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

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 '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.