Mechanical characterization and modeling of the heavy tungsten alloy IT180

Pure 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 approach

Digital campaigning: day of reckoning

With the rise of the Tea Party and the decline in President Obama’s ratings, the upcoming US mid-term elections are going to be crucial. So two years after the triumph of Obama’s Internet-fuelled campaign it is a good time to debate the role of digital politics. Polis brought together ePolitics founder Colin Delany, a veteran of 15 years in the digital politics space, and UK political analyst, Anthony Painter, who has written a book about the US 2008 campaign, too

Plastic behavior of laser‐deposited inconel 718 superalloy at high strain rate and temperature

2noNickel‐based superalloys have several applications for components exposed to high temperatures and high strain rate loading conditions during services. The objective of this study was to investigate the tensile properties of Inconel 718 produced using the laser metal deposition technique. Specimens with different heat treatments were investigated. Experimental tests were performed at the DYNLab at Politecnico di Torino (Italy). The temperature sensitivity was investigated between 20 °C and 1000 °C on a Hopkinson bar setup at a nominal strain rate of 1500 s−1. The specimens heating was obtained by means of an induction heating system, and the temperature control was performed by thermocouples, an infrared pyrometer, and a high‐speed infrared camera. The thermal images were analyzed to check the uniformity of the heating and to investigate the presence of adiabatic self‐heating. The results showed that the materials strength exhibited a significant drop starting from 800 °C. The strain rate influence was investigated at room temperature, and limited sensitivity was found covering six orders of magnitude in the strain rate. A preliminary analysis of the fracture mode was performed. Finally, different solutions for the strength material modeling were proposed and discussed with the aim of identifying models to be used in finite element simulations.openopenPeroni L.; Scapin M.Peroni, L.; Scapin, M

Numerical Modeling of Shockwaves Driven by High-Energy Particle Beam Radiation in Tungsten-Made Structures

The investigation of wave propagation in solids requires the development of reliable methods for the prediction of such dynamic events in which the involved materials cover wide ranges of different possible states, governed by plasticity, equation of state, and failure. In the present study, the wave propagation in metals generated by the interaction of high-energy proton beams with solids was considered. In this condition, axisymmetric waves were generated, and, depending on the amount of the delivered energy, different regimes (elastic, plastic, or shock) can be reached. Nonlinear numerical analyses were performed to investigate the material response. The starting point was the energy map delivered into the component as the consequence of the beam impact. The evolution of both hydrodynamic and mechanical quantities was followed starting from the impact and the effects induced on the hit component were investigated. The results showed the portion of the component close to the beam experiences pressure and temperature increase during the deposition phase. The remaining part of the component is traversed by the generated shockwave, which induces high values of strain in a short time or even the failure of the component

Temperature dependence of material behaviour at high strain-rate Proceedings of 24th DYMAT Technical Meeting 9-11 September 2019, Stresa (Italy)

In recent years, interest in material characterization at high strain-rates while varying the temperature has been continuously increasing. Consequently, the study and modelling of material behavior in such conditions has been promoted. In many applications such as machining, metal forming, high velocity impact or high energy deposition of metals, materials are deformed at very high rates. This produces self-heating to high temperatures due to adiabatic processes. In this case, the stress-strain response will be a balance between the effects of hardening (due to strain and strain-rate) and thermal softening. In other cases, the working temperature may be different to room temperature. Hence both the mechanical response of the material and the effect of strain-rate should be investigated in the domain of interest. At high temperature, materials generally become much more ductile and can also exhibit microstructural changes due to recrystallization phenomena. By contrast, at low temperatures the material strength usually increases and the mechanical behavior changes from ductile to brittle. From these considerations, it appears evident that temperature and strain-rate are variables of fundamental importance in the prediction of the mechanical response of materials, playing an important role in many deformation processes. Hence, it is clear there is a need to define proper material models which could be implemented in numerical Finite Element simulations from which it should be possible to predict and estimate the responses of structures, components and materials under different loading conditions and scenarios. Of course, the development of methodologies and facilities for the complete investigation of the mechanical response of materials in the whole temperature and strain-rate field of interest is required and has to be addressed, by also taking into account the fact that temperature and strain-rate are mutually related. This means that the thermal effects obtained from quasi-static tests cannot always be used to predict material response under dynamic loading conditions. Moreover, this reveals that in order to consider the coupled effects of temperature and strain-rate, material models should be used in which the thermal component of stress is also considered

An advanced identification procedure for material model parameters based on image analysis

Inverse methods for the material strength model calibration are widespread techniques, which allow taking into account for the actual strain, strain-rate, temperature and triaxiality fields inside the specimen. An optimization procedure generally starts from experimental measurement of force-stroke time history and is based on the minimization of the difference between experimental and numerically computed quantities. In this work, the strength model identification is performed also on the basis of the specimen shape recorded during the test. This information is imposed as boundary condition, which forces the experimental profile to the external surface of the specimen. The optimization is based on the minimization of the reaction force of the imposed boundary condition. This technique could be applied both to quasi-static and dynamic tests, also at different temperatures, since the only additional requirement is a video of the test with a good compromise in terms of spatial and time resolutions. The methodology is compared with a standard numerical optimization procedure, in order to evaluate the reliability of the method and the advantages/disadvantages of this new approach

Dynamic mechanical behavior of syntactic iron foams with glass microspheres

In this work, the mechanical behavior of syntactic foams made of hollow glass microspheres mixed in an iron matrix was investigated. This type of material is interesting since, when compared to other types of metal foams, it offers greatly increased quasi-static compressive strength, though at lower maximum porosity and thus higher density. Moreover it maintains the advantages and useful properties of metal foams such as thermal and environmental resistance. In particular, the strain-rate sensitivity response was studied. The experimental characterization was performed by means of compression tests at three strain-rate levels: at the highest strain-rate level a SHPB was used. Type and content of glass microspheres were also studied. The experimental results showed that the compression behavior of syntactic foams, similarly to the other types of foams, is strongly affected by all the examined factors. For what concerns the strain-rate, it was found to increase material characteristics in almost all cases. The influence of the matrix behavior on the composite was identified as the determining parameter in this respect. In order to evaluate the results obtained with the described tests campaign, the experimental data were further elaborated by means of an empirical analytical strain-rate sensitive model. The dependency of the material response on model parameters was widely discusse

ANALISI DELLA CURVA DI FLUSSO PLASTICO DI MATERIALI METALLICI BASATA SULLA MISURA SPERIMENTALE DEL PROFILO DI NECKING

L’identificazione dei modelli di flusso plastico a partire dalla realizzazione di prove di trazione, nonostante la semplicità della prova, presenta alcuni problemi legati allo sviluppo di condizioni di sollecitazione triassiale conseguenti al raggiungimento della condizione di necking. Numerosi approcci sono stati proposti in passato per correggere la misurazione canonica di forza e allungamento rilevata durante la prova, al fine di determinare la legge true stress-true strain del materiale e identificare, così, i parametri di modelli costitutivi elasto-plastici. Nel presente lavoro viene proposto un approccio basato su una combinazione di analisi digitale dell’immagine e ottimizzazione numerica di modelli FEM: l’evoluzione del profilo del provino dogbone rilevato durante una prova di trazione viene impiegata direttamente per l’identificazione della legge di flusso plastico del materiale. La metodologia presentata è stata applicata per il rame HDHC e comparata con le tecniche di indagine convenzionali

STUDIO DEL COMPORTAMENTO DI METALLI REFRATTARI AD ELEVATI STRAIN-RATE E TEMPERATURE

L’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 temperature