Papers by Author: Erwin Hack

Abstract: The term damage can be generally defined as a change introduced into a system that affects its performance. Its identification and characterization is a valid help in deciding amongst continuing the operation or performing a repair or replacement of the system. A valid support to this decision is based on the use of well-known measurement techniques from Non-Destructive Testing and Evaluation (NDT&E). A well-established correlation between damage and features extracted from the measured data makes these techniques capable of providing information about the extent of the damage. However the prediction of the remaining useful life of a system by comparing full-filed measurements techniques and FEM analysis results is the challenge of an increasing number of research studies. The need of a guide for enumerating the extent of the damage has been the thrust to perform this work. A common methodology developed for both numerical and experimental studies will be presented. It consists of three main parts: proper selection of the parameter capable of describing the damage in a quantitative manner; several approaches to obtain results from measurement techniques and FEM analysis; and damage assessment making use of a quantitative comparison of FEM results only, full-field experimental results only, or comparison of FEM to experimental results. A different approach in damage assessment will be also presented making use of Zernike moment descriptors from which the severity of the damage is inferred. An example to illustrate the methodology will be shown.

Abstract: A reference material is defined as material, sufficiently homogeneous and stable with respect to one or more specified properties, which has been established to be fit for its intended use in a measurement process. Reference materials provide a simple definition of the measured quantity that can be traced to an international standard and can be used to assess the uncertainty associated with a measurement system. Previous work established a reference material and procedure for calibrating full-field optical systems suitable for measuring static, in-plane strain distributions. Efforts are now underway to extend this work to the calibration of systems capable of measuring three-dimensional deformation fields induced by dynamic loading. The important attributes for a dynamic reference material have been identified in a systematic and rational fashion, which have been subsequently translated into a generic design specification. Initial prototypes of candidate designs have been produced and evaluated using experimental modal analysis and digital speckle interferometry, and the results have been compared with finite element analyses. Based on the outcome of this initial evaluation, further refinements in design and manufacturing are proposed.

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.