Steve D. Sharples, Wenqi Li, Richard Smith, Matt Clark and Mike Somekh
Applied Optics Group, Electrical Systems & Optics Research
DivisionFaculty of Engineering, University of Nottingham.
AFPAC, January 2011
Orientation imaging using spatially resolved acoustic spectroscopy (SRAS)
What is SRAS?EBSD image courtesy of University of Wales, Swansea SRAS surface acoustic wave velocity image
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 = f λ to get the velocity
The patch under the grating is the patch which is measured
f-SRAS: taking a velocity measurement
A few nice pictures…
Austenitic stainless steel weld L-R
Austenitic stainless steel weld U-D
Example images showing the capabilities of SRAS:
Scalability from large to small (titanium alloy)
Resolution: 400μmResolution: 400μm
10mm
Resolution: 25μmResolution: 25μmResolution: 400μmResolution: 400μm
84mm 700μm
Resolution: 25μmResolution: 25μm
ms-1
108μm
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)
(1) 3rd generation SRAS instrument
New dedicated SRAS system funded by emda (East Midlands Development Agency).
Completion due April 2011.
Smaller, faster, more capable
Example images from new instrument (1)
Ti-6Al-4V
170x80mm
25x250μm pixel size
2.2 megapixels
48 minutes scan time
>750 points/sec
Example images from new instrument (2)
(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
Forward model: calculating SAW velocities from known orientation and known elastic constants
Define elastic constants, and multiply
by rotation matrix
Define propagation direction l1, l2 and
velocities
substitute into |jk-jkv2| = 0
choose the 3 lower half plane roots of l3 and its 3
plot the curve of |d mn |= |cm3klk(n)ll(n) | vs.
velocities
choose the minima of |d mn | to determine velocities
calculate the out of plane displacement of velocities
l1, l2 = propagation direction
= density
V = phase velocity
C = stiffness tensors
jk = lillcijkl
d mn = determinant of |jk-jkv2|
3 = eigenvectors of displacement
First the forward problem for cubic Nickel
SAW velocity as a function of orientation:cubic crystal: Nickel
Propagation in multiple directions – single crystal Ni
Fit analytic curves to data to get orientation
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
Propagation in multiple directions – single crystal Ni
Orientation imaging on nickel
Supposedly “single crystal” nickel, actually consists of two large grains
SAW velocity left-right
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
Acknowledgements
Steve SharplesWenqi LiRichard Smith
RCNDEEMDARR AeroenginesEPSRC
University of Wales (Swansea)
For more information or if you have an interesting sample, please email:[email protected]
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