Traceability of Optical Techniques for Strain Measurement

Abstract

Strain is a dimensionless quantity derived from displacement. To measure strain, devices such as resistive strain gauges and extensometers but also imaging methods are applied in engineering and experimental mechanics. The lack of standards and reference materials related to optical methods of strain measurement is tackled by the SPOTS project. It is intended to make full use of the modularity of different methods in order to develop a unified approach to standardisation. In this contribution we focus on the problem of traceability of optical strain measurement values and underline the role of strain measurement standards. We clarify the terminology and outline routes for traceability in analogy to widespread procedures for single value strain measurements.

A Method for Calibrating a Digital Image Correlation System for Full-Field Strain Measurements during Large Deformations

“…A material measure is "a device intended to reproduce or supply, in a permanent manner, values of a given quantity" [22]; in this case, deformation values. The material measure facilitates a meaningful measurement calibration by being traceable back to a suitable standard level, i.e., the inaccuracy of the material measure is known.…”

Experimental Validation of Simulations Using Full-field Measurement Techniques

“…Similarly, for full-field techniques of strain, displacement or vibration measurement the need arises for an artefact that allows a full-field calibration of the quantity in question. The realization of such an artefact for static strain measurement was a main output of the European SPOTS project [4], and its use for calibrating an optical instrument is described elsewhere [5,6]. The artefact is based on a monolithic frame including a beam under four point bending.…”

Standard Test for the Evaluation of Optical Strain Measurement Systems

“…The standard metre appears to be the most appropriate standard for strain. The concepts of standardisation and traceability are discussed in more detail by Hack et al [2]. The ability to make comparisons along a chain implies the requirement for the measurement to be as simple as possible.…”

“…There is insufficient space here to include the derivation of the expressions so instead a brief description is included below. Function (1) is the classical Lame's equations [8] using a polar co-ordinate system and function (2) is the transformation of these into Cartesian co-ordinates expressed in terms of strain for a pair of cylinders with an interface radius and pressure of b and p respectively, the inner radius of the inner cylinder equal to a and the outer radius of the outer cylinder equal to c. Function (3) describes the light intensity, i in a circular polariscope using a six phase step approach [9] where I is the amplitude of the light emitted by the polariser, is Young's modulus and υ is Poisson's ratio. Function (4) is the solution of the phase-stepping equations in (2) where α is retardation and θ is angle.…”