Manufacturing with Light Phase 2: Photoelasticity for sub-surface stress measurements in structural ceramics coating systems

Lead Research Organisation: Heriot-Watt University
Department Name: Sch of Engineering and Physical Science


Birefringence is a difference in refractive index that occurs along different axes in a material. In some materials this effect is intrinsic due to the atomic structure. In other materials, artificial birefringence can be induced by a mechanical stress that produces anisotropies in the material. Polarized waves travel at different velocities through the stressed regions depending on their polarization direction. This phenomenon is exploited in the well-established technique of photoelasticity, in which a model of the component of interest is made in an optically transparent plastic material and placed between polarizing optics. The induced birefringence is directly proportional to the stress experienced at a given point: contours of constant difference in the principal stresses and contours of the principal stress direction appear as fringe patterns. The technique has played a fundamental role in experimental mechanics, design and manufacturing.

This project is concerned with measuring the stress-induced birefringence in materials that are opaque at visible wavelengths. We will use THz illumination up to 7.5 THz where some fraction is transmitted through a range of non-polar materials including ceramics, plastics and composites. Measuring the stress-induced birefringence will provide information on the internal stress distribution in real components that are opaque at visible wavelengths, removing the need to model it in transparent plastic as in photoelasticity. Measurement from the real components also enables direct validation of numerical models. These new techniques will enable in-process control during manufacturing applications and in-service quality assurance, for a range of materials where this is not currently available, enabling step changes in the manufacturing processes used and the components that can be produced.

Planned Impact

This 'Manufacturing with Light 2' project will have a transformative impact on the current manufacturing state-of-the-art, particularly in the aero-gas turbine business. It will lead to the development of a new measurement technology that will allow the quality of as-manufactured coatings to be assessed non-destructively. Adoption of a pro-active monitored service regime should allow the remnant life of coating systems to be measured on industrial components as part of in-service inspection. In particular the remnant lifetime of thermal barrier coatings can be assessed and measured. This would have a significant commercial impact on our prime OEM industrial sponsor, Rolls Royce. Further, success would lead to the development of new instrumentation to make such measurements possible in a commercial environment. Such instrumentation will be developed in collaboration with Renishaw, as a follow up to this programme.


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Description The aim of this project is to devise optical instrumentation using THz illumination that can measure sub-surface stress distributions with a spatial resolution that is useful for manufacturing applications. We will use this instrumentation to improve the performance of ceramic thermal barrier coatings through the development of better and more robust manufacturing methods and the development of through life models that permit the prediction of imminent ceramic thermal barrier failure. To date, we have:
• To devise optical instrumentation for sub-surface stress measurements with ~100 µm spatial resolution;
• To determine the stress-optic coefficient in-situ for a range of coating ceramics, and hence measure how the residual stresses introduced during their manufacture are affected by material choice, manufacture process and thermal history;
Exploitation Route Project partners Rolls-Royce use ceramics extensively in thermal barrier coatings (TBCs): coatings account for approximately 30% of the cost of a high-pressure blade in a turbine. They are particularly interested in this project for the potential to reduce variation in TBC properties during manufacture and determining TBC properties for aged conditions.

Project partners Renishaw's interests are very broad and cover turbine blade refurbishment (in which they already work extensively with Rolls Royce), ceramic dental inserts and additive manufacture. Renishaw is interested in the long-term potential to devise instrumentation to test the integrity of ceramic TBCs (and other ceramic components) and believe that such a development would produce a step-change in the manufacture and testing of ceramic coatings in a number of sectors. They note that "no inspection technique for TBCs has yet proved a viable commercial solution and that this could be the technology that makes it to the mainstream".
Sectors Aerospace, Defence and Marine,Energy,Manufacturing, including Industrial Biotechology,Transport

Description Renishaw 
Organisation Renishaw PLC
Country United Kingdom 
Sector Private 
PI Contribution EPSRC-funded research project.
Collaborator Contribution Specimens and staff time.
Impact See related projects.
Description Rolls Royce (Composites and Ceramics) 
Organisation Rolls Royce Group Plc
Country United Kingdom 
Sector Private 
PI Contribution We are testing thermal barrier coatings (TBCs) manufactured and supplied by Rolls Royce.
Collaborator Contribution Professor David Rickerby of Rolls Royce is manufacturing and supplying thermal barrier coatings (TBCs) to be tested. He is regularly attending review meetings.
Impact Publications.
Start Year 2015