Investigation of the nonlinearity of transducer acoustic couplants for nonlinear elastic measurements

The study of nonlinear elasticity of materials often relies on attaching contact transducers to a sample to achieve the required strain amplitudes. When measuring and quantifying the degree of nonlinearity, it is assumed that the nonlinearity of the bond between the material and the transducer is negligible. Here, we aim to characterize the nonlinearity of common acoustic couplants used in nonlinear measurements. Other factors are also reported since bond nonlinearity is not the only important property when choosing an acoustic couplant, i.e., amount of energy transfer, thermal properties, cure time, toxicity, and ease of application and/or removal. Various types of acoustic couplants including, epoxies, glues, thermal adhesives, and coupling agents were studied using Nonlinear Resonance Ultrasound Spectroscopy (NRUS), which is a measure of the hysteretic nonlinearity of a material. Measurements were made on cylindrical samples of acrylic, aluminum alloy 6061-T651, and Crab Orchard sandstone, chosen for their range of sound speeds, attenuation, and nonlinearity.