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The meaning of «sras»

SRAS (spatially resolved acoustic spectroscopy) a non-destructive acoustic microscopy microstructural-crystallographic characterization technique commonly used in the study of crystalline or polycrystalline materials. The technique can provide information about the structure and crystallographic orientation of the material.[1] Traditionally, the information provided by SRAS has been acquired by using diffraction techniques in electron microscopy - such as EBSD. The technique was patented in 2005, .mw-parser-output .citation{word-wrap:break-word}.mw-parser-output .citation:target{background-color:rgba(0,127,255,0.133)}EP patent 1910815 .

SRAS measures the surface acoustic wave velocity across a specimen, the surface acoustic wave (SAW) velocity is in turn a function of the material state, including parameters such as crystallographic orientation, elastic constants, temperature and stress.

In a SRAS measurement, as in most laser ultrasound techniques, two lasers are used, one for the generation of acoustic waves and one for the subsequent detection of these waves.  Considering first the generation of acoustic waves, an optical amplitude grating, illuminated by the a short pulse pump laser (typically ~1ns), is imaged onto the sample surface. The incident light is thermoelastically absorbed, creating surface acoustic waves, such as Rayleigh waves. As the laser pulse contains a broad range of frequencies, only the frequencies which match the grating spacing and acoustic velocity of that sample point will be generated. Using a second, continuous wave, laser these surface acoustic waves can then be measured through a number of interferometry techniques. Detection is usually achieved by optical beam deflection.

As Rayleigh waves are non-dispersive the phase velocity of the acoustic wave can be found by

v = f λ {\displaystyle v=f\lambda }

where λ {\displaystyle \lambda } is the distance between the grating fringes imaged onto the sample surface and f {\displaystyle f} is the dominant frequency of the wave packet, found by fast Fourier transform.

As the measurement probes the frequency of the wave packet, which does not change along the propagation length, the measured SAW velocity is determined by only the properties of the specimen at the area where the grating pattern is imaged, unlike more traditional time of flight measurements that are influenced by the sample properties along the propagation length. This makes SRAS robust and immune to the aberrating and scattering effects of the microstructure.

By raster scanning the sample, making measurements at several points across the surface, multi-megapixel images of the SAW velocity can be built up - providing rich microstructural maps. On samples with a good surface finish measurements can be made without averaging, allowing samples to be rapidly scanned. In-theory, means the acquisition rate is limited only by the repetition rate of the pump laser; modern laser repetition rates can exceed 10 kHz. As the measurements do not require a vacuum chamber or acoustic couplant there is little restriction, beyond the limit of scanning stages, to the size of sample which can be interrogated. The elastic anisotropy of most engineering materials means the acoustic response is a function of the loading direction. Hence, a unique velocity map exists for each propagation direction of the SAW direction. It is possible to combine multiple velocity maps to improve contrast between grains.

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