Papers by Author: Joao P. Nobre

Abstract: Rolling contact fatigue twin-disc tests were performed on rail/wheel steels from Spanish high velocity trains (AVE). Residual stress profiles were determined using X-ray diffraction before and after cyclic loading. The evolution of residual stress profiles, due to cyclic loading, was analysed in order to study how they affect the rolling contact fatigue behaviour of these materials. This study is included in a major project where other related phenomena and materials’ properties have been studied.

Abstract: Twin disc tribological tests were performed in wheel and rail materials, with specimens taken from a Spanish AVE train wheel and a UIC60 rail, in a program intended to characterize their contact fatigue behavior. The X-ray diffraction technique was used to characterize the residual stress distribution at the initial and damaged stages, as well as in intermediate stages, since existing residual stresses in the surface layers of the railways steels and its evolution during contact loading can have a major influence on crack initiation and propagation.

Abstract: A hybrid experimental-numerical methodology is applied to evaluate the unwanted stresses induced by hole-drilling in two 5000 and 7000 series aluminium alloys. The influence of the cutting speed of ultra-high speed drills powered by turbine systems by compressed air, which are commonly used in the hole-drilling equipments for residual stress measurements, is analyzed. The comparison of the effect of different drilling conditions on the drilled material using a quantitative approach is now possible. The applied methodology can play an important role on the improvement and optimization of the hole-drilling technique for residual stress measurements in particular and the drilling process in general.

Abstract: The samples studied in this paper were performed from carbon steel plates, cladded in one of the faces with stainless steel filler metals by submerged arc welding (SAW). After cladding work, the samples were submitted to post-weld heat treatments at different conditions and afterwards stainless steel coating surfaces were milled and mechanically polished, as in the industrial application. The residual stress analysis was performed by X-ray diffraction (XRD) and incremental hole-drilling methods (IHDM). The residual stresses profiles presented different in depth values in each sample, depending on the heat treatment conditions. The hole-drilling method was applied in several points of each stainless steel sample surface and the results presented similar evolution profiles. However compressive stresses increase with the increase of heat treatment temperature.

Abstract: The incremental hole-drilling method is the method of choice to determine residual stress depth distributions with limited costs and minor destruction of the investigated component. With a spatial resolution of commonly two millimeters in diameter and one millimeter in depth especially the effects of frequently used surface treatments like e.g. shot peening or deep rolling can be reliably detected if the in depth residual stress gradients are relatively smooth. Nevertheless up to now the quantitative accuracy of the method is poor for residual stress analyses close to the materials surface up to depths of approximately 0.2 mm and in the case of steep in-depth residual stress gradients or oscillating residual stress depth distributions. In this paper, residual stress depth distributions of a broad range introduced by mechanical surface-treatments in flat specimens were analyzed with the hole-drilling method and compared with the results measured by X-ray diffraction as the reference. It comes out, that arbitrary residual stress depth distributions can be successfully determined with a modified differential evaluation formalism. For this purpose, often neglected well known weak points of the hole-drilling method were considered and improved, e.g. hole geometry, numerical calibration and data conditioning. Especially, the proposed strategy of data conditioning results in an almost user-independent evaluation formalism.

Abstract: In this work, a methodology to quantify the effect of the drilling operation, during the application of the incremental hole-drilling technique (IHD) for measuring residual stresses in laminate composites, in particular, the polymer matrix composites (PMC), is presented. This technique will allow the optimization of the drilling procedures and its parameters, enabling the quantification of the drilling effect. This quantification is obtained by using an experimental calibration procedure followed by a numerical simulation of the whole process. The direct comparison of the experimental and numerical results will allow quantifying the effect of the drilling operation. As example, the methodology was applied to the case of carbon/epoxy cross-ply laminate [0°/90°]_5s. The holes have been made by using two different drilling procedures, but the same tool geometry. High speed milling powered by air compression, a process usually employed in the case of the application of hole-drilling technique to metal alloys and a conventional computer numerically controlled (CNC) milling machine, were used. The results seem to show that incremental hole-drilling could be a reliable technique to determine residual stresses in fibre-reinforced polymers.

Abstract: The influence of the heat treatment on the residual stress fields of weld cladded samples is discussed in this paper. The samples were elaborated from carbon steel plates, cladded in one of the faces with stainless steel filler metals by submerged arc welding. After the cladding process some of the samples were submitted to heat treatments with different parameters: one at 620° C for a holding time of 1 hour and the other at 540° C for a period of ten hours. The in‑depth residual stress profiles were determined by neutron diffraction. The results shown that the sample treated to 620 °C, presented the highest residual stress relaxation. The corresponding heat treatment has the industrial benefit to be shorter than the other heat treatment.

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.

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.

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.