Papers by Author: Eann A Patterson

Abstract: Thermoelastic Stress Analysis (TSA) has been used to obtain the stress field in bolted T-Stub joint models (as defined by Eurocode 3) subject to cyclic loading which were employed to validate a Finite Element (FE) model. It was concluded from the results of the experiments and modeling that the behavior of the T-Stub is more complex than claimed by Eurocode due to contact forces, bolt interaction and plastic behavior.

Abstract: Crack tip plasticity has been investigated using thermoelastic stress analysis (TSA) and digital image correlation (DIC). The plastic zone size at the tip of a propagating fatigue crack was measured using both techniques. At longer crack lengths, the results compared well with Dugdale’s and Irwin’s models for crack tip yielding. The TSA methodology requires careful observation of the adiabatic assumption.

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.

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.

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: This paper presents a very brief overview of the philosophy underlying a plastic inclusion approach to defining the boundary stresses imposed on the applied elastic stress or displacement field by the plastic deformation attendant on crack growth in a ductile material. It leads to two new fracture mechanics parameters, KR and KS. KR defines a retardation component arising from wake contact and the Poisson’s contraction associated with the plastic zone, whilst KS describes a compatibility-induced component arising from shear at the elastic-plastic interface. These additional components imply that KF is not directly comparable with KI, as it describes the net driving force on the crack from the applied load.

Abstract: The achievement of high levels of confidence in finite element models involves their validation using measured responses such as static strains or vibration mode shapes. A huge amount of data with a high level of information redundancy is usually obtained in both the detailed finite element prediction and the full-field measurements so that achieving a meaningful validation becomes a challenging problem. In order to extract useful shape features from such data, image processing and pattern recognition techniques may be used. One of the most commonly adopted shape feature extraction procedures is the Fourier transform in which the original data may be expressed as a set of coefficients (coordinates) of the decomposition kernels (bases) in the feature space. Localised effects can be detected by the wavelet transform. The acquired shape features are succinct and therefore simplify the model validation, based on the full-field data, allowing it to be achieved in a more effective and efficient way. In this paper, full-field finite element strain patterns of a plate with a centred circular hole are considered. A special set of orthonormal shape decomposition kernels based on the circular Zernike polynomials are constructed by the Gram-Schmidt orthonormalization process. It is found that the strain patterns can suitably be represented by only a very small number of shape features from the derived kernels.

Abstract: This paper presents an outline of the development, verification and application of a new model of crack tip stress fields in the presence of a plastic enclave around a growing fatigue crack. The approach taken rests on capturing the effects of this ‘plastic inclusion’, comprising the crack tip and crack wake plastic zones, via elastic stress distributions applied at the elastic-plastic boundary. The model is therefore independent of the mechanisms of plastic deformation and potentially applicable to a variety of materials. A Muskhelishvili complex potential extension to the Williams crack tip stress field is found for four stress parameters representing a K-stress, a T-stress, a crack retardation stress and a compatibility-induced shear stress at the elastic-plastic boundary. This model is validated via full field fitting to photoelastic stress fringe patterns, obtained from epoxy resin and polycarbonate specimens. It has also been extended to the strain fields measured in digital image correlation techniques, which allows its application to metallic alloys.

Abstract: The fatigue life of metallic materials is strongly influenced by crack closure effects. Finite element (FE) methods allow the study of crack closure with great detail and can provide valuable information about phenomena occurring in the bulk of the material. In this work the distribution of stresses through the thickness of a cracked specimen has been studied using 3D FE simulations. It was found that the transition between the interior of the specimen (plane strain) and the surface (plane stress) differs from that predicted by 2D plane stress models. In addition, an attempt is presented to experimentally validate the results at the surface level. For this purpose full-field image correlation technique was utilized. This allowed direct comparison between the displacement field predicted by the numerical simulations and the experimental results measured by digital image correlation.

Abstract: A novel methodology based on a combination of experimental and analytical methods is used for monitoring the stress intensity factor in fatigue cracks subjected to constant amplitude loads. Full-field displacement information is fitted, following a multi-point over-deterministic approach, to an analytical model. This is developed from Muskhelishvili’s complex formulation. The methodology allowed accurate monitoring of the stress intensity factor during three fatigue cycles when small-scale yielding conditions were achieved. Moreover for larger loads where important plastic deformation occurs around the crack tip, Dugdale’s correction accounted for the differences between theoretical and calculated stress intensity factors. Accordingly the tool provides an indirect approach for measuring crack tip plasticity. Due to the fact that image correlation is relatively simple to use and is a non-contacting technique, the approach pioneered in this work seems ideal for monitoring fatigue cracks in industrial applications.