The average longitudinal size of ultrasonic speckles back-scattered from a rough interface is investigated theoretically and experimentally in this paper. When a rough interface is insonified by ultrasound pulses, the average longitudinal size of the speckles in the space is found to be related not to the spatial coherence but to the temporal coherence of the ultrasound waves on the interface. Further theoretical analysis shows that the average longitudinal size of speckles is dependent on the sound speed and the time duration of the ultrasound pulse incident on the interface. To verify this theoretical conclusion, experiments are performed with a special experimental set-up. The experimental results are in good agreement with the theoretical ones. IntroductionWhen a rough interface is lighted by a laser beam, spots with random intensity in the backscattering space are produced due to the interference of the scattered laser waves. These spots are called laser speckles, and they can be viewed as an information carrier of the characteristics of the interface. Since the 1970s, the statistics and movement of laser speckles have been intensively studied, [1]$[4]. As a result of these studies, the laser-speckle interferometry was successfully established and widely used in fundamental scientific research and engineering, [5] [6]. Similarly, when a rough interface is insonified by ultrasound, ultrasonic speckles are formed in the back-scattering space. Ultrasonic speckles have many useful features that laser speckles do not possess. For example, ultrasonic speckles can exist within a solid object, and are not easily affected by the environment, such as dust, frost, etc. Recently, a discussion of the ultrasonic speckle interferometry has been given in [7]; however, the method has not yet been fully established due to the lacking understanding of the basic properties of ultrasonic speckles. Nevertheless, it can be predicted that ultrasonic speckles interferometry will be increasingly used more effectively and conveniently than the laser speckle interferomtry. In order to achieve this goal, the basic statistical properties of ultrasonic speckles, such as the in average size in space, should be thoroughly investigated. In paper [8], we presented our research on the average transverse size of ultrasonic speckles, showing that it is similar to that of the laser speckles. It is dependent on the space coherence of the ultrasound waves. Further theoretical analysis and experiments showed that the average transverse size of the ultrasonic speckles is dependent on the ultrasound wavelength, the aperture and the focus length of the receiving probe.

A high temperature bolt stress monitor, based on the testing of the Transit Time of Longitudinal wave and Shear Wave (TTLSW for short) and on the calculation of the stress through the established stress vs TTLSW equation, has been developed and tested. Lithium Niobate was used for piezoelectric material, gold or silver films were used for coupling material and clamping together method was used to install the transducer. Time interval averaging, zero crossing detect and several other techniques were used to improve the accuracy of TTLSW measurement. Using this method within the range of 30-360'C, and 0-443MPa the accuracy of stress is better than 9.8MPa. Nous avons develop6 et 6ssaiye un moniteur 5 haute temperature de contrainte dans boulon. Son principe est la mesure du temps de transmission des ondes longitudinal et transversale (breivement TTLSW) et la calculation de contrainte par I'equation itablie entre contrainte et TTLSW. Niobate de Lithium est utilise comme material piezoelectrique, feuille de l'or ou de l'argent sont utilise' comme mate'rial couplant, et le transducteur est instalk par la method de "pressis en ensemble". Moyenne des lintervales du temps, dktection par le zero croissant et certains autres techniques sont utilisges pour am6liorer la precision measure du TTLSW. Dans la region de 30 a 360C, cette methode a une precision de contrainte meilleur que 9.8MPa.