Published paper Maurice Whelan Institute for Health and Consumer Protection, European Commission DG-JRC, Italy ABSTRACTThe design and testing of a reference material for the calibration of optical systems for strain measurement is described, together with the design and testing of a standardized test material that allows the evaluation and assessment of fitness for purpose of the most sophisticated optical system for strain measurement. A classification system for the steps in the measurement process is also proposed and allows the development of a unified approach to diagnostic testing of components or sub-systems in an optical system for strain measurement based on any optical technique. The results described arise from a European study known as SPOTS whose objectives were to begin to fill the gap caused by a lack of standards.

The advent of digital imaging technology and huge increases in data-processing power have led to enormous advances in optical techniques for evaluating strain fields. A standardised test material designed for evaluating the capabilities of the most sophisticated optical systems using these techniques is described. The standardised test material presents a significant challenge to optical techniques for full-field strain evaluation by employing complicated and reproducible strain fields for which analytical solutions are available. The design philosophy and process employed in developing the standardised test material are described and sample results are presented. The relationship between a standardised test material and reference material for calibration is discussed.

For sometime, reflection photoelasticity has been used in a qualitative manner as an aid to the placement of strain gauges in vibration tests on turbine and compressor blades. Often, the motivation for such tests is the validation of numerical models used in lifetime predictions. Strain gauges supply data at a small number of discrete points, whereas the photoelastic fringe patterns provide information over the whole field of view. Digital photoelasticity based on phaseshifting allows these fringe patterns to be processed into maps of surface strain data that can be used to verify the computational results. Digital photoelasticity has been developed over the last two decades utilising spectral or phase approaches sometimes combined with Fourier analysis; and in most reported applications only idealised case studies are considered. Recently, the application of digital photoelasticity in high frequency blade tests has been developed as a robust methodology that can be applied using standard equipment or using solid-state polariscopes. The paper includes descriptions of the methodology and exemplar results to demonstrate the efficacy of this advanced application of photoelasticity for full-field validations of numerical modelling.