Recent issues on development of reference materials and standardized tests of optical methods of strain measurement

The need for standards in optical methods of strain measurement has been discussed previously and attention has switched to the creation of reference materials and standardised tests. Reference materials provide a means of calibrating a measurement system by comparison to a standard that is traceable to an international standard. In this way an unbroken chain of comparisons between the measurement system and the international standard with defined uncertainties in each comparison is created. A standardised test allows the performance of the measurement system to be assessed against a number of known quantities and such tests should be as challenging as the applications for which the measurement system has been designed. The preliminary design of a reference material for optical techniques of strain measurement are presented. Results obtained from the tests of these physical reference materials using digital image correlation, ESPI, grating (moire) interferometry, photoelasticity, strain gauges and thermoelasticity support the design hypothesis and have aided the refinement of the design. The first set of results produced with the new design showed remarkable correlation despite being obtained independently in four different laboratories in four different countries using six different techniques. Initial designs for a set of standard tests have also been created and some preliminary results will be presented. The concept of virtual standardised test materials has been introduced to allow the performance of the algorithms within a measurement system to be assessed so that a standard and comprehensive diagnostic and evaluation framework will be available to system designers, manufacturers and end-users.