Measurement of local contact stiffness using ultrasound

Friction in built-up structures has a nonlinear impact on the overall dynamics and is difficult to predict due to the different phenomena occurring at various length scales. A major challenge is observing and quantifying the interfacial behavior in-situ. One solution that has long been studied is the use of ultrasound to measure the interfacial contact stiffness. However, existing research requires complete immersion of the setup in water or permanently attaching the transducer to the test article using epoxy to ensure good, consistent coupling between the transducer and metal. Immersion would change the interface conditions whereas epoxy-based coupling would mean that an array of transducers is required to measure larger contact surfaces. This work proposes a cost-effective, roving-transducer method that uses a conformable delay-line to couple the transducer to the metal surface. The accuracy of the proposed method is tested against static measurements of contact stiffness from a universal testing machine. Furthermore, the contact stiffness across the interface of two bolted stainless-steel blocks is measured using the proposed approach.