Photoelasticity for sub-surface stress measurements in structural ceramics and ceramic coating systems.

Lead Research Organisation: Cranfield University
Department Name: Sch of Aerospace, Transport & Manufact

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.

Publications

10 25 50
 
Description Teraherz radiation can be used to detect changes in the microstructure of EB-PVD thermal barrier coatings that results from high temperature thermal aging of such coating systems.
Exploitation Route These observations could be used as part of a structural health monitoring stratergy for TBC systems.
Sectors Aerospace, Defence and Marine,Manufacturing, including Industrial Biotechology

 
Description Discussions have commenced with Rolls-Royce plc over the use of Tera-Hetz radiation for the non-detructive inspection of Thermal Barrier Coatings. Some preliminary trials are planned.
First Year Of Impact 2019
Sector Aerospace, Defence and Marine,Transport
Impact Types Economic

 
Description Multi-layered CMAS Resistant Thermal Barrier Coatings
Amount £79,896 (GBP)
Funding ID STC 32729 
Organisation Rolls Royce Group Plc 
Sector Private
Country United Kingdom
Start 06/2018 
End 12/2018
 
Description Multi-layered CMAS Resistant Thermal Barrier Coatings (phase 2)
Amount £62,180 (GBP)
Funding ID STC 32872 
Organisation Rolls Royce Group Plc 
Sector Private
Country United Kingdom
Start 04/2019 
End 12/2019
 
Description Collaboration with Heriot Watt University 
Organisation Heriot-Watt University
Country United Kingdom 
Sector Academic/University 
PI Contribution Cranfield University's contribution to this collaborative programme 'Manufacturing with Light' is the manufacture of EB-PVD thermal barrier coatings and bond coat systems, their aging at temperature to produce part-life microstructures and the modelling of life duration,
Collaborator Contribution Heriot Watt's contribution is the analysis and characterisation of the TBC systems, using terahertz radiation to determinine if defects are generated during thermal aging and to measure the stress optic coefficient for these thermal barrier coatongs and how they change with high temperature thermal exposure.
Impact Research is still on-going.
Start Year 2016
 
Description Rolls Royce Plc 
Organisation Rolls Royce Group Plc
Country United Kingdom 
Sector Private 
PI Contribution Discussions with Rolls-Royce over the use of THz radiation to non-destructively measure thermal barrier coating thickness. Provision of reference samples to Heriot-Watt University to calibrate measurements.
Collaborator Contribution Rolls-Royce provided high pressure turbine blade samples to Heriot-Watt University. Cranfield University provided reference samples. Heriot-Watt University to measure thermal barrier coating thickness on commercial HP turbine blade samples, calibrated against thickness data provided by Cranfield University manufactured samples.
Impact Non that are published. Validation exercise in progress.
Start Year 2019