1,721,046 research outputs found
L'USO DI EQUAZIONI DI STATO MULTI-FASE NELLA SIMULAZIONE NUMERICA DI ONDE D'URTO
Full text linkQuesto lavoro è volto a mettere in evidenza l'importanza dell'utilizzo di Equazioni di Stato (EOS) multi-fase in eventi caratterizzati da onde d'urto. L'obiettivo è la descrizione del comportamento del materiale in seguito alla deposizione di energia derivante dall'impatto con un fascio di particelle subatomiche ad elevata energia. Siccome i tempi di deposizione sono talmente brevi (ns o mus) da essere inferiori al tempo caratteristico di risposta idrodinamica del sistema, nel materiale gli incrementi di energia, e di conseguenza, di temperatura e pressione avvengono con una trasformazione pressoché isocora. Il materiale tende, perciò, ad esplodere dall'interno: una volta che il processo di rarefazione inizia, il materiale, che si trova a monte del fronte di shock, è libero di espandere, e raggiunge valori inferiori di pressione e densità. Al contrario, l'arrivo del fronte di shock nel materiale inizialmente quasi indisturbato, ne causerà l'aumento di pressione e densit
Mechanical characterization and modeling of the heavy tungsten alloy IT180
Full text linkPure tungsten or its alloys (WHA) find applications in several fields, especially due to the fact that these materials show a good combination of mechanical and thermal properties and they are commonly used in aerospace, automotive, metal working processes, military and nuclear technologies. Looking at the scientific literature, a lack in the mechanical characterization over wide ranges in temperature and strain-rates was found, especially for W-Ni-Cu alloys. In this work, the mechanical characterization and the consequent material modeling of the tungsten alloy INERMET® IT180 were performed. The material is actually used in the collimation system of the Large Hadron Collider at CERN and several studies are currently under development in order to be able to numerically predict the material damage in case of energy beam impact, but to do this, a confident strength model has to be obtained. This is the basis of this work, in which a test campaign in compression and tension at different strain-rates and temperatures was carried out. The dynamic tests were performed using Hopkinson Bar setups, and the heating of the specimen was reached using an induction coil system. The experimental data were, finally, used to extract the coefficient of three different material models via an analytical approac
L’IMPIEGO DI ESTENSIMETRI A SEMICONDUTTORE IN PROVE AD ELEVATO STRAIN-RATE
No full textNegli ultimi anni, l’impiego di estensimetri a semiconduttore per la misura della deformazione in prove sperimentali ad elevata dinamica, svolte mediante Split Hopkinson Pressure Bar (SHPB), ha subito un forte incremento. La motivazione principale è essenzialmente legata alla possibilità di eliminare dalla catena di misura lo stadio di amplificazione del segnale, in quanto questi estensimetri presentano un gage factor (fattore di taratura) molto elevato. Eliminando tali componenti elettronici, innanzitutto, si aumenta la banda passante del sistema, che altrimenti sarebbe limitata; inoltre si elimina una delle possibili cause di rumore dei segnali. Un alto fattore di taratura, però, implica che la relazione fra il segnale misurato e la variazione relativa di resistenza non sia lineare: la calibrazione del sistema di misura diventa, quindi, fondamentale. In questo lavoro viene descritta una procedura che permette la calibrazione del sistema in-situ, direttamente sul setup di prova
Experimental analysis and modelling of the strain-rate sensitivity of sheet niobium
No full textThe niobium is currently used for the construction of the superconducting radio frequency (RF) Crab Cavity for the particle accelerator LHC at CERN in Geneva. An alternative technique to traditional forming methods is the electrohydraulic forming (EHF), in which ultrahigh-speed deformation of blank sheets is performed by using shockwaves electrically induced in water. A big effort is made for the analysis of the forming processes by FEM simulations, which require the definition of an appropriate flow stress material model. With this aim, in the present work, a testing campaign was performed in tension on sheet specimens with a rectangular cross-section at different strain-rates, up to 103 s-1. The obtained results showed the material is strongly sensitive to strain-rate, as expected for a pure BCC metal. The data, were processed via a reverse engineering procedure, based on finite element simulations of the experimental tests. This methodology allowed the identification of a tabular flow stress model (MAT_224 implemented in LSDYNA) for the prediction of the material behaviour as a function of the plastic strain, strain-rate and temperature
