1,721,198 research outputs found
The local strain energy density approach applied to pre-stressed components subjected to cyclic load
No full textAhead of sharp V-notches, residual stresses, arising from the solidification of a fusion zone, have the same asymptotic nature of the stress field induced by mechanical loads. This stress field significantly affects the engineering properties of structural components, notably fatigue life and corrosion resistance of welded joints. Tensile residual stresses can reduce the fatigue strength of welded joints particularly in the high-cycle regime, where no stress redistribution due to local plasticity phenomena is expected to be present. The aim of this work is to analyse, by means of the numerical simulation, the residual stress redistribution near a V-notch tip induced by cyclic loads and to propose a method, based on the local strain energy approach, for the fatigue resistance estimation of pre-stressed components. The numerical solutions of the problem were carried out under the hypothesis of generalized plane strain conditions by means of SYSWELD and SYSTUS codes
Molten pool in welding processes: Phenomenological vs fluid-dynamic numerical simulation approach
No full textThe metallurgical and mechanical properties of fusion welded joints are influenced, among others phenomena, by the weld pool dimension and shape. Weld pool shape is important in the development of grain structure and dendrite growth selection process as well as in the development of residual stresses. For these reasons, significant advances have been made in recent years to understand, in greater detail, the dynamics of the heat and fluid flow in the weld and the subsequent development of the pool shape. In numerical simulation of welding processes, there are two different approaches used to model the fusion zone. If the prediction of distortions and residual stresses is the primary objective of the simulation (computational weld mechanics simulation), the phenomenological approach is the most suitable method used to model the fusion zone. Otherwise, when the weld pool shape has to be predicted, the fluid-dynamic equations must be solved at the expense of a significant ‘computational load’ increase. In this work, after a brief description of weld pool characteristics, such two different approaches are described and compared
Un modello bidimensionale per lo studio delle tensioni indotte dal processo di saldatura
No full textIn questo lavoro è stato sviluppato un modello bi-dimensionale che consente di stimare sia la reale distribuzione del calore durante il processo di saldatura, sia il campo di tensione in corrispondenza della regione del giunto che raggiunge la condizione di campo termico quasi-stazionario. In particolare è stata definita una funzione, implementata nel codice numerico, che tiene conto della velocità della sorgente mediante un progressivo riscaldo della zona in prossimità del cordone di saldatura, mentre il calcolo tensionale è stato eseguito utilizzando uno stato di deformazione piano di tipo generalizzato. Scopo del lavoro è stato quello di mettere a punto un modello numerico in due dimensioni che, grazie ad alcune ipotesi semplificative, superi le sue intrinseche difficoltà nel simulare il processo di saldatura. Ciò ha permesso di evitare l’enorme mole di calcolo tipica dei modelli numerici 3D e i limiti dei modelli analitici che, anche se molto efficienti, sono applicabili solo a geometrie molto semplici e a materiali non interessati da trasformazioni di fase durante il processo. Il modello proposto ha dato dei risultati in buon accordo con i valori analitici, sperimentali e numerici provenienti da un’analisi 3D e può considerarsi quindi assai utile per la simulazione e progettazione dei processi di saldatura. Le analisi 2D sono state eseguite con il codice Ansys5.7®, quelle 3D con il codice Sysweld2002®
A dissolution kinetics model and its application to duplex stainless steels
No full textThis work proposes a semi-empirical model for isothermal particle dissolution. An impingement factor in the form yc+1 is introduced to generalize the exponential law found in the recent literature and numerically obtained by means of the phase field method. A procedure is described for calibration of the model parameters that uses information from isothermal experiments. The model was applied to isothermal dissolution of r phase in duplex stainless steels. A very good agreement was obtained between experimental and analytical results
Influence of phase transformations on the asymptotic residual stress distribution arising near a sharp V-notch tip
No full textIn this work, the residual stress distribution induced by the solidification and cooling of a fusion zone in the vicinity of a sharp V-notch tip is investigated. The intensity of the residual asymptotic stress fields, quantified by the notch stress intensity factors, was studied for two different V-notch specimen geometries under generalized plane-strain conditions. In order to analyze the influence of phase transformations on the obtained results, simulations with and without the effects of phase transformation were carried out on ASTM SA 516 steel plates. Thanks to the possibilities of numerical modelling, additional analyses were performed without taking into account the transformation plasticity phenomenon.
It was found that phase transformation effects (both volume change and transformation plasticity) have a great influence on the intensity and sign of the asymptotic stress fields at the sharp V-notch tips. This result is believed to be very important for the correct numerical determination (and future applications) of notch stress intensity factors resulting from asymptotic residual stress distributions induced by transient thermal loads. The analyses were performed with the finite element code SYSWELD
The use of Matlab in the advanced design of bonded and welded joints
No full textThe development of new models for joining design assumes a great importance in the industrial and research field. The prediction of residual stresses, temperature distributions, phase transformations, asymptotic stress fields near the weld toe or near the interface between the matrix and the adhesive in bonded joints, may be a fundamental step for a good joining plan and operation. Aim of this work is to describe some analyitical models developed for bonded and welded joints and solved by means of Matlab code. In the first part, models for temperture ditributions, phase transformations diagrams and residual stresses prediction will be presented with particular attentions both to conventional and innovative welding precesses such as laser and friction stir welding. In the second part, after a brief description of the ‘stress function approach’ used in advanced joints planning, the application of Matlab code in such field will be treated
The strain energy density approach applied to cyclic and transient thermo-mechanical problems
No full textProprieta’ statiche e di resistenza a fatica di una ghisa sferoindale contenente Grafite Chunky
No full textModelling of the Carburizing and Quenching process applied to caterpillar track bushings
No full textThe carburizing-quenching process applied to caterpillar track bushings was studied by means of experimental and numerical analyses. The numerical model was developed on the basis of the real cycle. The purpose of this work is to predict the carbon profiles, microstructural phase changes, hardness and residual stress that occur during quenching using finite element techniques. Good agreement was obtained between the experimental and numerical results in terms of carbon diffusion and hardness profiles. The Sysweld® numerical code was used to perform the simulations
Microstructural, texture and residual stress investigations on a twin roll cast AZ31 magnesium alloy
No full textMagnesium alloys are interesting for both structural and functional applications where the mass of components is of concern. For the production of panels or frames whose shape develops mainly according to a 2-D configuration, a sheet forming sequence may be considered rather than pressure die casting, squeeze casting or thixotropic casting. An interesting method to obtain semi-finished sheets is twin roll casting. The rolled-cast sheet usually requires further rolling to achieve the required thickness.
Aim of the work is the characterization of a piece of “as twin roll cast” AZ31 sheet. To this end usual microstructural investigation techniques were used, such as light and scanning electron microscopy, energy dispersive spectrometry and X-ray emission maps, in association with crystallographic texture determination by X-ray diffraction and residual stresses determination by means of the Hole-Drilling Strain-Gage Method. The investigations showed an heterogeneous distribution of structural components associated with an inhomogeneous distribution of residual stresses through the thickness of the sheet. This confirms the absolute necessity to carry out effective solution treatments before performing any further reduction of the thickness of the sheet by hot rolling or whatsoever forming step aimed at achieving the required final configuration of a workpiece
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