Papers by Author: Alessandro Soprano

Abstract: Among several problems which might affect the passenger safety during an accidental crash event, the deceleration pulse is one of the most critical. For this reason vehicles are designed to convert the Kinetic Energy occurring in an impact in plastic deformation and to spread the loads due to such events through designed structural load paths. An important role in the kinetic energy absorbing at high velocities is played by the energy absorbers. The energy absorption capability of a crashworthy element or system is largely affected by material properties and structural design. This work deals with a numerical investigation on the energy absorbing capability of a new concept of energy absorber made out of the combination of metal parts and carbon composite materials. A numerical investigation on the parameters which increase the crash performance as well as decrease the weight of such device has been presented in this paper.

Abstract: In the recent years, the study of the behaviour of damaged structures has been focused on cracked components in presence of an extensive material yielding at the crack tip; under this condition, linear elastic fracture mechanics theory (LEFM) is not able to describe the real plastic zone shape and size. Within this work, an extensive numerical analysis, based on elastic plastic fracture mechanics theory (EPFM), of the plastic zone size at the tip of a Mode I pre-crack at the notch edge in a plate is presented.

Abstract: The scientific community involved in the study of the gas storage processes faces with the problem of the gas hydrates. In fact when the natural gas goes in contact with liquid water the methane hydrates take place; such hydrates can be considered as a solid structure which could abrupt partially or totally a gas pipeline. In such cases, the differential pressure across the plug can put it in movement producing catastrophic scenarios in the whole plant. In this work a parametric analytical analysis was conducted in order to define the plausible kinematic conditions of the hydrates mass. Once selected an opportune set of boundary conditions, some of the consequences of the motion of hydrates were taken into account, such as the amplification of the displacements related to resonance phenomena, the inertial effects and impacts against the pipe internal walls.

Abstract: In this work, a parametric numerical investigation on the plastic zone size and shape at the crack front of a through crack in a plate is presented. The thickness of the plate, the size of the crack and the applied remote load values are considered as parameters. The obtained results allow assessing the limits of the linear elastic fracture mechanics in presence of an extensive material yielding at the crack front and the relationship between the plastic zone size and other parameters of the elastic-plastic fracture mechanics theory is highlighted.

Abstract: Gas hydrates are known to form plugs in pipelines. Gas hydrates are crystalline compounds that form when hydrocarbons such as methane come in contact with water under thermodynamical opportune conditions, as high pressure and low temperature. Hydrates, like any obstruction in a pipeline, reduce flow, increase back pressure in the system and increase the differential pressure across the obstruction. When the line section is obstructed by a plug, the differential pressure can put the hydrate in movement and quickly accelerate it up to a speed approaching that of sound; in this case, the moving mass can cause serious mechanical damages at downstream locations where the plug can also meet restrictions or obstacles such as valves, elbows or tees. In this paper a real case of a gas pipeline failure, due to the presence of a moving mass of methane hydrate, has been investigated by considering an analytical and numerical modeling of the motion of the hydrates, as well as of their impact against the pipes.

Abstract: The mechanical properties of geopolymers can be obtained through different kinds of experimental tests: this paper is focused on the compressive strength (i.e. in a direction parallel to the loading axis) for the case of uniaxial compression. The compressive strength of such materials is traditionally characterized by the 28^th-day value, but their strength is expected to increase in time at a continuously decreasing rate. The knowledge of the strength vs. time law is of importance when a structure is subjected to a certain type of loading at a later age. In this work inorganic polymers from activated metakaolin (alumina silicate inorganic polymers, obtained from alkali activation of powders containing SiO₂+Al₂O₃ > 80%_wt) are reported. In order to improve their compressive strength a percentage of polyethylene glycol has been added, thus obtaining a hybrid (organic-inorganic) geopolymer. Many factors can influence significantly the compressive strength of such materials e.g. w/c ration, aggregate content, water curing period, polyethylene/glycol ratio. Afterwards experimental compressive tests (performed in a Zwick-Roell^® testing machine) have been carried out varying the polyethylene/glycol ratio and the main dimensions of the samples.

