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
Abstract
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.
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.
Publications
Diederich G
(2016)
Stress measurements in YTZP ceramic using GHz and THz radiation
Isern L
(2021)
Non-destructive thickness measurement of thermal barrier coatings using terahertz radiation
in Emergent Materials
Schemmel P
(2017)
Monitoring stress changes in carbon fiber reinforced polymer composites with GHz radiation.
in Applied optics
Schemmel P
(2017)
Measurement of direct strain optic coefficient of YSZ thermal barrier coatings at GHz frequencies.
in Optics express
Waddie AJ
(2020)
Terahertz optical thickness and birefringence measurement for thermal barrier coating defect location.
in Optics express
Description | In this project, we devised optical instrumentation using THz illumination to measure sub-surface stress distributions with a spatial resolution that is useful for manufacturing applications. We used this instrumentation to explore the performance of ceramic thermal barrier coatings and the prediction of imminent coating failure. Specifically we measured optical thickness and birefringence immediately after manufacture and monitored its change through accelerated aging trials of thermal barrier coatings through to their failure (spalling). Variations in coating thickness during the manufacturing process were revealed. 'Hot-spots' in coating optical thickness and birefringence can apparently be related to positions of coating failure. These results are being prepared for publication. |
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 | This project measured the stress-induced birefringence and thickness of materials that are opaque at visible wavelengths. We concentrated on thermal barrier coatings (TBCs) which are applied to metal surfaces in gas turbine jet engines to enable the temperature of combustion chamber gas flows to exceed the melting point of the metallic superalloys used in the fabrication of the gas turbine. Higher combustion chamber operating temperatures improve the gas turbine efficiency and reduce greenhouse emissions. The non-destructive measurement of TBC material parameters, and their evolution with aging, is of critical importance to their deployment and in-service monitoring. We devised a full-field technique using THz illumination to determine the optical thickness and birefringence of yttria-stabilized zirconia (YSZ) thermal barrier coatings. The THz-measured optical thickness and its variation through the depth profile of the YSZ coating were shown to be in good agreement (<4%) with scanning electron microscope cross-sectional thickness measurements. The technique was successfully deployed on large surfaces, and across different coatings of the same material produced under the same deposition conditions and has the potential for in-line control of TBC manufacture. In addition, the position of discontinuities in both the optical thickness and birefringence were correlated to coating failure points observed during accelerated aging trials. |
Sector | Aerospace, Defence and Marine,Energy,Manufacturing, including Industrial Biotechology |
Impact Types | Economic |
Description | THz measurements for ceramic coatings |
Amount | £62,456 (GBP) |
Organisation | Renishaw PLC |
Sector | Private |
Country | United Kingdom |
Start | 01/2020 |
End | 09/2020 |
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 |