Photoelasticity for opaque objects

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 between 0.3 and 1.5 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. This new unique stress visualisation technique might be considered as 'photoelesticity for opaque objects', although more accurate techniques will be used to measure the phase difference that arises between the polarized components of the illumination. 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 will provide the underpinning research to determine if measuring stress-induced birefringence at THz frequencies is feasible. The phenomenon has not been reported in the literature. Based on the fundamental measurements of the stress-optic coefficients, THz systems will be built to measure residual stress distributions and stresses produced by direct loading in ceramic and polymer materials. Non-spectroscopic imaging at THz frequencies is not well developed, enabling novel phase measurement techniques to be implemented with single point detectors and start-of-the-art line detectors. The project brings together research expertise in optical instrumentation for industrially relevant metrology and industrial collaborators with strong track records in innovation for high value manufacturing applications.

As a feasibility study, this project has the potential for significant scientific impact. If the objectives of the project are fully met, the existence of stress-induced birefringence for a range of ceramics and polymers at THz frequencies will have been shown and used to measure internal stress distributions, for the first time. Such measurements would then become widespread in modern experimental mechanics. The economic and societal beneficiaries would be pervasive, analogous to the use of polarimetry in the manufacture of optical systems (for research, healthcare, energy) or polarisation control in optical coherence tomography (for healthcare, engineering).

Beneficiaries of the research include:
UK industry through increased scientific knowledge that will enable new manufacturing processes and products that reduce waste and increase efficiency.
The industrial partners will gain from the research undertaken in this project and from the relationships form which will continue after this project is completed.
Academic research in the manufacturing, engineering, materials and physics communities from the cross-disciplinary research. Applications in healthcare and energy are feasible.