Abstract: Coupling techniques for components of different materials is spreading in mechanical industry; the test case studied in this work deals with the connection of an aluminium alloy component with a carbon fibre composite one. In particular, the first component is made of an aluminium-zinc alloy and exhibits an isotropic behaviour, while the second is made of a carbon fibre reinforced polymer (CFRP) and shows a strongly anisotropic behaviour; both materials are widely used in engineering applications. A titanium bolt connects the parts. This work is focused on the influence of the geometrical parameters which characterize the coupling between the components. In particular, a study has been carried out on the influence of the shank-hole clearance, the bolt head size, the bolt preload and the shape of the bolt head. A numerical model has been built and statically tested; the results have been compared with the experimental ones from literature. Once validated, the same numerical model has been used to evaluate the performance of the joint in presence of a change of the above mentioned characteristic parameters. The required numerical analyses have been performed using Abaqus/Standard® numerical code.

Abstract: In this work, results from a study on bolted joints made of unidirectional, quasi isotropic Carbon Fiber Reinforced Polymer (CFRP) composites, subjected to tensile loads, are reported. CFRP composite materials are widely used in the mechanical industry, such as that of aerospace, where requirements of weight reduction and structural high performances are very compelling. Composite materials generally present a high resistance to fatigue and corrosion; however, the presence of joints produces the major problems and a poor design of joints leads to a drastic reduction of the reliability of structures made of these materials. A hybrid bolted joint involving a metal plate, made of aluminum alloy, and a CFRP composite plate has been considered; the plates are held together by a titanium bolt. Experimental results from literature are compared with those obtained through a numerical analysis developed with Abaqus code. Once the CFRP composite has been analyzed and the numerical model validated through numerical-experimental correlations, other possible configurations have been numerically analyzed in order to ensure the highest strength of the examined hybrid joint. Afterwards the effects of bolt-hole clearance on the stiffness and strength of the same joint have been investigated.

Abstract: In the present paper the authors refer about a series of experimental tests, where an aluminium alloy square tube, filled with an aluminium foam, was crushed by a longitudinal load at a speed of 10 m/s. The test apparatus consisted of a sled installed on a very stiff frame moving on appropriate guides, as the specimen was set on a home-made fixture. Two arrangements of square tubes were considered as specimens: a “standard” one and an “optimized” one. Both crushing behaviours and energy absorption capabilities were analyzed experimentally and numerically simulated by means of the explicit FE code LS-DYNA^®; the complete numerical model consisted of the striker, the assemblage of square tubes and the base. A high-speed video recording system was used to capture the images from the physical test. The results from the numerical analyses were compared to those obtained from the experiments: those results showed that the force–deflection response had been overestimated by the numerical model. The authors attempted to justify this inconsistency by considering the influence of the strain rate parameters of the considered Cowper-Symonds analytical model on the results. It was shown that the “optimized” energy absorber exhibited a more desirable force–deflection response than the standard one due to some easy design changes, which involved the insertion of aluminium foam dampers.

Abstract: The energy absorption capability of an exposed crashworthy element or system is largely affected by material properties and structural design: prismatic sandwich structures, made of foam or honeycomb core between two metallic or laminated composite face plates, are good candidates. This work deals with a numerical investigation on the energy absorbing capability of such a structural component. There are several difficulties associated with the numerical simulation of a composite impact-absorber, such as high geometrical non-linearities, boundary contact conditions, failure criteria, material behaviour; that is because the main objectives of any numerical investigation are the calibration of the model with experimental results and the evaluation of the sensitivity of the variables with respect to the geometrical and physical parameters which influence the study at hand. The latter is a very relevant aspect for designers if the application of the model to real cases has to be a robust one from both a physical and a numerical point of view. In this paper a preliminary calibration of a numerical model for a composite impact absorber is presented, on the basis of experimental data found in literature; then a sensitivity analysis of the same model to the variation of the main geometrical and material parameters, developed by using the explicit finite element algorithms implemented in the Ls-Dyna code, is illustrated.