OS1-1-1 SPOTS standard for calibrating and evaluating strain measurement systems

Abstract

Over the past few decades, there has been a rapid growth in both innovative techniques and commercially available instruments for performing full-field strain measurements by optical means. The need for procedures that would allow the calibration of such systems and for methodologies for comparing their performances either against one another or a manufacturer's specification has been recognized for some time. This led to the formation of the international Technical Working Area (TWA26) on full-field optical methods of strain measurement under the auspices of VAMAS in the late 1990s. A little later a European consortium of research labs, instrument designers and manufacturers and end-users was formed and obtained funding to develop a prototype standard to address this need. In 2006 the consortium published a draft standard for the calibration and evaluation of optical systems for strain measurement. This paper will provide a brief overview of the draft standard including an explanation of the different approaches taken to calibration and evaluation. These are based on the requirement to form a continuous chain of comparison to a national standard for calibration and the demand to assess the most advanced features of the most sophisticated instrument in the case of evaluation. The manner in which these requirements drove the design philosophies employed in the reference material and standardized test material will also be described. An example will be used to illustrate the calibration of a system and quantification of the uncertainty and confidence levels associated with the data subsequently acquired using the calibrated instrument. The introduction of these "standard" methodologies represents a substantial step in the development of reliable instrumentation operating within well-defined limits.