Spatially resolved acoustic spectroscopy imaging (SRAS)


Measuring material microstructure


An exciting new ultrasonic technique known as Spatially Resolved Acoustic Spectroscopy (SRAS) has been developed at the University of Nottingham. This technique, which can replaced EBSD, can direct image the microstructure of materials ? for example titanium, steel and aluminium and be used to measure properties such as the grain orientation, grain size distribution and texture. It can also be used to measure and image coating thickness. The ability to map the material microstructure ? in effect to image the grains ? quickly and in a non-destructive manner is useful for non-destructive evaluation and process control.

Key Benefits

? Similar capabilities to Electron microscope (EBSD) but faster, cheaper and can be used in the field and on large samples.
? No difficult and expensive sample preparation.
? Non contact
? Validated 25 microns resolution with potential for 3 microns
? Measures grain structure and other structural properties of industrially-relevant materials e.g. metal alloys(titanium, nickel, aluminium and steel), ceramics, silicon, Si3N4
     o mean grain size;
     o degree of randomness of both grain and orientation;
     o crystallographic orientation.
? Measurement of film thickness.
? Can be used to reveal hidden layers.
? All the key data without cutting up your specimen!


Spectroscopic image obtained from dwell fatigue Titanium 685                    Spectroscopic image obtained from Aluminium

Market Sectors

? Aerospace, marine, power, rail and automotive
? Process development and process control for manufacturing - e.g. metals, forgings, photovoltaics and coatings.
? Diagnostics and NDE/T

Technical Information

Common engineering materials, such as steel and titanium, have a material microstructure consisting of many grains of random orientation. The orientation determines the velocity of surface acoustic waves (SAWs). As the acoustic source is moved, the waves pass through different grains and a different velocity is observed. These different observations form the basis of the SRAS technique where acoustic spectroscopy is used to determine the velocity of SAWs for a small region on a test sample. This measurement represents a one point on a corresponding velocity image.


  Contrast due to variation in thin film coating thickness, in this case an extra 30nm of gold buried under a uniform coating 500nm thick

SAW velocity does not only depend on grain orientation, it can also be used as a contrast mechanism to measure thin film coating thickness, or surface stress.

In the system developed, the SAWs are excited from a fixed frequency source ? a pulsed laser ? using a grating of regularly spaced lines. The fringe spacing of this grating is swept over a certain range, corresponding to the likely range of SAW wavelengths at the fixed frequency. The SAWs are then detected and analysed to determine the phase velocity at that point. To build up a velocity map for the whole test sample, it is raster scanned with respect to the excitation grating and the detection point. By plotting this velocity map, knowledge of the grain pattern within the sample is obtained.


Diagram illustrating how repeating the acoustic spectroscopy measurement above at different positions on the sample
allows a velocity map to be built up.

By propagating the SAW waves in two or more orthogonal directions it is possible to start and build up knowledge of crystallographic orientation of a material. With knowledge of the crystallographic structure of a material and how Rayleigh velocity varies with orientation it is possible to assess the likely crystallographic orientation of the grains. Propagating in more than two directions allows the measurements to be more precise.

IP Status

The University of Nottingham has filed a patent application, WO 2007/003952, which is in national phase and is actively seeking commercial and research partners to further develop this technology.




(EN) A sample (10) is measured by generating ultrasound at (12), for example by using a laser (22) and spatial light modulator (26). The ultrasound is detected at (16), for example by optical beam deflection techniques. A characteristic of the generation at (12) is swept across a range of values to vary the efficiency of generation of ultrasound. The value of the characteristic, which corresponds with the peak amplitude detected at (16), is identified to provide a measure of the acoustic velocity at the region (12). The method is executed at a plurality of sites (12, 20) to provide a set of spatially resolved measurements of the sample (10). This allows an image of the sample to be created.

Patent Information:
For Information, Contact:
George Rice
Commercial Manager, Engineering & Physical Sciences
The University of Nottingham
0115 82 32190
Matthew Clark
Steve Sharples
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