With a Pulse of Light, Focused Ultrasound Spots Small Cracks
Dual lasers and a steel mirror help evoke nonlinear elastic response in water.
What you can’t see can hurt you — at least indirectly. Surface defects and cracks are among the causes of material failures in cars, aircraft and buildings, and they sometimes have a hand in the demise of biomedical and microelectronic devices. The challenge, which is the basis for an entire industry, has been in coming up with methods to evaluate materials nondestructively so that small problems can be spotted before they become big ones.
Using light and sound helps to sense microcracks in materials. A laser pulse strikes a stainless steel mirror, generating localized heating and a subsequent ultrasound pulse that strikes a glass plate. A second laser beam probes the surface deflection on the plate. In this scheme, the presence of a crack changes the deflection, resulting in detection. Images courtesy of Peter Hess, University of Heidelberg.
Now researchers Victor V. Kozhushko and Peter Hess of the University of Heidelberg in Germany have demonstrated that laser-induced focused ultrasound could be a useful technique in this quest. Hess, a physical chemistry professor, noted that the method requires only a laser pulse and a strongly absorbing metal surface of the right shape. “There is no piezoelectric material, acoustical lens, voltage or sophisticated electronics,” he said.
Kozhushko, a postdoctoral researcher, added that this approach offers better spatial and temporal resolution, enables the generation of higher strains in solids and extends the dynamic range as compared with conventional ultrasound methods. Moreover, he said, the propagation of the focused ultrasound energy involves a nonlinear elastic response of the water, which increases the high-frequency part of the spectrum and improves resolution.
The technique works because of the photoacoustic transformation of laser pulses at the interface between a metal and a liquid. The researchers fired 8-ns pulses from a B.M. Industries 1064-nm Nd:YAG laser. These pulses passed through a 1-mm-thick glass plate, traversed a water-filled chamber and hit a stainless steel mirror with a 14-mm radius that was tilted at 15°. In response, a thin layer of water at the interface overheated, and its transient thermal expansion mainly was responsible for the excitation of a pressure pulse that traveled through the water.
These maps of the magnitude of the transient amplitude as measured by an optical probe show how the response of the glass varies with location (a). When the angle of incidence of the ultrasound beam is optimized, the spatial resolution is 10 to 20 μm (b). The dark spots result from the focused ultrasound beam.
The shape of the mirror focused the waves onto a spot on the glass plate, which contained an isolated microcrack. Hess noted that the ultrasound pulses striking the crack did not pass through it; rather, they reflected off the crack’s walls or the free fracture surface. That reflection caused a change in the phase of the pulse, altering its transient profile.
“Optical methods allow us to register locally such changes of the transient profile,” Kozhushko said.
To do this, the researchers used a 532-nm probe beam from a Coherent Inc. Nd:YAG laser, focusing it onto an ∼5-μm spot size on the sample surface — the top of the glass. The beam deflecting off the surface was sensitive to mechanical discontinuities in the sample.
The investigators used this approach to probe the response of the sample at various points, moving the sample in 20-μm steps. They detected the microcrack over a 200-μm area around the crack line. The measured profiles carried information about the crack size and its penetration depth, enabling the researchers to localize and image the discontinuity.
They also noted that the technique provides spatial resolution comparable to that of a state-of-the-art acoustical microscope.
However, it offers potentially significant advantages compared with the pulse-echo microscope method, which is sensitive to surface defects alone. Because the focused beam traverses the solid, it interacts with — and can detect — bulk defects such as bubbles and voids.
Potential applications of the technique for nondestructive evaluation are being explored by the group.
“Our next point is the application of the method to silicon wafers with and without coatings,” Hess said.
Applied Physics Letters, Nov. 26, 2007, Vol. 91, 224107.
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