Strength Model Evaluation Based on Experimental Measurements of Necking Profile in Ductile Metals
No full textThe reliability of the numerical results strongly depends upon the choice of the constitutive models, which sometimes requires an ad-hoc calibration of the parameters starting from experimental results. Often, the tension test is chosen since it allows obtaining a big amount of data, but in case of ductile materials, in which the specimen could be subjected to high value of plastic strain before the fracture, it is necessary to properly manage the experimental results. For low values of deformation, the deformation of the gage length of the specimen is uniform and the true stress can be easily derived from the longitudinal one and considered as the equivalent stress. At the onset of necking, geometrical instability and concentration of deformation are developed. In the scientific literature several different approaches have been proposed in order to estimate the equivalent stress distribution in the post-necking regime. In this work, a new approach was proposed by the combination of digital image analysis and numerical inverse method. The evolution of the profile deformation of the specimen observed during the test is directly used to identify the constitutive relation of the material in case of dynamic tensile tests
Numerical simulation of landmine explosions: comparison between different modelling approaches
Full text linkUntil decade ago the design of mechanical structures, having to resist to explosive events, was mainly performed using experimental tests with explosive materials. In the last years, numerical methods are assuming importance thanks to the following advantages: high cost reduction, flexibility in investigating different scenarios and the chance to study explosive phenomena without risks. An explosion is a complex and multidisciplinary subject. It involves a large number of physical parameters which influence the amount of energy transferred to the target above the detonation. The aim of this paper is to describe numerical models to simulate landmine explosion and blast loading on structures, using different approaches: an Arbitrary Lagrangian Eulerian (ALE) mesh and a pure Lagrangian mesh. For what concerns the ALE simulations, three different cases are analyzed. First of all, the numerical model of the landmine explosion is validated through the comparison with experimental data. The same model is then used to evaluate the effect of detonations against two structures, using a fluid-structure algorithm: a steel plate and a human leg. For this type of simulations, an Eulerian approach is needed, in order to reproduce the expansion of the mix of sand, air and gas against the target. When the gas encounters the target a fluid structure interaction algorithm (FSI) determines the pressure values, which are transferred from the Eulerian parts to the Lagrangian ones. The main disadvantage of an ALE approach is the large computational time, which is further aggravated by the need to use quite fine mesh resolution to adequately reproduce the air shock. For this reason it is interesting to use 2D modeling. The second approach is based on empirical airblast equations developed by Kingery and Bulmash, for the application of pressure loads due to explosives in conventional weapons, and was implemented in LS-DYNA by Randers-Pehrson and Bannister. This methodology is applied to simulate the detonation against the plate and the results are compared with the corresponding results obtained using an ALE approac
THERMO-MECHANICAL MODELLING OF DYNAMIC TENSILE EXTRUSION TEST
No full textRecently, the researchers of the Los Alamos National Laboratory (LANL) have been developed a technique to investigate high strain-rates in which a sphere is launched at high speed (300-800 m/s) through a conical die (Dynamic Tensile Extrusion test). The system can use a set-up similar to a Taylor test (like a light gusgun) but induces in the material high levels of tensile stresses and allows to investigate the fracture and damage of the material in these extreme conditions. Due to the high strain-rates and hydrostatic component of stress that occur during the extrusion in the die, the material is also subject to a rapid heating. In this work, the DTE test has been simulated with the explicit Lagrangian code LSDYNA. In order to correctly reproduce the various aspects of the test it is necessary to use an elasto-plastic material model that takes into account strain-rate, thermal softening, pressure influence and damage and that adequately describes the thermal coupling effects and the equation of state (EOS) of the material. Besides, due to the high distortion of the Lagrangian mesh, the use of a mesh adapting technique is absolutely necessar
