Laser-based Apparatus for Measuring the Elastic Constants of Materials

The resonant ultrasonic spectroscopy (RUS) system is a useful device for measuring the elastic properties of materials. The system is based on the principle that the mechanical resonant response of a sample depends on the elastic moduli, density, and shape. Thus, by measuring the density and shape, the elastic moduli could be obtained by solving the inverse problem of relating the frequency response to elastic constants. RUS has many advantages over other ultrasonic techniques in that there is no need for corrections due to ultrasonic diffraction, the transducer properties play a secondary role and all the elastic constants are obtained with a single measurement. This technique is widely used in the fields of materials science, geophysics and nondestructive evaluation. Resonant mode excitation and detection using conventional piezoelectric transducers is well established. In this approach, a test sample is placed with a minimal contact force between the transmitting and the receiving transducers. A frequency generator is used to sweep the frequency of the excitation transducer and the receiving transducer detects the mechanical response. While it is a simple and cost effective approach, there are three key drawbacks to this approach. First, due to transducer contact, the vibrations are not fully free which causes signal attenuation and systematic error such as frequency shifts and peak splitting. Second, contact transduction is inadequate for application to soft materials or operation in hostile environments. Third, the detection of RUS using a contact transducer does not enable high resolution spatial imaging. However, this issue is currently being addressed by using a laser interferometer for ultrasonic detection. In these cases the spatial resolution is determined by the optical spot size of the laser probe beam at the sample surface. Recently, laser resonant ultrasonic spectroscopy (laser-RUS), a fully non-contact approach, has been studied as an improved tool for measuring the mechanical properties of materials. Laser-RUS entails thermo-elastically exciting resonant modes by irradiating a sample with a pulse laser. A second continuous wave laser interferometer is used to measure the out-of-plane surface motion associated with the vibrating sample. There are two distinguishing features of laser-RUS: 1) All of the resonant modes are excited simultaneously (broadband excitation), 2) Mechanical coupling to the sample is minimized (i.e. the sample must be supported with small acoustically mismatched pins). The non-contact nature of this approach has great utility for the in-situ evaluation of mechanical properties in harsh environments (e.g. high temperature, high radiation) and for measuring the elastic properties of a soft material. Though the signal-to-noise ratio of this system is low, it has many advantages by virtue of having no mechanical coupling. In this paper, a fully non-contact apparatus for measuring elastic constants based on the laser beams is fabricated. Here, a pulse laser beam is used to generate resonant laser ultrasound in a specimen and a confocal Fabry-Perot laser interferometer is used to measure the resonant surface vibrations of a specimen. The unstably varied gain of the confocal Fabry-Perot interferometer maintained a constant value within a limited gap by using a designed static stabilizer. This configured apparatus is adequate for measuring the elastic constants for a small sample. The measured signal and experimental result are described through experiments in this paper