Standardisation project for optical techniques of strain measurement

A set of recommended data formats for image data, numerical data, and processed data have been developed. The range of optical techniques available for assessing strain and stress is broad and the techniques are diverse in nature. Since neither stress nor strain are quantities that can be measured directly all techniques are based on measurements of deformation and, or displacement of materials. Consequently, a plethora of formats have been introduced for presenting data, which makes comparisons difficult or impossible. The adoption of the recommended data formats is expected to facilitate data exchange and comparison between techniques and between organisations on a pan-European scale.

The development of unified methodologies and standards gives rise to the opportunity for more rigorous validation of numerical results from simulations. A feasibility study has been conducted on the development of a unified methodology for the comparison of results from experimental and numerical strain / stress analysis. This will be of significant benefit in assessing the viability of designs and could have a major impact on the acceptability of optical techniques in the design process.

Reference materials have been designed and tested for the calibration of optical systems for strain measurement, in addition standardized test materials of the evaluation of optical systems and their sub-components have been developed. Methodologies for the use of both sets of materials have been developed.

The utility of the reference materials and standard guides has been demonstrated via a set of industrial case studies from the aerospace, automotive and electronics industries. This is being disseminated through the technical and scientific literature.

The results of the first round robin are available and have been reported in a special session at the 12th International Conference on Experimental Mechanics being held in Bari, Italy in August 2004. The second round robin has just been completed and the results will be disseminated in the engineering and scientific literature.

The diversity of optical techniques implies that the results can be dependent on the method used to acquire data. A set of draft standard guides has been generated for the following techniques: digital fringe pattern/image analysis, ESPI, grating interferometry, photoelasticity, image correlation, and thermoelasticity. The draft standard guides for photoelasticity and grating interferometry are based on existing guidelines and technical notes. They provide recommendations for best practice and optimised methodologies for the measurement of strain. They should lead to better quality of data being generated, so that engineering decisions are better informed. The target areas are all industries concerned with load-bearing components, from turbine blades to oilrigs.

Confidence in the reliability and accuracy of a measuring system is in part derived from the ability to demonstrate or calibrate its performance against norms or reference materials, which are founded on reliable, calibrated sources. The route through which the calibration of the reference material can be traced is not straightforward for strain, which is a relative change in length, and hence is not measured but derived from some other measurement e.g. birefringence and optical path length. Possible routes for trace ability for calibration of systems have been proposed to cover applications in all areas of engineering where strain levels are of interest. They are being promulgated on the project website.

Exploitable results

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