Hot rolling of bars issued from continuous-casting aims at refining the material structure and guaranteeing the central soundness of the metallurgical product. The rolling route must be designed to achieve the complete closure of the shrinkage porosity inherent in the continuous casting process. To predict the void evolution, many models exist that can be implemented in the finite element simulation of the process. Nevertheless, these models need parameter adjustments to be adapted to the forming process, the formed material, and the real geometry of the void. Real scale tests being very expensive in the long product rolling mill, an improved representativeness experimental configuration was designed to reproduce at the laboratory scale the key characteristics of the thermomechanical path driving the void closure phenomenon. This testing consists of successive forming stages with shaped anvils applied to samples containing a shrinkage cavity. The shaped anvils and the forming conditions are calibrated to reproduce the levels of strain and the stress triaxiality of rolling stands, and the alternation of the forming direction of the industrial process. The geometry of the voids before and after the forming paths are measured by tomography. The simulation of the test with an explicit modelling of the void is developed parallel to the experiments. The simulation/experiment comparison allows the validation of the numerical model. The obtained model will be used in future works to perform a more extended design of experiments to characterise void closure during hot rolling of bars.

CEA-Gramat studies the sensitivity of energetic materials to enhance their security and reliability. The conditions leading to the initiation of an explosive must be understood to control its sensitivity. According to the hot spots theory, the shock initiation of heterogeneous explosives is related to their microstructure: the shock interacts with the heterogeneities of the microstructure (pores and inclusions, morphology of grains and fragments, debonding, etc.) and creates local deposits of energy. To describe these hot spots, energetic materials have to be modeled at a scale allowing the discretization of their microstructure: the mesoscale. Micro-computed tomographies of energetic materials are done at CEA-Gramat and analyzed to build geometric models used in finite element simulations. Two kinds of models are studied:-Real models are directly built on the real microstructures extracted from micro-computed tomographies.-Virtual models are based on the same microstructures but simplified to study independently the effects of microstructural parameters (granulometry, porosity, filler content{\ldots}) on the creation of hot spots. Compositions based on different kind of RDX particles in an inert binder are studied through numerical simulation. The influence of particle shape on the inert shock response is investigated at the mesoscale. Local heterogeneities of pressure and temperature fields appear intimately related to the morphological properties of the microstructures. Particles with sharp edges create more hot spots than spherical particles.

Cemented tungsten carbide, with its very high density and high strength, is known to be the material composing several small calibre armour piercing ammunitions. The impact of a tungsten carbide core projectile onto a high efficiency armour often leads to the fracture of the tungsten carbide core. Thus, the pertinence of material models used in numerical simulations to describe the behaviour and the damage of the target is not sufficient to well predict a ballistic impact. In this work, the GEPI high-pulsed power generator is used to conduct dynamic characterization of the behaviour of a cemented tungsten carbide under both compressive and tensile loadings. The Hugoniot Elastic Limit of this material has been identified (HEL = 5.8 GPa). Moreover, Lagrangian analysis allowed the complete loading path to be identified up to 18 GPa.

Subjected to a dynamic traction, a ceramic material will eventually fail as a consequence of the triggering, propagation and coalescence of a distribution of microcracks. The DFH (Denoual-Forquin-Hild) anisotropic damage model considered in this study is based on a description of three physical phenomena activated all along the fragmentation process occurring in brittle materials. The first one concerns the activation of the population of preexisting defects distributed in the material, described by a Weibull law. The second one corresponds to the propagation of microcracks at a constant velocity. The last one is the so-called occultation phenomenon. It is based on the observation that in the vicinity of a crack, tensile stresses are relaxed, hence precluding the triggering of another crack in the same direction. The performance of DFH model has been assessed with the aim of improving the modeling of damage associated to spalling in ceramic materials. Dynamic tensile loadings have been performed at different strain rates, by means of a plate impact and a quasi-isentropic compression followed by a release wave. The characteristics of different patterns of damage observed experimentally have been accurately reproduced by means of three-dimensional Lagrangian calculations.

