Eliminating the effect of couplant on ultrasonic characterisation of solids: Experimental and theoretical investigations

Abstract

Up to now, in-situ monitoring of earthquake-resistant bearings for bridges or nuclear power plants has been limited to visual inspection or inspection of witness samples. In the context of exploring non-destructive measurement methodologies for such monitoring, ultrasonic techniques have emerged as a promising approach. However, ensuring optimal coupling between the transducer or waveguide and the material to be characterised is of crucial importance to guarantee the reliability of the measurement. Typically, the coupling agent employed acts as a liquid, which can exert a considerable effect upon measurement outcomes. The objective of this study is to examine the impact of the coupling agent on the assessment of the mechanical properties of viscoelastic solids. Polymer in rubbery state (polychloroprene) was characterised using the Surface Reflection Method with two different coupling agents (honey and a commercial coupling agent named SWC2). The results obtained in terms of storage and loss moduli exhibited considerable variability depending on the couplant under consideration. G′ values were found to be 258 MPa with honey and 287 MPa with SWC2 at 2 MHz. In addition, G″ values were determined to be 213 MPa with honey and 501 MPa with SWC2 at the same frequency. Consequently, an inverse method based on combining the results obtained using two different couplants was developed. This method was used to determine the storage and loss moduli of polychloroprene at high frequencies in a reliable way, which, to the authors' knowledge, does not exist in the literature: for the storage modulus, the estimated value is higher than those obtained previously (G’ = 339 ± 17 MPa), and the loss modulus lies between the two values aforementioned (G” = 312 ± 40 MPa).