Advances in Experimental Mechanics VIII

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Authors: Wolfgang Osten
Abstract: Optical metrology has shown to be a versatile tool for the solution of many measurement and inspection problems. The main advantages of optical methods are the non-contact nature, the non-destructive and areal working principle, the fast response, high sensitivity, resolution and accuracy. Consequently, optical principles are increasingly being considered in many areas where reliable data about the shape, the surface properties, the state of stress and the strength of the object under test have to be acquired. However, these advantages have to be paid with some serious disadvantages that are mainly connected with the poor features of identification problems. In this article several examples are presented where optical metrology is helpful for experimental mechanics. The presentation is mainly focused on two topics: the acquisition of quantitative data for experimental stress analysis and the solution of inspection problems by holographic non-destructive testing. Some current aspects such as modern approaches for the solution of identification problems, the installation of remote laboratories and the calibration of measurement set-ups by specially engineered calibration samples are discussed finally.
Authors: Jacob Notbohm, Jin Hong Kim, Anand Asthagiri, Guruswami Ravichandran
Abstract: With increasing understanding of the important role mechanics plays in cell behavior, the experimental technique of traction force microscopy has grown in popularity over the past decade. While researchers have assumed that cells on a flat substrate apply tractions in only two dimensions, a finite element simulation is discussed here that demonstrates how cells apply tractions in all three dimensions. Three dimensional traction force microscopy is then used to experimentally confirm the finite element results. Finally, the implications that the traction distributions of cell clusters have on the study of inhibition of proliferation due to cell contact and scattering of cells in a cluster are discussed.
Authors: Janice M. Dulieu-Barton
Abstract: Polymer composites are increasingly being used in high-end and military applications, mainly due to their excellent tailorability to specific loading scenarios and strength/stiffness to weight ratios. The overall purpose of the ongoing research at the University of Southampton is to develop an enhanced understanding of the behaviour of fibre reinforced polymer composites when subjected to a range of loading scenarios. The measurements lecture reviews progress in using the thermoelastic stress analysis technique as a tool that enables a better understanding of the behaviour of polymer composite single skin and sandwich structures.
Authors: Andrea Davighi, Richard L. Burguete, Mara Feligiotti, Erwin Hack, Simon James, Eann A Patterson, Thorsten Siebert, Maurice P. Whelan
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.
Authors: Wei Zhuo Wang, John E. Mottershead, Christopher M Sebastian, Eann A Patterson, Thorsten Siebert, Alexander Ihle, Andrea Pipino
Abstract: Recent advances in measurement techniques, including digital image correlation, automated photoelasticity, electronic speckle pattern interferometry and thermoelastic stress analysis, permit full-field maps of displacement or strain to be obtained easily. They provide large volumes of mostly redundant data, which should be condensed to the essential information to permit straightforward processes such as validations of computational models or damage assessments. A way to do this is by image processing, an important aspect of which is the definition of an orthogonal basis (orthogonal kernel functions). Generally, this is problem dependent and requires some skill from the analyst if the number of image features (the coefficients of the orthogonal basis) is to be restricted to a suitably small number. Advantage may be taken of patterns of symmetry, for example cyclically symmetric patterns are well-suited to treatment by Zernike polynomials and rectangular patterns are well-suited to treatment by Fourier series. The Zernike and Fourier kernels are continuous polynomials with orthogonality properties that require integration and must be discretised. The discrete Tchebichef polynomials are ideal for the treatment of full-field information at multiple discrete data points. In many cases the data field is localised around a particular feature, such as local strain around a hole in a tension-test specimen. In this case, the polynomial basis should similarly be localised by various forms of scaling – this requires the application of the Gram-Schmidt procedure to maintain orthogonality. The image features (sometimes called shape features) are meaningful and may be used to identify particular patterns in the data – e.g. for detecting cracks or other forms of damage. When assembled in a feature vector, the distance between feature vectors from measured and numerical results are useful for refining numerical models. In this paper the principles of image analysis, as applied to full-field displacement/strain data are explained and experimental examples are used to illustrate the practical usefulness of the method. The applications include (i) vibration mode shapes of laminated honeycomb structures and, (ii) strain in an aluminium plate with a central hole in tension.
Authors: Thorsten Siebert, Wei Zhuo Wang, John E. Mottershead, Andrea Pipino
Abstract: For the analysis of vibrations and mode shape extraction in particular the use of optical full-field measurement techniques has grown during the last years. Beside techniques like Digital Speckle Pattern Interferometry, Moiré, Thermography or Photoelasticity the Digital Image Correlation techniques have already been successfully proven to be an accurate displacement analysis tool for a wide range of applications.
Authors: Mara Feligiotti, Erwin Hack, George Lampeas, Wei Zhuo Wang, John E. Mottershead, Richard L. Burguete
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.
Authors: George Lampeas, Vasilis Pasialis, Thorsten Siebert, Mara Feligiotti, Andrea Pipino
Abstract: Innovative designs of transport vehicles need to be validated in order to demonstrate reliability and provide confidence. The most common approaches to such designs involve simulations based on Finite Element (FE) analysis, used to study the mechanical response of the structural elements during critical events. These simulations need reliable validation techniques, especially if anisotropic materials, such as fibre reinforced polymers, or complex designs, such as automotive components are considered. It is normal practice to assess the accuracy of numerical results by comparing the predicted values to corresponding experimental data. In this frame, the use of whole field optical techniques has been proven successful in the validation of deformation, strain, or vibration modes [1]. The strength of full-field optical techniques is that the whole displacement field can be visualized and analyzed. By using High Speed cameras, the Digital Image Correlation (DIC) method can be applied to highly non-linear dynamic events and deliver quantitative information about the three-dimensional displacement field [2].
Authors: Christopher M Sebastian, Eann A Patterson, Donald Ostberg
Abstract: Image decomposition is used to address the problem of accurately and concisely describing the strain in an inhomogeneous composite panel that is bolted to a vehicle structure. In-service, the composite panel is subject to structural loads from the vehicle which can cause unintended damage to the panel. Finite element simulations have been performed with the plan to establish their fidelity using full-field optical strain measurements obtained using digital image correlation. A methodology is presented based on using orthogonal shape descriptors to decompose the data-rich maps of strain into information-preserved data sets of reduced dimensionality that facilitate a quantitative comparison of the computational and experimental results. The decomposition is achieved employing the Fourier transform followed by fitting Tchebichef moments to the maps of the magnitude of the Fourier transform. The results show that this approach is fast and reliably describes the strain fields using less than fifty moments as compared to the thousands of data points in each strain map.

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