The impact response of silicon carbide ceramics is a key issue due to the fact that they are increasingly used in lightweight armor solutions. In the present work, four silicon carbides with different microstructural properties are obtained by varying the sintering process (pressureless or spark plasma sintering in liquid or solid state). The dynamic fragmentation activated due to impact is investigated through three specific experiments: Edge-on Impact experiments are conducted in open, and sarcophagus configurations. Additionally, normal impact tests are performed with a sandwich configuration. The damage growth is observed with an ultra-high speed camera (open configuration), and the failure pattern and crack density are analysed by means of post-mortem observations of the recovered specimens after impact, or through an analysis of the fragment-size distribution (sarcophagus and sandwich configurations). The tests highlight the major role of the microstructure on the fragmentation process. The influence of microstructure is examined based on a closed-form analytical solution resting on the Denoual-Forquin-Hild (DFH) damage model. This micromechanical model predicts the fragmentation characteristics of ceramics based on a statistical description of the flaw population. A good correlation between the model predictions and experimental data confirms that the impact response of a ceramic is primarily determined by the flaw population disseminated in its microstructure.

Ceramics are particularly interesting as protective materials due to their high compressive strength. However the maximum tensile stress withstood by these materials is usually lower by one order of magnitude. To study the dynamic strength in tension, spalling tests are commonly performed. In this work, a new spalling configuration is presented to characterize brittle materials such as ceramics at ultra-high strain-rates of about 103–104 s−1 in a one-dimensional stress state. To do so, a specific test is designed in which the ceramic specimen is a bar with a 3 mm radius. This geometry is chosen to ensure a one-dimensional stress state in the cylindrical specimen avoiding wave dispersion during the propagation of the loading pulse. Finally, the first experimental validation tests conducted at the CEA-Gramat with the pulsed power generator called GEPI are reported.

Chapter 12 Shockless Characterization of Ceramics Using High-Pulsed Power Technologies Jean-Luc Zinszner, Jean-Luc ZinsznerSearch for more papers by this authorBenjamin Erzar, Benjamin ErzarSearch for more papers by this authorPascal Forquin, Pascal ForquinSearch for more papers by this author Jean-Luc Zinszner, Jean-Luc ZinsznerSearch for more papers by this authorBenjamin Erzar, Benjamin ErzarSearch for more papers by this authorPascal Forquin, Pascal ForquinSearch for more papers by this author Book Editor(s):David Edward Lambert, David Edward LambertSearch for more papers by this authorCrystal L. Pasiliao, Crystal L. PasiliaoSearch for more papers by this authorBenjamin Erzar, Benjamin ErzarSearch for more papers by this authorBenoit Revil-Baudard, Benoit Revil-BaudardSearch for more papers by this authorOana Cazacu, Oana CazacuSearch for more papers by this author First published: 31 December 2018 https://doi.org/10.1002/9781119579311.ch12 AboutPDFPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShareShare a linkShare onFacebookTwitterLinked InRedditWechat Summary Due to the wealth and diversity of their properties, advanced ceramic materials are well known for a wide range of applications, such as biomedical implants, electrical insulators, semiconductors or conductors, filters and membranes against aggressive environment. The microstructure of a ceramic plays a major role in its ballistic performance. Depth-of-penetration experiments are often conducted to characterize the ballistic resistance of a ceramic. The GEPI generator, and in general, all the high-pulsed power generators characterized by a rise time of hundreds of nanoseconds, can be used to perform isentropic compression experiments. The Lagrangian analysis is based on a comparison of signals measured on two specimens of different thicknesses of the tested material. The dynamic tensile strength of ceramics is usually characterized by spalling using planar impact experiments. The access to the damage pattern created by the dynamic loading constitutes a unique insight into the mechanisms driving the fragmentation process in brittle materials. Dynamic Damage and Fragmentation RelatedInformation

To evaluate the vulnerability of ultra-high performance fibre-reinforced concrete (UHPFRC) infrastructure to rigid projectile penetration, over the last few years CEA-Gramat has led an experimental and numerical research programme in collaboration with French universities. During the penetration process, concrete is subjected to extreme conditions of pressure and strain rate. Plasticity mechanisms as well as dynamic tensile and/or shear damage are activated during the tunnelling phase and the cratering of the concrete target. Each mechanism has been investigated independently at the laboratory scale and the role of steel fibres especially has been analysed to understand their influence on the macroscopic behaviour. To extend the experimental results to the structural scale, penetration tests on UHPFRC slabs have been conducted by CEA-Gramat. The analysis of this dataset combined with material characterization experiments allows the role of steel fibres to be identified in the different plasticity and damage mechanisms occurring during penetration. In parallel, some improvements have been introduced into the concrete model developed by Pontiroli, Rouquand and Mazars (PRM model), especially to take into account the contribution made by the fibres in the tensile fracture process. After a primary phase of validation, the capabilities of the PRM model are illustrated by performing numerical simulations of projectile penetration into UHPFRC concrete structures.This article is part of the themed issue 'Experimental testing and modelling of brittle materials at high strain rates'.

