Shockless spalling damage of alumina ceramic

Ceramic materials are commonly used to build multi-layer armour. However reliable test data is needed to identify correctly models and to be able to perform accurate numerical simulation of the dynamic response of armour systems. In this work, isentropic loading waves have been applied to alumina samples to induce spalling damage. The technique employed allows assessing carefully the strain-rate at failure and the dynamic strength. Moreover, specimens have been recovered and analysed using SEM. In a damaged but unbroken specimen, interactions between cracks has been highlighted illustrating the fragmentation process

An analysis of the propagation of front shock in concrete

The aim of the authors is to model the shock response of concrete structures submitted to the effects of projectile penetration or contact detonation, in a range of pressure levels from 0 to 20 GPa. The limitations of current computers imply the need to homogenize the response of the different constituents of concrete into a single macroscopic model. Though concrete is widely used as a construction material, the knowledge concerning its response under shock loading response remains rather modest. An exhaustive review of the research effort in this field shows that the limited available data is affected by an important dispersion. As a consequence, any general rule correlating the composition of concrete to its shock properties have not yet been evidenced. A simple method to predict the shock Hugoniot of concrete based on a mixture theory is developed considering concrete as a mix of cement hydrates, free water, rock and voids. Experimental results obtained on special concrete compositions are presented. They illustrate the relation between the wave structure and the size of the aggregates, and so, the level of heterogeneity of the composition. Numerical simulations demonstrate the ability of a mesoscopic model to describe this phenomenon, and the failure of the homogenized approach to do so

Shock characterization of an ultra-high strength concrete

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

Modelling of concrete under high confinement : a mesoscale approach with damage behaviour

International audienc

Comportement du béton sous fort confinement: Effet du rapport eau/ciment

cited By 1International audienceThis study concerns the identification of concrete behaviour under severe triaxial loadings (rock falls, explosions, ballistic impacts). In order to reproduce high stress levels with well controlled loading paths, static tests are carried out on concrete samples by means of a very high capacity triaxial press (stress levels of the order of the GigaPascal). In addition, it is known for a long time that the water/cement ratio (W/C), entering the concrete composition, is one of the major parameters conditioning the porosity and the strength of the cement matrix of the hardened concrete. The objective of this article is to quantify the effect of this ratio on the concrete behaviour under high confinement. From the composition of a reference “ordinary” concrete (W/C = 0, 6), two other concretes were formulated with W/C ratios equal to 0,4 and 0,8 respectively. This article presents experimental results concerning the effect of the water/cement ratio (W/C) of the concrete on its behaviour under high confinement. The test results presented in this article show that under high confinement, the concrete behaves as a granular stacking which the concrete constitutes without any influence of the cement matrix strength. © 2008 Taylor & Francis Group, LLC

Concrete behavior under very high stresses

cited By 0International audienceThis communication concerns the concrete behavior under extreme loading situations (near field detonation or ballistic impacts). During such loadings, concrete material is subjected to very high-intensity triaxial stress states. The validation of concrete behavior models, which simultaneously take into account the phenomena of brittle damage and irreversible strain such as compaction, thus requires test results that enable reproducing complex loading paths. Since 2004, the University of Grenoble has launched in collaboration with the French Ministry of Defense (DGA, Centre d'Etudes de Gramat) a research program on the vulnerability of concrete infrastructures. This presentation is devoted to the results of this program. Triaxial tests on a low-strength plain concrete have been performed, using a large capacity triaxial press named "GIGA". Stress levels overpassing one GigaPascal with homogeneous, static and well controlled loading paths have been reached. The results show that contrary to what is observed in simple compression, when placed under high confinement, concrete behaves like a granular stacking. There is not any influence of the level of the cement matrix strength whereas the saturation ratio exerts a major influence. The concrete strength capacity can be divided by 5 if the concrete is saturated instead of dried. ©2009 Society for Experimental Mechanics Inc

Influence of aggregate size and free water on the dynamic behaviour of concrete subjected to impact loading

Concrete is a material widely used in civil engineering. Thus the knowledge of its mechanical behaviour is a major safety issue to evaluate the ability of a structure to resist to an intense dynamic loading. In this study, two experimental techniques have been applied to a micro-concrete and a common concrete to assess the influence of the aggregate size on the dynamic response. First, spalling tests on dry and wet specimens have been performed to characterize the tensile strength of concrete at strain rates in the range 30 – 150/s. Then, edge-on impact tests in sarcophagus configuration have been conducted. The cracking pattern of the micro-concrete and the concrete plates in wet and dry conditions have been compared to appraise the influence of aggregate size and free water on the damaging process

Tensile strength of dried mortar over a wide range of strain rate

To investigate the tensile strength of a dried mortar over a wide range of strain-rate (1e-5 s−1 – 5e3 s−1), quasi-static tests and dynamic tests have been conducted on dry specimens. The device used for the quasi-static experiments is a hydraulic press armed with ball-and-socket joints on both sides of the loading platens between which the mortar specimen is glued. In parallel, based on the spalling technique, a Hopkinson bar device has been used for intermediate loading rates (range of strain-rate from 40 s−1 to 140 s−1). Complementary, based on pulsed power technology, spall experiments have been performed on the mortar with the GEPI device at Centre d'Etudes de Gramat to reach the highest strain rates. The experimental results show a significant increase of strength with strain-rate over the range of loading rate considered

A mesoscopic model for the behaviour of concrete under high confinement

cited By 50International audienceWhen impact loaded, concrete is submitted to high triaxial stresses. The experimental response of concrete under quasi-static triaxial compression is studied using a triaxial press capable of applying a mean pressure greater than 1GPa on cylindrical samples measuring 7 cm in diameter and 14 cm high. A numerical analysis of these previous experiments is performed herein at a mesoscopic scale. Concrete is modelled as a biphasic material consisting of a mortar (cement paste and fine aggregates) and roughly spherical aggregates (with a diameter exceeding 2 mm) whose characteristics are applied on a regular cubic finite element mesh. A damage-plasticity model is then used to model the behaviour of mortar. An identification of model parameters on mortar samples and the subsequent comparison between numerical and experimental tests will be presented for hydrostatic and triaxial compression. Copyright © 2009 John Wiley & Sons, Ltd