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Steve D. Sharples, Wenqi Li, Richard Smith, Matt Clark and Mike Somekh

Orientation imaging using spatially resolved acoustic spectroscopy (SRAS). Steve D. Sharples, Wenqi Li, Richard Smith, Matt Clark and Mike Somekh Applied Optics Group, Electrical Systems & Optics Research Division Faculty of Engineering, University of Nottingham. AFPAC, January 2011.

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Steve D. Sharples, Wenqi Li, Richard Smith, Matt Clark and Mike Somekh

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  1. Orientation imaging using spatially resolved acoustic spectroscopy (SRAS) Steve D. Sharples, Wenqi Li, Richard Smith, Matt Clark and Mike Somekh Applied Optics Group, Electrical Systems & Optics Research Division Faculty of Engineering, University of Nottingham. AFPAC, January 2011

  2. What is SRAS? EBSD image courtesy of University of Wales, Swansea SRAS surface acoustic wave velocity image • Spatially Resolved Acoustic Spectroscopy • A Laser ultrasound technique for imaging microstructure** • Uses SAW velocity as contrast • varies with grain orientation • varies with SAW direction • Produces nice images of grains and tells us how they are orientated • Tells us all sorts of nice information about the sample microstructure

  3. f-SRAS: frequency spectrum SRAS • Excite with short (ns) laser pulses projected through optical grating. • The grating generates narrowband SAWs. Only one wavelength, λ (the grating period). • Detect the SAWs with a broadband optical detector. Measure the frequency on a scope. • Use v = fλto get the velocity • The patch under the grating is the patch which is measured

  4. f-SRAS: taking a velocity measurement

  5. A few nice pictures…

  6. Austenitic stainless steel weld L-R

  7. Austenitic stainless steel weld U-D

  8. Resolution: 25μm 700μm 10mm Resolution: 25μm Resolution: 400μm Example images showing the capabilities of SRAS:Scalability from large to small (titanium alloy) 84mm Resolution: 400μm ms-1 108μm

  9. What’s new since last AFPAC? • 1. Instrumentation • A dedicated SRAS microscope • Smaller, much faster, cheaper, simpler • Will have ability to scan on “rough surfaces” next month! • Higher spatial resolution • 2. Determination of orientation from SAW velocities • cubic crystals (e.g. nickel, aluminium)

  10. (1) 3rd generation SRAS instrument • New dedicated SRAS system funded by emda (East Midlands Development Agency). • Completion due April 2011. • Smaller, faster, more capable

  11. Example images from new instrument (1) • Ti-6Al-4V • 170x80mm • 25x250μm pixel size • 2.2 megapixels • 48 minutes scan time • >750 points/sec

  12. Example images from new instrument (2)

  13. (2) From “contrast” to orientation measurement • The velocity depends on the crystallographic orientation • Ok to go from orientation to velocity (forward) • Trickier to invert this problem So… • Solve the forward problem v=f( orientation ) • Fit the data to the forward problem to find the orientation

  14. Forward model: calculating SAW velocities from known orientation and known elastic constants Define propagation direction l1, l2 and velocities Define elastic constants, and multiply by rotation matrix l1, l2 = propagation direction  = density V = phase velocity C = stiffness tensors jk = lillcijkl d mn = determinant of |jk-jkv2| 3 = eigenvectors of displacement substitute into |jk-jkv2| = 0 choose the 3 lower half plane roots of l3 and its 3 plot the curve of |d mn |= |cm3klk(n)ll(n) | vs. velocities choose the minima of |d mn | to determine velocities calculate the out of plane displacement of velocities

  15. First the forward problem for cubic Nickel

  16. SAW velocity as a function of orientation: cubic crystal: Nickel

  17. Propagation in multiple directions – single crystal Ni Fit analytic curves to data to get orientation

  18. Getting the orientation… Analytically calculated velocity as a function of orientation + Measure velocity as a function of propagation direction on surface + Simple fitting algorithm = Orientation of the crystals

  19. Propagation in multiple directions – single crystal Ni

  20. Orientation imaging on nickel • Supposedly “single crystal” nickel, actually consists of two large grains SAW velocity left-right

  21. SRAS: Conclusions • SRAS is faster and fancier than ever before! • We got a nice new machine thanks to EMDA • It will have optically rough surface capability shortly • We can go from measurement to orientation Next: • More forward modelling • Slicker fitting • Strategies for speed vs information • Higher resolution

  22. Acknowledgements Steve Sharples Wenqi Li Richard Smith RCNDE EMDA RR Aeroengines EPSRC University of Wales (Swansea) For more information or if you have an interesting sample, please email: steve.sharples@nottingham.ac.uk

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