Abstract
A simple reference material for establishing the minimum measurement uncertainty of optical systems for measuring 3D surface displacement fields in deforming objects is described and its use demonstrated by employing 3D digital image correlation as an exemplar technique. The reference material consists of a stepped bar, whose dimensions can be scaled to suit the application, and that can be clamped rigidly at its thick end to create an idealized cantilever. The cantilever was excited at resonance to generate out-of-plane displacements and, in a separate experiment, loaded statically in-plane to provide in-plane displacement fields. The displacements were measured using 3D digital image correlation and compared to the predicted displacement fields derived from tip deflections obtained using a calibrated transducer that provided traceability to the national standard for length. The minimum measurement uncertainties were evaluated by comparing the measured and predicted displacement fields, taking account of the uncertainties in the input parameters for the predictions. It was found that the minimum measurement uncertainties were less than 3% for the Cartesian components of displacement present during static in-plane bending and less than 3 µm for out-of-plane displacements during dynamic loading. It was concluded that this reference material was more straightforward to use, more versatile and yielded comparable results relative to an earlier design.
Highlights
Most modern engineering analysis is based on computational models, and establishing conidence in and credibility of these models is essential
Since no independent values for the measurement uncertainty of digital image correlation (DIC) are available— it is the purpose of this work to obtain these—the calibration uncertainty for a measurand, mE is the combination of its residual standard deviation from the comparison to the reference material, u(d) from equation (12) and the corresponding uncertainty in the predicted displacements, u(mT ), which previously has been referred to as the uncertainty in the reference material, uRM [5, 6, 8, 9], such that umeas(m) = u2(d) + u2(mT )
The focus of this study is to assess the utility of a proposed design of reference material for establishing the measurement uncertainty of optical systems capable of measuring in-plane and out-of-plane deformations of components subject to static and dynamic loading. This process is known as calibration [7]
Summary
Most modern engineering analysis is based on computational models, and establishing conidence in and credibility of these models is essential. In-plane strain ields using camera-based optical instruments The design of this reference material featured a beam subject to four-point bending within a monolithic frame, and it has been used successfully to establish measurement uncertainties for electronic speckle pattern interferometer (ESPI) [8] and digital image correlation (DIC) systems [9]. When digital image correlation or any other optical measurement technique, such as ESPI or shearography, is to be used in industry within a regulatory environment, e.g. the aerospace or nuclear industries, it is usually important to perform the measurements within a standards framework in order to assist in establishing conidence in the measurements In this context, it is relevant to consider the traceability of the measurements, which is a component of a quality assurance system, and should allow an unbroken chain of comparisons or calibrations to an international reference. The results reported here demonstrate the suitability of the reference material for evaluating uncertainties in measurements of static in-plane displacements and dynamic out-of-plane displacements
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