Papers by Author: E.A. Patterson

Abstract: The paper describes the work towards a set of standardised tests for the evaluation of the performance of optical systems for measuring strain. In particular, the standardised tests are designed to allow the components of a system to be evaluated both in terms of hardware and software. The intention is to allow designers of optical systems to assess the efficacy of their innovations in instrumentation and algorithms, to assist manufacturers of systems in the quality assurance process, to permit end-users to evaluate system capabilities prior to purchase, and to support both manufacturers and end-users in setting up and fault diagnosis. Two standardised tests are proposed and described of which one is physical, a disc in contact with a half-space, and the other is virtual, a pair of shrinkage-fit thick rings or cylinders.

Abstract: There are no standard reference materials suitable for the calibration of full-field optical strain measurement systems. This is hindering the uptake of the technology by industrial end-users since optical metrology instrumentation and procedures cannot be easily integrated into quality assurance systems. The EU-funded SPOTS project is developing a physical reference material (PRM) and measurement protocol that should provide the basis of a calibration standard for establishing the traceability of strain values obtained with optical devices. This paper describes a PRM based on a parametric design of monolithic four-point bend test that can reliably generate a known strain field over a range of specimen sizes. Measurements acquired from strain gauges and LVDTs compared well with data obtained from ESPI, digital image correlation, photoelasticity and thermoelasticity studies, demonstrating excellent repeatability and inter-laboratory reproducibility.

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.

Abstract: The surface displacement fields of a fatigue precracked compact tension sample under tensile load were registered by electronic speckle pattern interferometry and image correlation. The in-plane elastic strain fields calculated from the displacement data were used to obtain the first stress invariant, which for the case of plane stress is proportional to the real part of the first complex potential in Muskhelishvili's approach. Solutions for the stress fields around the crack tip, KI and KII were sought in the form of the Fourier series using Muskhelishvili's complex stress functions. The Fourier series coefficients were calculated from the displacement data using multiple point overdeterministic method (MPODM). The nominal and inferred KI values differ by around 10%; this is probably due in part to mixed mode (KII) loading introduced by some degree of misalignment during the experiment.