Ceramic materials are commonly used to design multi-layer armour systems thanks to their favourable physical and mechanical properties. However, during an impact event, fragmentation of the ceramic plate inevitably occurs due to its inherent brittleness under tensile loading. Consequently, an accurate model of the fragmentation process is necessary in order to achieve an optimum design for a desired armour configuration. In this work, shockless spalling tests have been performed on two silicon carbide grades at strain rates ranging from 103 to 104 s(-1) using a high-pulsed power generator. These spalling tests characterize the tensile strength strain rate sensitivity of each ceramic grade. The microstructural properties of the ceramics appear to play an important role on the strain rate sensitivity and on the dynamic tensile strength. Moreover, this experimental configuration allows for recovering damaged, but unbroken specimens, giving unique insight on the fragmentation process initiated in the ceramics. All the collected data have been compared with corresponding results of numerical simulations performed using the Denoual-Forquin-Hild anisotropic damage model. Good agreement is observed between numerical simulations and experimental data in terms of free surface velocity, size and location of the damaged zones along with crack density in these damaged zones.This article is part of the themed issue 'Experimental testing and modelling of brittle materials at high strain rates'.

The dynamic response of brittle materials like ceramics is usually studied by means of flyer plate impact experiments. In this work, original tests has been carried out on several ceramics (alumina and silicon carbides) using a high pulsed power generator named GEPI. This electromagnetic device allows obtaining a ramp loading, spread over 500 ns. This particularity is a great advantage to get data on brittle fragmentation through spalling tests in which strain-rate can be accurately determined. Several samples have been recovered partially damaged, giving an interesting insight into the fragmentation process. The ramp loading has also been used to study the compressive response of ceramics thanks to lagrangian analysis. Data have been gathered up to more than 15 GPa on a silicon carbide.

To evaluate the vulnerability of ultra-high performance fiber reinforced concrete (UHPFRC) infrastructure to rigid projectile penetration, CEA-Gramat has led since few years an experimental and numerical research program in collaboration with French universities. During the penetration process, concrete is subjected to extreme conditions of pressure and strain-rate. Plasticity mechanisms as well as dynamic tensile and/or shear damages are activated during the tunneling phase and the cratering of the concrete target. Each mechanism has been investigated independently at the laboratory scale and the role of steel fibers has been specially analyzed to understand their influence on the macroscopic behavior. In parallel, some improvements have been introduced into the concrete model developed by Pontiroli, Rouquand and Mazars (PRM model), especially to take into account the fibers contribution in the tensile fracture process. The capabilities of the PRM model have been illustrated by performing numerical simulations of material characterization experiments. Next step will be to assess the concrete model to simulate projectile penetration into UHPFRC concrete structures.

Nowadays, the design of protective structures may imply ultra-high performance concretes. These materials present a compressive strength 5 times higher than standard concretes. However, few reliable data on the shock response of such materials are available in the literature. Thus, a characterization of an ultra-high strength concrete has been conducted by means of hydrostatic and triaxial tests in the quasi-static regime, and plate impact experiments for shock response. Data have been gathered up to 6 GPa and a simple modelling approach has been applied to get a reliable representation of the shock compression of this concrete.

Ceramic materials are commonly used as protective materials for infantry soldiers and military vehicles. However, during impact, intense fragmentation of the ceramic material is observed. This fragmentation process has to be correctly numerically simulated if one wants to accurately model the dynamic behaviour of the ceramic material during impact. In this work, shockless spalling tests were performed on an alumina ceramic using the high-pulsed power generator (GEPI) equipment. These spalling tests allowed us to master the experimental strain-rate magnitude of the tensile loading applied to the specimen. The spall strength is observed to be rate dependant and the experimental configuration allowed for recovering damaged but unbroken specimen which gives further insights about the fragmentation process initiated in this ceramic material. The collected experimental data has been compared with corresponding numerical simulations conducted with the DFH (Denoual–Forquin–Hild) anisotropic damage model. This modelling approach relies on the description of the main basic micromechanisms activated at high loading rates using physical parameters related to the population of defects that produces multiple cracking in the ceramic material at high strain-rates. Very good agreement was observed between numerical simulations and experimental data in terms of free-surface velocity, size and location of the damaged zones along with crack density in these damaged zones.

