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
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
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)
84mm
10mm
Resolution: 400μm
Resolution: 400μm
Resolution: 25μm
ms-1
108μm
700μm
Resolution: 25μm
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-jkv2| = 0
l1, l2 = propagation
direction
 = density
V = phase velocity
choose the 3 lower half plane roots of l3 and its 3
C = stiffness
tensors
plot the curve of |d mn |= |cm3klk(n)ll(n) | vs. velocities
jk = lillcijkl
choose the minima of |d mn | to determine velocities
d mn = determinant
of |jk-jkv2|
calculate the out of plane displacement of velocities
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 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