The Interaction of Ultrasound with a Partially Contacting Solid-Solid Interface in the Low Frequency Regime

Abstract

INTRODUCTIONWhen real engineering surfaces touch, contact occurs between the asperities of the surface roughness. Forthis reason the true area of contact between components can be significantly less than the apparent contact area and the stresses at the asperities are considerably higher than the average (nominal) contact pressure. Measurement ofthe degree of contact between solids is important in a number of applications such as the design of contacting elements (e.g. gears and bearings) [1] and the detection of 'kissing' bonds [2].The proportion of the amplitude of the incident uhrasonie wave reflected at a solid-solid interface is dependent on, amongst other things, the degree of contact between the two surfaces. This is because where asperity contact occurs the ultrasonic wave is transmitted across the interface and where the asperities arenot in contact (i.e. an air gap exists) negligible energy is transmitted. When no Ioad is applied across the interface the percentage contact will be low and so the energy transmitted across the interface will be low. As the applied Ioad is increased, the transmission of uhrasound across the interface will increase as the percentage contact increases. The proportion ofthe amplitude ofthe uhrasonie wave which is reflected, termed the reflection coefficient, can therefore be used to interrogate a partially contacting interface and to extract some information about the degree of contact. This paper describes the use of low frequency longitudinal uhrasonie waves to interrogate the partially contacting interface between two rough aluminium specimens. Firstly, experimentswill be described which were performed to validate the use of spring models to describe the interaction of ultrasound with partially contacting solid-solid interfaces. Secondly, contact modelswill be described which allow the interfaceial stiffness of a solid-solid interface to be predicted. The predictions obtained using this model are then compared to experiment. MODELLING THE REFLECTION OF ULTRASOUND FROM SOLID-SOLID INTERFACES USING SPRING MODELSIfthe wavelength ofthe ultrasonic wave is comparable to the sizes ofthe air gaps in the plane ofthe interface then complex scattering phenomena occur [3] in which resonances are set up between neighbouring gaps. In this wavelength regime the precise shape of each air gap can significantly affect the scattered field and hence the proportions of energy reflected and transmitted. This is, therefore, not a useful regime in which to study the contact. Ifthe wavelength is increased further until it is large compared to the sizes ofthe air gaps then the proportions of the ultrasonic wave transmitted and reflected are no Ionger dependent on the exact shape and size of each air gap but on the stiffness, and to a small extent on the effective mass and damping ofthe interface. Ifthe sizes ofthe gaps are in the range 5-50!-lm then a wavelength of above 500!-lm is required to operate in this long wavelength region. This corresponds to a frequency ofbelow 13MHz in aluminium. The ...

On the Relationship Between Ultrasonic and Micro-Structural Properties of Imperfect Interfaces in Layered Solids

“…The interaction of ultrasonic waves with interfaces formed by two non-conforming, rough surfaces in contact has been the subject of numerous investigations [1][2][3][4][5][6][7][8][9][10]. The motivations behind these studies have been various: from the assessment of the real area of contact between two rough surfaces [I], to the modeling of crack closure near the tip of a fatigue crack [4]; from the identification of the nature of interfacial imperfections in kissing and partial bonds [6], to the generation of ultrasonic waves [8].…”

“…The motivations behind these studies have been various: from the assessment of the real area of contact between two rough surfaces [I], to the modeling of crack closure near the tip of a fatigue crack [4]; from the identification of the nature of interfacial imperfections in kissing and partial bonds [6], to the generation of ultrasonic waves [8]. In most of these studies, the characterization of the interfacial properties has been attempted by studying the reflection of longitudinal and shears waves at normal incidence.…”

Nondestructive Testing System to Assess Lack-Of-Bond in Brazed Generator Coils by Ultrasonic Retro-Reflection

“…Clearly, solid rubber coupling would be more desirable in these applications. Solid coupled ultrasonic transducers employ a soft solid, typically rubber, padding between the transducer and the test piece, which conforms to the surface contour [7][8][9]. However, special lowattenuation rubbers have inherently high stiffness requiring relatively thick rubber cushions to conform to the irregular surface therefore the transmission loss is still significant.…”

Ultrasonic Spectroscopy of two Parallel Imperfect Interfaces

“…Possible corrections are discussed in [18,19). The interfacial stiffness between two contracted rough surfaces has been measured ultrasonically by several authors [12,14,15]. Also, several studies have been performed to estimate it theoretically [18,19]).…”

“…When the interphasiallayer containing imperfections is thin and the imperfections are flat the interphasiallayer can be modeled as an infinitely thin interface with distributed springs to account for interfacial stiffness reduction due to imperfections [1]. Significant effort has been put into experimental and theoretical studies of ultrasonic wave interaction with imperfect interfaces [1]- [15] with the goal of characterizing interface imperfections by the ultrasonic signature. It has been shown that by measuring the frequency response of the coefficient of reflection from an interface with spring boundalY conditions one can determine the interfacial spring stiffness.…”