Investigation and Mechanical Modelling of Pure Molybdenum at High Strain-Rate and Temperature
Full text linkThis work shows the results obtained from the investigation of the mechanical behavior of two batches of pure molybdenum specimens (≥99.97 % Mo, Mo1 supplied by Plansee and Mo2 supplied by AT&M) under static and dynamic loading conditions at different temperatures, both under tensile and compressive loading conditions. Due to its properties molybdenum has applications in several fields including nuclear. At this moment, it is a good candidate for structural material application for Beam Intercepting Devices of the Large Hadron Collider at CERN, Geneva. The experimental tests in tensile loading condition were performed on small dog-bone specimens. A series of tests at room temperature and a range of strain-rates was performed in order to obtain information about the strain-rate sensitivity of the material. A series of tests at different temperatures in both static and high dynamic loading conditions was performed in order to obtain information about the thermal softening of the material. The dynamic tests were performed using the Hopkinson Bar technique, and the heating of the specimen was performed using an induction coil system. The experimental tests in compression were carried out on cylindrical specimens at room temperature and a range of strain-rates. The experimental data were analyzed via a numerical inverse method based on Finite Element numerical simulations. This approach allows to obtain the effective stress versus strain curves, which cannot be derived by using standard relations since instability and necking were present. Moreover, it also allows the non-uniform distribution of strain-rate and temperature inside the specimen to be accounted for. The results obtained from compression tests confirm the data obtained in tension in terms of strain-hardening and strain-rate sensitivity, even if the material exhibits a tension–compression asymmetry of the behavior. The analysis of the hardening, temperature and strain-rate sensitivities reveals that a unique standard visco-plastic model could not be defined to reproduce the material strength behavior under different loading conditions, especially over wide range of variation of the variables of interest
STUDIO DEL COMPORTAMENTO DI METALLI REFRATTARI AD ELEVATI STRAIN-RATE E TEMPERATURE
Full text linkL'introduzione di acceleratori di particelle ad elevata energia, come il Large Hardon Collider (LHC) realizzato al CERN, ha richiesto lo sviluppo di metodi avanzati per predire il comportamento di particolari dispositivi che possono essere soggetti all'impatto con il fascio di particelle, e che, quindi, sono progettati per operare in un ambiente estremamente radioattivo e fortemente sollecitato da un punto di vista termo-strutturale. I materiali coinvolti devono soddisfare una serie di requisiti, quali: elevata resistenza, buona duttilità alle alte velocità di deformazione ed elevata stabilità alle alte temperature, oltre ad una buona resistenza alla corrosione e agli ambienti radioattivi. Ne deriva che i metalli refrattari e le loro leghe risultano essere degli ottimi canditati e lo studio del loro comportamento meccanico deve essere effettuato in un ampio intervallo di velocità di deformazione e di temperatur
An efficient shape-based procedure for strain hardening identification in the post-necking phase
Full text linkNowadays, finite element (FE) codes are increasingly employed for simulating large deformation problems. Thus, to reliably represent the strain hardening behavior, a proper calibration of constitutive laws is essential. Focusing on tensile tests, the main issue with ductile metals is necking occurrence, because of the consequent triaxiality and non-uniformity of the strain and stress states. Over the past decades many strain hardening identification approaches have been proposed. Among them, FE-based inverse methods are widely used, but computationally expensive and time consuming. Hence, the authors propose an efficient method which exploits a database for relating the plastic flow rule and the specimen necking profile. The explicit solver of the nonlinear FE code LSDYNA was used to build the database, whose size could be limited thanks to physical considerations. The developed methodology was applied to experimental quasi-static tensile tests performed on different metals. The predicted hardening laws showed good agreement with those identified with FE-based inverse methods, thus verifying the applicability of the proposed strategy. This study paves the way for machine learning tools having as main input the necking shape: indeed, the present work suggests their feasibility and provides insights into how to establish datasets for a proper and efficient training
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