Researchers at the University of Nottingham have developed SRAS, a new technique that evaluates the elasticity matrix of a material by measuring the speed of sound across the surface of the material.
The innovation, which is claimed to be a world first, uses high-frequency ultrasound to produce microscopic resolution images of the microstructure and maps the relationship between stresses and strains in the material (the elasticity matrix). By accurately measuring the speed of sound across the surface of these crystals, their orientation and the elasticity of the material can be revealed.
According to Nottingham University, this EPSRC-funded technology is beginning to be used in areas such as aerospace to understand the performance of new materials and manufacturing processes. The progress will also launch a new area of research as the technique is being used as an entirely new way to evaluate materials for improving safety in systems such as jet engine turbine blades, or in developing new designer alloys with tailor-made stiffness.
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In a statement, Paul Dryburgh, co-lead of the study in the Optics and Photonics Research Group at Nottingham University, said: “Many materials are made of small crystals. The shape and stiffness of these crystals are essential to the material’s performance. This means that if we try to pull on the material, such as a spring, its extensibility depends on the size, shape and orientation of each of these hundreds, thousands, or even millions of crystals.This complex behavior makes it impossible to estimate the inherent microscopic stiffness. This has been a problem for over 100 years because we didn’t have enough resources to measure this property.”
“The development of SRAS++ is a remarkable breakthrough as it is the first method to measure the elasticity matrix without knowing the distribution of crystals in the material,” added co-author, Professor Matt Clark, also of the Optics and Photonics Research group. “SRAS does not require demanding preparation of a single crystal; it is fast and offers unparalleled measurement accuracy. The speed of the technology is such that we estimate that we can repeat all historical elasticity measurements from the past 100 years in the next six months.”
Previously, the only way to measure the elasticity matrix was to cut the part or grow a single crystal of the material, a process that cannot be performed for many materials, including titanium alloys used in jet engines. Estimates are that less than 200 of the many thousands of materials have their elasticity measured. As a result, the elasticity of most industrial materials is unknown, with significant – and potentially dangerous – uncertainty in the performance of the material used.
With laser ultrasound, ultrasound can be created in a 200 µm area. By measuring the speed of sound across each crystal, the researchers can tell the shape of the crystals and the elasticity matrix of the material on a microscopic scale. Sound travels 10 times faster over the surface of metals than through air (at ~3000m/s).
The team’s findings are reported in a new paper entitled “Measurement of the single crystal elasticity matrix of polycrystalline materials‘, published in Acta Material†