Photoelasticity: visualising strain in polymer films

(supervisor: Rudi Winter)

Nature of project: experimental, data analysis

Available to full-time physicists only.

Project description and methodology

Birefringent materials have different refractive indices in different directions. As a result, they produce colourful patterns when viewed through a pair of crossed polarisers. Birefringence can be induced or, if already present, enhanced by applying mechanical stress to the material. In this case, the birefringence patterns observed reflect the distribution of strains in the sample; the phenomenon is known as photoelasticity[1,2].

In this project, birefringent polymer thin films[3] will be applied to flexible transparent polymer substrates (acetate film). Controlled stresses will be applied to these samples using a clamping mechanism, and the resulting photoelasticity patterns will be recorded and analysed using image analysis software.

A successful project will develop beyond the above in one/some of the following directions:
Once the method is established and data have been obtained for a range of stress configurations, the project can be extended to polymer films with embedded nano-particles[4] in order to understand how the presence of the particles stabilises (or otherwise) the films. This is motivated by applications where polymer films are used as a matrix to support functional particles, e.g. in photovoltaic devices.

When considering where to take your project, please bear in mind the time available. It is preferable to do fewer things well than to try many and not get conclusive results on any of them. However, sometimes it is useful to have a couple of strands of investigation in parallel to work on in case delays occur.

Additional scope or challenge if taken as a Year-4 project: A Y4 student may wish to compare the experimental results with those obtained using finite element modelling using one of several available freeware packages

(e.g. in-browser SimScale).

Initial literature for students:

  1. A Ajovalasit et al.; Optics Lasers Eng 68 (2015) 58
  2. M Solaguren-Beascoa Fernandez et al.; J Strain Anal Eng Design 45 (2010) 1
  3. CW Frank et al.; Science 273 (1996) 912
  4. G Schmidt, MM Malwitz; Curr Op Coll Interf Sci 8 (2003) 103

Novelty, degree of difficulty and amount of assistance required

Photoelasticity has been known for a long time and is well studied. The challenge of this project lies in the sample preparation (thin films) and the observation of photoelasticity under changing loads.

Project milestones and deliverables (including timescale)

milestoneto be completed by
Identify suitable high-birefringence, soluble polymerend of November
Design plan for clamping mechanismChristmas
Image analysis procedure established and testedend of February
Comprehensive range of photoelasticity images taken and analysedEaster

Students taking this project will have to submit a full risk assessment form