Papers by Author: Chiara Colombo

Abstract: The use of additive manufacturing (AM) technique is recently increased due to its ability in producing complex-shaped components. AlSi10Mg alloy is largely employed for AM and, in particular, for selective laser melting (SLM). Hence, the interest on this alloy is growing, together with the studies to control its mechanical performances, which can be increased by microstructure modification. With this focus, low temperature heat treatments to enhance AlSi10Mg mechanical behavior have been proposed in the recent literature. The present work focuses on two post-additive thermal treatments, with temperatures specifically selected for SLMed AlSi10Mg alloy. The study investigates their effect on mechanical performances, with a particular attention to residual stresses. Experimental measurements of residual stresses obtained by an X-ray diffractometer (XRD) are presented, considering samples produced with their main dimension along the in-plane scanning directions (XY configuration) and the Z direction. Different conditions are accounted: 1) as-built; 2) after heat treatment at 244°C for 180 minutes; 3) after heat treatment at 290°C for 45 minutes. Tensile properties are correlated with the measurements of the residual stresses, allowing for a critical discussion and for a deeper insight on the correlation of the mechanical performance with the process parameters and the following thermal treatments.

Abstract: Flanged joints arefundamental components of pipelines and they must be optimized to avoid leaks,bolts overstress and flanged joints damage. Overcoming standards indications asover-torqueing, and combination of different parameters, as difference in hardnessbetween flanges and gasket, can cause groove impressions. Aim of this study isto provide an efficient tool to optimize the choice of characteristicparameters of flanged joints, by numerical simulations. Geometries of a realtest case are simulated by FE models, considering different values of flangeand gasket hardness, as well as tightening torques, in order to cover possibleranges. From the analyses, it is possible to highlight that a particularcombination of hardness difference and over-torque is responsible for permanentflange impression.

Abstract: Pipeline working environment is characterised by corrosive conditions, able to develop hydrogen formation. The presence of atomic hydrogen localized in correspondence of crack tip, where the plastic strain reaches the maximum value, is responsible for life reduction. For this reason, it is important to estimate and predict the mechanical properties decay, in terms of toughness and crack propagation, when steel is in contact with hydrogen. Aim of this study is to develop FE models of two carbon, low-alloyed steels used in pipelines applications: X65 and F22. A complex model including three simulations steps is presented. It considers the combined effect of plastic strain and hydrogen concentration on the material toughness. The results of the model are validated by a comparison with experimental tests carried out on the two low-alloyed steel.

Abstract: According to classical definition of crack deformation modes, the constant stress term (Tstress) exists only in presence of mode I. However, some studies show that this term can exist in mode II problems as well; and significantly affect the elastic stress field around the crack tip. Based on the previous analytical results, T-stress changes the photoelastic fringe patterns from symmetric closed shapes to asymmetric and discontinuous loops. In this research, the effects of T-stress on the fringe patterns in mode II cracks is investigated experimentally. Test specimens are Brazilian disks made of polycarbonate, and thermal treatment is performed to remove the residual stresses after generation of the cracks. Observed isochromatic fringes are in good agreement with theoretical predictions. Also, experimental results indicate that this specimen contains a negative T-stress in pure mode II condition.

Abstract: Aim of this paper is to study the impact between the sailing boat keel and the ground and to look for the optimal configuration of the bulb-keel-hull system able to direct the damage in the bulb-keel zone avoiding the hull region. The research, which is carried out in collaboration with Wally shipyard, requires firstly an accurate investigation of the bulb-keel-hull system geometry, of the boat inertial properties and of the impact condition. Secondly, a numerical dynamic analysis by means of the finite element method allows to model the impact and to determine how, during the collision, the state of stress varies and how the kinetic energy is absorbed/dissipated. Starting from these results, the performance of the present configuration is evaluated in several conditions and some proposals for new design configuration are made. In particular the results carried out from the new design analyses were evaluated comparing themselves to results obtained from experimental studies.