Ceramic materials are commonly used as protective materials particularly due to their very high hardness and compressive strength. However, the microstructure of a ceramic has a great influence on its compressive strength and on its ballistic efficiency. To study the influence of microstructural parameters on the dynamic compressive behaviour of silicon carbides, isentropic compression experiments have been performed on two silicon carbide grades using a high pulsed power generator called GEPI. Contrary to plate impact experiments, the use of the GEPI device and of the lagrangian analysis allows determining the whole loading path. The two SiC grades studied present different Hugoniot elastic limit (HEL) due to their different microstructures. For these materials, the experimental technique allowed evaluating the evolution of the equivalent stress during the dynamic compression. It has been observed that these two grades present a work hardening more or less pronounced after the HEL. The densification of the material seems to have more influence on the HEL than the grain size.

To better evaluate the vulnerability of reinforced concrete infrastructures to rigid projectile penetration, CEA-Gramat has conducted for a few years a research program with the help of French universities. The different phases appearing during the penetration process, like tunneling, scabbing or cratering, are respectively associated to several loadings like compression under high confinement, dynamic tension or shear. Each mechanism has been studied independently in laboratory at the scale of concrete material, and some parameters like curing conditions, saturation ratio, water/cement ratio, paste volume have been specially analyzed to understand their influence on the concrete behaviour. To translate the experimental results from the material scale to the structure scale, penetration tests have been performed at CEA-Gramat. Using an ogival nose projectile embedding an accelerometer system recorder, about twenty impact experiments have been carried out for velocities ranging from 230 m/s to 450 m/s. Targets were cylindrical concrete slabs with different concrete compositions. The analysis of these penetration results coupled with material characterization experiments allow identifying the material parameters playing a major role in the different mechanisms occurring during penetration. This entire experimental database can be used to validate our numerical concrete model called PRM model (Pontiroli, Rouquand, Mazars).

To be able to model accurately the interaction of a bullet with a hard armor system including a ceramic layer, one has to have a good understanding of the damage mechanisms involved. The dynamic behavior of ceramic is usually studied by means of flyer plate impact experiments. In this work, a generator applying high pulsed power technologies allows obtaining data on fragmentation of ceramics, on strain-rate sensitivity of tensile strength and it can also be used to perform compression tests at very high stresses (more than 20 GPa). The experimental configurations are presented and illustrated with experimental results gathered on two silicon carbides.

With the exponential increase of computational power, numerical simulations are more and more used to model the response of concrete structures subjected to dynamic loadings such as detonation near a concrete structural element or projectile-impact. Such loadings lead to intense damage modes resulting from high strain-rate tensile loadings in the concrete structure. However, the modelling of the post-peak tensile response of concrete still remains difficult due to the lack of experimental data at high strain-rates. This work aims at improving the modelling of the softening behaviour of concrete based on the following statement: despite the propagation of unstable cracks in the tested specimen cohesion strength exists in the vicinity of triggered cracks and is driving the whole softening behaviour of concrete. This statement is justified in the present work by means of experiments and Monte-Carlo calculations: firstly, concrete samples have been subjected to a dynamic tensile loading by means of spalling experiments. Several specimens have been recovered in a damaged but unbroken state and have been subsequently loaded in quasi-static tensile experiments to characterise the residual strength and damage level in the sample. In addition, Monte-Carlo simulations have been conducted to clarify the possible influence of cohesion strength in the vicinity of cracks. Finally, the DFH (Denoual-Forquin-Hild) anisotropic damage model has been adapted to take into account the cohesion strength in the damaged zone and to describe the softening behaviour of concrete. Numerical simulations of experiments conducted on dry and saturated samples at different levels of loading-rate illustrate the new capability of the model. (C) 2014 Elsevier Ltd. All rights reserved.

During the lifetime of a structure, concrete and mortar may be exposed to highly dynamic loadings, such as impact or explosion. The dynamic fracture at high loading rates needs to be well understood to allow an accurate modeling of this kind of event. In this work, a pulsed-power generator has been employed to conduct spalling tests on mortar samples at strain-rates ranging from 2 × 104 to 4 × 104 s−1. The ramp loading allowed identifying the strain-rate anytime during the test. A power law has been proposed to fit properly the rate-sensitivity of tensile strength of this cementitious material over a wide range of strain-rate. Moreover, a specimen has been recovered damaged but unbroken. Micro-computed tomography has been employed to study the characteristics of the damage pattern provoked by the dynamic tensile loading.

In the framework of its deterrence missions, CEA has developed the so-called "Standards of Simulation", a methodology to guaranty the precision of High Performance Computation (HPC) predictions. Recently, a similar need has been expressed by the French Defence community concerning the modelling of the mechanical and thermal effects of conventional weapons. This paper presents key elements of the transposed methodology, as well as an illustration of its application to a configuration of blast wave diffraction on a two-storey building resulting from an air-to-ground strike.