuse state (15% for the energy integral metric, ηarea and 1 for the phase coherence metric, ηphase) and identifying the convergence start point.
Highlights
A diffuse acoustic field in which multiply scattered waves propagate is guaranteed to be produced if all the boundaries or heterogeneities within the material volume are diffuse reflectors [1, 2]
The identification of a diffuse acoustic field in a specific structure is important for acoustic techniques based on the measurement of diffuse ultrasonic or seismic waves, such as nonlinear ultrasonic diffuse energy imaging (NUI) [7, 8] and acoustic emission (AE) testing [9, 10] used in structural health monitoring
The results demonstrate that the metrics, ηarea and ηphase, can be used to identify gate start time tr and window length T by comparing their values with the indicated diffuse state, in which the ηarea is close to 15% and the ηphase is approximate to 1
Summary
A diffuse acoustic field in which multiply scattered waves propagate is guaranteed to be produced if all the boundaries or heterogeneities within the material volume are diffuse reflectors [1, 2]. Note that the selection of transmitters will be discussed later and the window length, T, was thought to be 0.12 ms, the empirical value used in NUI as stated in [8, 11] Their averaged value (termed ηarea and ηmax) were plotted against gate start time, tr, in order to represent the level of diffusivity with corresponding wave energy at different times. Selection of the most representative wave sources to examine their relative phase coherence significantly determines the effectiveness of this method This is because two neighbouring transmitters in an array are preferred over two transmitters far away from each other in terms of the similarity of their received signals in the same time window in a coherently scattered field.
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