The use of ultrasonic longitudinal critically refracted (LCR) waves is one approach used for near surface material characterization. It has been shown to be sensitive to stress and, in general, less sensitive to the effects of the texture of the material. Although the LCR wave is increasingly widely applied, in experiments the factors that influence the formation of the LCR beam are seldom discussed. This paper reports a new numerical model used to investigate the transducers' parameters that can contribute to the directionality of the LCR wave and hence enable performance optimization when used for industrial applications. An orthogonal experimental method is used to study the sensitivity to the transducer parameters which influence the LCR wave beam characteristics. This method provides a design tool used to study and optimize multiple parameter experiments and it can identify which parameter or parameters are of most significance. The effects of incident angle, the aperture and the center frequency of the transducer were all studied. It is shown that the aperture of the transducer, the center frequency and the incident angle are the most important factors in controlling the directivity of the resulting LCR wave fields. The model was validated by comparision of data to those obtained with a finite element model. Experiments were also performed to confirm the numerical results. The model and experimental data provided improve understanding of the transducer selection and positioning in the optimization of LCR wave fields in experiments, which can be used to give signals which exhibit higher sensitivity for near-surface stress characterization.
Keywords: Directivity; Longitudinal critically refracted (LCR) wave; Models; Orthogonal experimental method; Residual stress.
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