Abstract
The ability to monitor the viscosity of lubricating oils within metallic products is of interest to many industries, these being the automotive, aerospace and food industries to name a few. Acoustic mismatch at the metallic-liquid interface restricts ultrasonic signal transmission and so limits applicability and sensitivity of the technique. In this work, we propose the use of a continuously repeated chirp (CRC) shear wave to amplify the measurable acoustic response to liquid viscosity. The technique enables multiple reflections to superimpose inside the component and form a quasi-static standing wave whose amplitude spectrum depends on the condition at the solid-liquid boundary. Bare element shear ultrasonic transducers of 5 MHz resonant frequency were bonded to the lower surface of an aluminium plate in a pitch-catch arrangement to measure liquid in contact with the upper surface. Transducers were pulsed using a continuously repeated frequency sweep, from 0.5 to 9.5 MHz over 10 ms. The amplitude spectrum of the resulting standing wave was observed for a series of standard viscosity oils, which served as a calibration procedure, from which the standing wave reflection coefficient (S), was obtained. Measurements of 17 blended oils ranging in viscosity from 1080 to 6.7 mPa s were made. The technique was also evaluated with the addition of a polyimide matching layer (ML) between the metallic and liquid interface. Ultrasonic viscosity measurement values were then compared to measurements made using a conventional laboratory viscometer. The CRC method was found to significantly improve the sensitivity of viscosity measurement at a metal-liquid interface when compared to a single frequency burst with the benefit of low cost signal generation and acquisition hardware requirements. The CRC method is also capable of instant rapid response measurements as the signal responds in real time without the need to wait for a returning pulse.
Highlights
Ultrasonic methods offer a promising approach for industrial in-situ determination of viscosity made from the exterior surface of a component in real time
The Doppler viscometer uses the frequency shift of longitudinal ultrasonic pulses to measure the viscosity of a fluid [12,13] while longitudinal waves have been combined with shear wave modes to increase the proportion of shear ultrasonic energy incident upon the liquid [14]
The use of surface waves to increase the surface area exposed to the liquid, and physical additions to ultrasonic apparatus such as the development of wave guides, matching layers and Perspex wedges have all been developed for potential in-situ use [15,16,17]
Summary
Ultrasonic methods offer a promising approach for industrial in-situ determination of viscosity made from the exterior surface of a component in real time. Conventional ultrasonic techniques rely on pulsed ultrasound to investigate tribological properties such as lubricant film thickness [1,2], contact stress [3,4], surface wear [5,6], and lubricant properties [7,8,9]. Whilst techniques still use phase to measure the viscosity of liquid [11], viscosity can be correlated to amplitude [7] and frequency change. The Doppler viscometer uses the frequency shift of longitudinal ultrasonic pulses to measure the viscosity of a fluid [12,13] while longitudinal waves have been combined with shear wave modes to increase the proportion of shear ultrasonic energy incident upon the liquid [14]. The use of surface waves to increase the surface area exposed to the liquid, and physical additions to ultrasonic apparatus such as the development of wave guides, matching layers and Perspex wedges have all been developed for potential in-situ use [15,16,17]
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