Difference between revisions of "SRAS"

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Spatially resolved acoustic spectroscopy (SRAS) is 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. Traditionally, the information provided by SRAS has been acquired by using diffraction techniques in electron microscopy.
  
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= Laser ultrasound =
  
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= SRAS: Spatially Resolved Acoustic Spectroscopy =
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In a SRAS measurement two lasers are used, one for the generation of acoustic waves and one for the subsequent detection of these waves. 
  
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We can use the [[OSAM|O-SAM]] to map the SAW velocity in one or more directions, if we use a technique we have called spatially resolved acoustic spectroscopy (SRAS). This allows us to image micro- and macro-structure, probe coating thicknesses, and potentially image residual surface stress and porosity. The lateral resolution achievable is currently of the order of 50µm, and the velocity resolution of the order of 1ms<sup>-1</sup>.
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An optical amplitude grating, illuminated by the generation laser, is imaged onto the specimen surface. The incident light is thermoelastically absorbed, creating surface acoustic waves, such as Rayleigh waves.  
  
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More information is available at the [[SRAS for materials characterisation]] wiki page.
 
  
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<!-- [http://www.ndt.net/article/laser-ut2008/papers/Sharples%20LU2008.pdf [1]] Steve D. Sharples, Matt Clark, Wenqi Li and Mike G. Somekh, "Rapid imaging of microstructure using spatially resolved acoustic spectroscopy," presented at the ''1st International Symposium on Laser Ultrasonics,'' Montréal, Canada, July 16-18, 2008.
 
  
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[http://papers.icultrasonics.org/1620_sharples.pdf [2]] Steve D.  Sharples, Matt Clark, Mike G.  Somekh, Elizabeth E.  Sackett, Lionel Germain and Martin A.  Bache, "Rapid grain orientation imaging using spatially resolved acoustic spectroscopy," presented at the ''International Congress on Ultrasonics,'' Vienna, April 9-13, 2007.
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= Microstructure imaging =
  
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[http://www.opticsinfobase.org/abstract.cfm?URI=oe-14-22-10435 [3]] S. D. Sharples, M. Clark, and M. G. Somekh, "Spatially resolved acoustic spectroscopy for fast noncontact imaging of material microstructure," ''Optics Express'' '''14''', 10435-10440, 2006.
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The velocity of acoustic waves in a material is a function of many essential
  
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[http://optics.eee.nottingham.ac.uk/optics/papers/Laser_Ultrasound/papers/2005_ieee_sds_sras.pdf [4]] S. D. Sharples, M. Clark and M. G. Somekh, "Fast noncontact imaging of material microstructure using local surface acoustic wave velocity mapping," in ''IEEE International Ultrasonics Symposium,'' '''1-4''', 886-889, 2005.
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= Orientation mapping =
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Having measured the SAW velocity in multiple directions the challenge is then to convert this information into the measurement of crystallographic orientation. The direct calculation of the orientation from velocity is a difficult problem. However, the numerical calculation of the SAW velocity as a function of SAW velocity is relatively simple, as first outlined by Farnell. Therefore, a database of possible slowness surfaces can be pre-calculated and compared to the measurement values.

Latest revision as of 09:44, 12 August 2021

Spatially resolved acoustic spectroscopy (SRAS) is 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. Traditionally, the information provided by SRAS has been acquired by using diffraction techniques in electron microscopy.

Laser ultrasound

In a SRAS measurement two lasers are used, one for the generation of acoustic waves and one for the subsequent detection of these waves.

An optical amplitude grating, illuminated by the generation laser, is imaged onto the specimen surface. The incident light is thermoelastically absorbed, creating surface acoustic waves, such as Rayleigh waves.


Microstructure imaging

The velocity of acoustic waves in a material is a function of many essential


Orientation mapping

Having measured the SAW velocity in multiple directions the challenge is then to convert this information into the measurement of crystallographic orientation. The direct calculation of the orientation from velocity is a difficult problem. However, the numerical calculation of the SAW velocity as a function of SAW velocity is relatively simple, as first outlined by Farnell. Therefore, a database of possible slowness surfaces can be pre-calculated and compared to the measurement values.

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