An important nondestructive evaluation (NDE) measurement technique is to record the vibration-mode spectrum of a material part or structure, excited by a random or a driven source (i.e., acoustic coupling or shaker table). The vibration-mode spectrum fully describes the part dimensions and material properties, including defects. Application of this technique has spawned a new NDE method known as resonant ultrasound spectroscopy (RUS) that has been found useful for characterizing components and structures of many diverse shapes and sizes.
At INEEL, a new optical method for implementing RUS analysis based on photorefractive frequency-domain processing has been developed. The method utilizes the photorefractive effect in bismuth silicon oxide for detection of the optical phase shift of an object beam scattered from a vibrating specimen surface. Flat narrow-bandwidth detection can be achieved at frequencies from the photorefractive response limit (~70 Hz) to the reciprocal of the photoinduced carrier recombination time (~10 MHz).
In this approach, all optical interference occurs inside the photorefractive crystal, resulting in an output beam whose intensity is proportional to the Bessel function of order one; therefore, linear for small amplitudes: 2π ξ/λ<Two modes of operation are possible: the power spectrum mode, for asynchronous vibration amplitude measurement, and the swept network mode, for synchronous vibration amplitude and phase measurement in analogy with common electrical spectral analysis methods. Very narrow-bandwidth detection (1 Hz) of the output signal intensity yields a minimum detectable amplitude of about 0.002 nm over the entire operating frequency range. Further improvements in sensitivity are possible and are being investigated. The figure shows the amplitude and phase measured on a vibrating surface operating around its fundamental resonance.
This method is applicable to full-field views of vibrating structures, thereby providing a significant improvement in sensitivity and speed over the conventional speckle interferometry method. An all-optical implementation, including the vibration excitation, is under development.