Development of Novel Coatings for Full Field Strain Measurements

Reflection Photoelastic Stress Analysis (RPSA) is an experimental technique used in industries to assist in a variety of design and manufacture processes. Users can benefit from immediate qualitative and quantitative information, for example, about maximum shear strains in complex geometries. One such user of RPSA, and the focus of this research, is Airbus that perform RPSA within the manufacturing process of their aircraft wing assemblies. Although insightful, RPSA procedures suffer from complicated application processes, a need for skilled engineers and large timescales for data collection. These issues all stem from one vital component — the photoelastic coating material. Common commercially available RPSA coatings are designed in such a way to compensate for failings in RPSA apparatus that were once insensitive to small changes in component surface strain. However, current commercially available automated photoelastic collection and analysis apparatus have given the potential for RPSA to be a truly effective method. This is due to high detection sensitivity of better than 0.1 nm that translates to a 20 micro strain resolution. RPSA techniques could benefit and compensate for using easy to apply, thinner (approximately 50 microns) but less sensitive coatings. As a result, this research aims to investigate and develop a novel, time-efficient and `smart' coating material that updates current RPSA methods. Whilst applications at Airbus are the focus of this research the wider applicability of this coating is extensive. Within this thesis, the key criteria for the new coating are outlined and explained by reviewing relevant theory, literature and all conducted experimental research. Coating development is divided into two main investigative topics: reflection photoelasticity (RPE) and mechanoluminescence (ML).

The RPE development topic utilised a range of UV curable resins that were rationally designed into an optimum formulation. The coating was designed and tested with the automated RPE imaging device — the Grey Field Polariscope (GFP). The choice of UV curable materials provides a new range of rapid application coatings boasting a 20 s cure time whilst possessing similar RPE sensitivities of existing commercial products. Additionally, the coating was designed with thickness correction capabilities within post-processing to further promote the ease of use of the coating. By introducing a red tint to the formulation and imaging with the GFP, it was shown that thickness differences could be corrected in order to provide accurate RPSA data. At targetted thicknesses of 50 microns, correction by manipulation of different wavelengths of light has not been attempted before. Performing calibration experiments, the clear (WF31) and red tinted (WF-Red1) UV curable materials possessed strain-optic coefficients indicative of medium-high sensitivity coatings (K = 0.8 - 0.1).

The ML topic investigated the introduction of a mechanoluminescent phosphor to the developed UV curable formulation to produce a novel dual-technique strain imaging coating. ML materials generate light under the influence of strain with the intensity able to provide a map of the surface's strain field. The objective was to capture the production of ML light whilst performing RPE simultaneously in order to provide additional information that would support both techniques. The commonly researched ML material strontium aluminate was utilised and was added at a 30 % weight percentage to the optimised UV curable formulation. It was demonstrated for the first time that concurrent measurement of both techniques was possible by filtering the green ML light against RPE performed monochromatically using red light. Qualitative comparisons demonstrated it utility, however challenges utilising ML within elastic conditions limits it use. Firstly, it was observed that the ML coating had a `one-shot' application with the intensity of produced light diminishing with subsequent loads. This was a result of the ML phosphor debonding with the carrier resin. Additionally, the light production within elastic conditions was poor requiring high strain rates to produce observable conditions. As a result, eliminating all ambient light is a prerequisite for successful application. However, ML demonstrated relativity high light intensities when at areas of high strain, singularities, and when the component breached its elastic limit. Therefore, it has been shown that ML's potentially usefulness could be a validation technique to go alongside photoelasticity.

The research has successfully demonstrated the high performance of a new reflection coating with the ability for application, data collection and processing, and coating removal to be performed in one single day — an order of magnitude better than existing techniques. Combination with ML materials for simultaneous RPE and ML capture has been demonstrated with the ability for RPE validation however not without difficulties. However, a novel technique/apparatus has been designed and implemented which allows for the first time PE and ML data to be measured simultaneously.