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- New mandrel design for ring hoop tensile testingPublication . KHALFALLAH, ALI; KTARI, Zied; Leitao, Carlos; Fernandes, José ValdemarThe determination of mechanical and fracture properties of anisotropic tubular materials along hoop direction needs the use of the ring hoop tensile test, for which, the obtained results are deceived by the effect of friction between the ring sample and the Dshaped block mandrel. Commonly, lubricants are applied to reduce the friction, which are inefficient in some specific cases; despite of that, it was noticed that scarce works have focused on the development of new mechanical mandrel designs or trying to improve the current ones to resolve the friction concern. The aim of this research is to correctly address the friction issue between the ring sample and the fixture mandrel to significantly reduce its effect on the ring hoop tensile test results without using any kind of lubricants. New mechanical design of D-shaped block mandrels are developed to carry out ring hoop tensile tests to simply characterize the mechanical behaviour of tubular materials. New mechanical D-shaped block mandrels were designed, manufactured and used to carry out experimental ring hoop tensile tests. An inverse identification method based on an artificial neural network trained by finite element simulation responses, was developed to efficiently segregate the flow stress curve from the influence of the friction, inherent in the global force-displacement curve for the classical ring hoop tensile test. The experimental force – displacement curves using five mandrel-types are established and quantitatively compared on the base of their ability to reduce the friction issue. The analysis of the finite element simulations, related to the investigation of the influence of the friction on the ring hoop tensile test results, shows that one of the new developed mandrels reduces the friction coefficient by about 10 times compared to that identified using the classical D-shaped block mandrel. It has been found that, the finite element simulation of ring hoop tensile test using the identified material parameters matches the experimental results. This investigation provides a useful fixture mandrel, which is able to drastically reduce the friction without resort to any lubricants to just determine the material flow stress curve using ring hoop tensile test, regardless the friction level between the sample and mandrel.
- Thermo-mechanical modelling of the Friction Stir Spot Welding process: Effect of the friction models on the heat generation mechanismsPublication . Hannachi, Nasra; KHALFALLAH, ALI; Leitao, Carlos; Rodrigues, DulceFriction Stir Spot Welding involves complex physical phenomena, which are very difficult to probe experimentally. In this regard, the numerical simulation may play a key role to gain insight into this complex thermo-mechanical process. It is often used to mimic specific experimental conditions to forecast outputs that may be substantial to analyse and elucidate the mechanisms behind the Friction Stir Spot Welding process. This welding technique uses frictional heat generated by a rotating tool to join materials. The heat generation mechanisms are governed by a combination of sliding and sticking contact conditions. In the numerical simulation, these contact conditions are thoroughly dependent on the used friction model. Hence, a successful prediction of the process relies on the appropriate selection of the contact model and parameters. This work aims to identify the pros and cons of different friction models in modelling combined sliding-sticking conditions. A three-dimensional coupled thermo-mechanical FE model, based on a Coupled Eulerian-Lagrangian formulation, was developed. Different friction models are adopted to simulate the Friction Stir Spot Welding of the AA6082-T6 aluminium alloy. For these friction models, the temperature evolution, the heat generation, and the plastic deformation were analysed and compared with experimental results. It was realized that numerical analysis of Friction Stir Spot Welding can be effective and reliable as long as the interfacial friction characteristics are properly modelled. This approach may be used to guide the contact modelling strategy for the simulation of the Friction Stir Spot Welding process and its derivatives.
- Mechanical design of ring tensile specimen via surrogate modelling for inverse material parameter identificationPublication . KTARI, Zied; Leitao, Carlos; Prates, Pedro; KHALFALLAH, ALIThe mechanical characterization of anisotropic thin walled-tubes along hoop direction is not a trivial task. It is necessary to develop experimental techniques, numerical methods and design test samples, which enable to determine the real tube properties along hoop direction without any external influences. In this study, first we propose a surrogate based-model for the mechanical design of the ring hoop tensile test (RHTT) specimen, in order to obtain the effective homogeneous stress and strain distribution of the uniaxial tensile test along hoop direction. Second, the optimized sample is used to carry out RHTT and to obtain the actual flow stress curve and the anisotropy coefficients of AA6063-O extruded tube. However, the experimental curve measured from RHTT (force –displacement) is a degenerate response, since it suffers from intermixture effects of the effective material behaviour with the friction between the sample and the sample-holding tool. Hence, we developed an inverse parameter identification method, which uses design of experiments, finite element analysis and artificial neural network to separate out the tubular material parameters from the friction coefficient. The assessment of the developed method is achieved by comparing the predicted material parameters and the identified flow stress curve obtained by artificial neural network algorithm. The finite element simulation results corroborate the obtained findings.