Joao P. Nobre

Deformation Asymmetry of AZ31 Wrought Magnesium Alloy

In this work, the mechanical behaviour of an AZ31 wrought magnesium alloy due to tensile, compressive and four-point bending tests was analysed. Several specimens of this material were taken in different directions from a rolled plate. Tensile and compression tests allowed to characterise the mechanical properties of the material in the rolling and cross rolling directions. To take the effect of the anisotropy induced by the rolling procedure itself into account, the bending tests were carried out using specimens with four different orientations. Six strain gages per specimen allowed to observe the evolution of the strain with the bending moment during the tests. All bending specimens were bent until a total compressive deformation of 2.5%. It was observed that the corresponding tensile strain was significant lower and slightly different for each orientation of the specimens. In addition, a characteristic non-uniform distribution of deformation twinning was observed. The induced residual stresses after bending were characterised by X-ray diffraction (XRD) and incremental hole-drilling (IHD). Due to the different mechanical behaviour in tension and compression, an asymmetric residual stress distribution after bending could be observed. The neutral axis was, in all cases, shifted towards the tensile side. This observation agrees with the strain measurements during bending tests. In addition, an example of the influence of the asymmetric deformation behaviour during fatigue was given.

Use of the hole-drilling method for measuring residual stresses in highly stressed shot-peened surfaces

The same shot-peening treatment was applied to five steels with different mechanical properties. The induced residual stress profiles were analyzed using X-ray diffraction and incremental hole drilling (IHD). The results of both techniques showed that IHD can still be successfully used for measuring shot-peening residual stresses, even if these exceed the yield strength of the bulk material. Expected errors due to the plasticity effect are reduced by the strain hardening of the surface. For an assessment of the reliability of IHD data, strain-hardening variation was quantified by microhardness measurements to estimate the yield strength of the plastified layer. All the main calculation methods for IHD were applied. The results were compared and discussed with respect to the characteristics of each method.

A Windows‐based software package to evaluate residual stresses by the incremental hole‐drilling technique

A new software package for the incremental hole‐drilling technique (IHD) is presented in connection with undergraduate mechanical engineering education. The main available technical residual stresses evaluation procedures for applying IHD in isotropic materials are reviewed and have been included in the package. It has an easy‐to‐use, friendly interface, and can be used as a tool to teach the IHD technique or as a laboratory to study its applicability to real world problems. A set of exercises have been prepared, which are freely distributed to our students, in order to show the full potential of the package, but the students can also do simulations and experiments with their own data.

Quantifying the Drilling Effect during the Application of Incremental Hole-Drilling Technique in Laminate Composites

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.

Comparative Analysis of Shot-Peening Residual Stresses Using Hole-Drilling and X-Ray Diffraction Methods

An identical shot-peening treatment was applied to steel samples with different mechanical properties. The in-depth distributions of the residual stresses were analysed by the incremental hole-drilling (IHD) method and compared to X-ray diffraction (XRD) results. The strain hardening of the near surface layers was quantified by microhardness measurements, allowing to estimate a local yield strength. Due to the shot-peening treatment, the local yield strength increased thus reducing the influence of the plasticity effect on IHD results. In some cases, IHD can still give satisfactory results even though residual stresses exceed by far 60% of the bulk materials yield strength, normally limiting the application range of this method. Taking the local yield strength into account, a simple criterion for the assessment of IHD measurements is proposed to predict the influence of the plasticity effect in the case of shot-peening residual stress analysis.

An Almost User-Independent Evaluation Formalism to Determine Arbitrary Residual Stress Depth Distributions with the Hole-Drilling Method

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.

Residual stress evaluation on X 36 Cr Mo 17 HSM finished mould steel

The presented study was carried out in an industrial environment, using existing equipments, tools and materials. A set of tests was performed based on production demands and restrictions, aiming to achieve the lowest surface roughness and beneficial residual stress state. The experimental test surfaces were obtained using high-speed milling (HSM) and precision surface grinding on a DIN X 36 CrMo 17 steel work piece, widely used in injection moulding industry. The grinded surface was considered as a reference surface in order to evaluate the high-speed milling performance. For HSM tests, two types of tools were selected: tungsten carbide end mills with and without a TiAlN multilayer coating. The selected HSM main parameters ranged an upper, middle and lower limit value, considering the standard working values. The residual stress state at machined surfaces was evaluated by X-ray diffraction (XRD). Experimental results are discussed in order to achieve a procedure to select the optimal machining parameters that meet manufacturing specifications.

Evaluation of welding residual stresses using the incremental hole-drilling technique

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.

A utilização da difração de neutrões na determinação do perfil de tensões residuais em revestimentos por soldadura

A tecnica de difracao de neutroes e usada neste artigo para estudar amostras ferriticas, revestidas comacos inoxidaveisausteniticos atraves de soldadura por arco submerso. Este procedimento e frequentemente usado no fabrico de equipamentos de processo para as industrias quimica e nuclear, por facilidade de execucao e razoes economicas. A principal desvantagem deste processo de revestimento e a fissuracao que frequentemente ocorre na interface material base/soldadura, potenciada pela presenca de tensoes residuais resultantes da operacao de soldadura, a qual pode ser minimizada com a realizacao de tratamentos termicos de relaxacao de tensoes. As amostras foram produzidas a partir de placas em aco ao carbono, tendo uma das superficies sido revestida com dois tipos de aco inoxidavel. Para a primeira camada foi usado um eletrodo EN 12072 - S 23 12 2 L e para a segunda e a terceira camadas foi usado um eletrodo EN 12072 - S 19 12 3 L. Apos a soldadura, as amostras foram submetidas a um tratamento termico de relaxacao de tensoes, durante 1 hora, a temperatura de 620oC. Os perfis de tensoes residuais obtidos por difracao de neutroes evidenciam a relaxacao de tensoes residuais apos o tratamento termico realizado. A tecnica de difracao de neutroes revelou-se muito adequada na avaliacao de tensoes residuais neste tipo de ligacoes.

Wear and surface residual stress evolution on twin-disc tests of rail/wheel steels

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.