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The fundamental study on the ultrasonic characterization of non-Gaussian rough surface of a material is carried out. In this study, a theoretical relationship among ultrasonic reflection coefficient at normal incidence, root mean square roughness Rq, and skewness Rsk (which is the measure of symmetrical level of a height distribution) has been derived. In the derivation, a Gaussian curve fitting to the non-Gaussian height distribution of a rough surface is effectively performed so that the Kirchhoff theory of wave scattering from a rough surface can be utilized. In order to verify the validity of the derived relationship, air-coupled ultrasonic pulse-echo measurements with normal incident configuration at center frequency of 0.35 MHz have been conducted to a series of sandpapers having various Rq and Rsk values. From the measured ultrasonic reflection coefficient of each sandpaper, the Rq values of the sandpapers have been estimated from the derived relationship with known values of skewness of the sandpapers and then compared to those measured by a mechanical stylus profiler. It is found that the Rq values estimated by the ultrasonic method and the stylus profiler almost agree with each other. Thus, the validity of the derived theoretical relationship is demonstrated.

The feasibility of ultrasonic in-situ measurement of friction surface temperature has been examined. The ultrasonic thermometry that is a method providing nondestructive temperature measurements by ultrasound is applied to temperature measurements of a friction surface at which temperature rise occurs due to friction, and an attempt is made to demonstrate in-situ monitoring of transient variations in the friction surface temperature and temperature distribution beneath the surface. Those temperatures are quantitatively determined by a combined method consisting of ultrasonic pulse-echo measurements and a finite difference calculation for estimating one-dimensional temperature distributions along the direction of ultrasound propagation. To demonstrate the practical feasibility of the method, the ultrasonic pulse-echo measurements at 2 MHz are performed for an acrylic resin plate of 10 mm thickness whose single side is being heated by friction with a felted fabric plate. The temperature profile near friction surface and its transient variation are measured during the friction heating under different applied loads to the friction surface. It has been observed that the temperature at friction surface increases quickly and significantly just after the friction started, and the maximum temperature rise at the friction surface increases markedly with the applied load. Thus, in-situ measurement of friction surface temperature by the ultrasonic thermometry has successfully been demonstrated.

We propose a novel measurement method for directly measuring the lateral displacement of structures by using air-coupled ultrasound transducers. The normally-distributed far field of an ultrasound transducer in a lateral direction is taken advantage of for the purposes of measuring lateral displacement. The measurement system is composed of three flat-type air-coupled ultrasound transducers and a steel wire as a target. The ultrasound transducers are immobilized at a fixed point, whereas the steel wire is separately arranged on the opposite side. When the steel wire is displaced laterally, the lateral displacement is calculated utilizing the intensity ratio of the reflected ultrasound waves. The accuracy of the lateral displacement measurement is experimentally assessed to be within ± 0.5 mm and the feasibility of the measurement system is discussed. The results show that a developed displacement measurement method will be usable in the field of structural health monitoring due to its accuracy and wide measurement range through the use of several ultrasound transducers.