Papers by Author: A. Morão Dias

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Authors: António Castanhola Batista, José P. Marinheiro, Joao P. Nobre, A. Morão Dias
Abstract: An inverse method for the characterisation of the elastoplastic behaviour of materials has been studied. The method is based on spherical indentation test data and numerical analysis of the indentation process, enabling to find a characteristic stress-strain curve. This method will be appropriate for elastoplastic behaviour study, mainly on surface hardened materials, when the standard methods cannot be applied. In this work, the method was applied to annealed and quenched steels, with homogeneous properties over the cross section. The obtained results are in good agreement with those obtained from the standard tensile tests. However, if the material does not follow a linear hardening law, the elastoplastic characteristics determined by the inverse method will depend on the indentation depth. For these cases a method for the evaluation of the actual behaviour law has been improved.
Authors: Joao P. Nobre, Martin Kornmeier, A. Morão Dias, Berthold Scholtes
Authors: Joao P. Nobre, U. Noster, Martin Kornmeier, A. Morão Dias, Berthold Scholtes
Authors: Joao P. Nobre, Altino Loureiro, António Castanhola Batista, A. Morão Dias
Abstract: In this work the reliability of the hole-drilling technique (HDT) for measuring welding residual stresses was analysed. HDT residual stress results were systematically compared with those determined by X-ray diffraction. A systematic overestimation of the residual stresses determined by HDT was observed, which was mainly attributed to the possibility of the so-called plasticity effect occurring. Experimental results were discussed taking the measurement principles of both techniques into consideration. In addition, preliminary results of a numerical study, using the finite element method, will be presented for a better understanding of the plasticity effect on HDT residual stress results.
Authors: Maria José Marques, J.C.P. Pina, A. Morão Dias
Abstract: The conventional Bragg diffraction geometry, normally used to characterize the residual surface stress state, it is not suitable to evaluate surface treated materials and thin films. The X-ray path lengths through a surface layer or thin film are too short to produce adequate diffraction intensities in relation to the bulk or the substrate. Another limitation of the conventional technique appears when a residual stress gradient is present in the irradiated surface. The technique only enables the evaluation of the mean value of this gradient. In these cases, a recently proposed Pseudo-Grazing Incident X-ray Diffraction method would be better applicable. In this study, the Pseudo-Grazing Incidence X-ray Diffraction is applied to characterize the residual stress depth profiles of several AISI 4140 samples, which were prepared, by mechanical polishing and grinding, in order to present different surface roughness parameters, Ra. The experimental results lead to the conclusion that the surface roughness limits the application of the Pseudo-Grazing Incidence methodology to a minimum X-ray incident angle. This angle is the one that enables a mean X-ray penetration depth with the same order of magnitude of the sample surface roughness parameter, Ra.
Authors: J.C. Outeiro, A. Morão Dias
Abstract: Machining residual stresses are considered as part of surface integrity and a consequence of the machining process. Theses stresses are closely correlated with the corresponding process parameters, including the work material properties. As it is well known, not only the mechanical but also the physical properties of the work materials have great influence on machining residual stress. This was demonstrated in the present work through studying the residual stress and work hardening induced by the turning of AISI 316L and AISI 1045 steels. The residual stresses were determined at the workpiece surface and in-depth using the X-ray diffraction technique. To understand the influence of the work material properties on the residual stress and work hardening distributions, the mechanical and thermal phenomena occurring during the cutting process were studied, using a t developed experimental procedure. The experimental setup included a piezoelectric dynamometer to determine the cutting forces, and thermal imaging equipment developed to assess the temperature distribution in the deformation zone in turning. The results showed that the cutting forces and temperatures in the machining of 316L steel are much higher than those in the machining of 1045 steel. Thus, machining 316L steel, when compared to 1045 steel, results in higher superficial residual stresses and stronger in-depth residual stress gradients, higher superficial work-hardening and greater thickness of the work hardened layer.
Authors: António Castanhola Batista, Joao P. Nobre, A. Morão Dias
Abstract: A characterisation technique based on the stress determination by X-ray diffraction has been developed. It enables the identification of elastoplastic stress-strain laws on materials presenting an in-depth gradient of mechanical properties on its cross section. This technique is especially suitable to the characterisation of surfaces due to the small X-rays penetration depth. The method was applied in the characterisation of a carbonitrided and shot-peened steel, allowing to evaluate the stress-strain laws of the material at the surface, the intermediate layers and the bulk material. In addition, the in-depth evolution of microhardness, residual stresses, diffraction peak broadening and retained austenite contents were analysed. This allowed to understand the results of the proposed technique.
Authors: Maria José Marques, A. Morão Dias, C. Gautier-Picard, Jean Lu Lebrun
Authors: J.C. Rebelo, Martin Kornmeier, António Castanhola Batista, A. Morão Dias
Authors: P. Matos, Pedro Miguel Guimarães Pires Moreira, J.C.P. Pina, A. Morão Dias, Paulo Manuel Salgado Tavares de Castro
Abstract: Cold working introduces a compressive stress field around rivet holes, reducing the tendency for fatigue cracks to initiate and grow under cyclic mechanical loading. As it is well known, for the accurate assessment of fatigue lifetimes a detailed knowledge of the residual stress profile is required. Powerful experimental and numerical tools are nowadays available for that purpose. In the present work both types of tools, X-ray diffraction and 3D Finite Element Analysis (FEA), were used in order to evaluate the residual stress profile. A comparison of experimental and numerical data is presented and discussed.
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