Measurement of absolute acoustic strain by non-contact technique

Some ultrasonic applications require non-contact techniques because the target material is not easily accessible. In such cases laser-based and air-coupled ultrasonic techniques play a major role but commonly significant transmission loss is known to occur especially at higher frequencies. Therefore, it becomes imperative to know the amount of absolute acoustic strain achieved for a given application. In this paper, we report on the use of laser-based techniques to measure absolute strain on the face of vibrating rods excited under various scenarios. These include contact and air-coupled excitation at frequencies at resonance, as well as a factor of 100 below and above the resonance. The limit of our out-of-plane displacement measurement appears to be about 5 nanometers. Strains as high as 10^-6 have been obtained. The paper will describe the details of the ultrasonic techniques and some of the applications. The data are compared to theoretical and simulated strain calculations.