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At: ENSAIT (Ecole Nationale Supérieure des Arts et Industries Textiles); International audience; During the three last decades several testing configurations have been developed to investigate the dynamic behaviour and damage modes in armour ceramics under high-strain rates loading conditions. The edge-on impact configuration and the pyrotechnic spherical expansion experimental method can be used to investigate the tensile damage and microplasticity mechanisms in ceramics at strain-rates ranging from 103 s-1 to 105 s-1. The Hugoniot Elastic Limit and the spall strength of ceramics can be analysed by means of plate-impact experiments or highpulsed power technologies (GEPI testing facility). The tandem configuration that consists in impacting a ceramic target previously subjected to an impulsive load aims at characterising the strength of a pre-fragmented ceramic under the impact of a penetrating projectile. In the present work, the principle of each testing configuration and the main results obtained with SiC ceramics are introduced. The main observed damage mechanisms and the obtained dynamic response are explained. These experimental data allows identifying a constitutive model of ceramic to be used in a numerical simulation describing the ballistic impact of an AP projectile against a bilayered configuration. This numerical approach is necessary to highlight the state of loading applied to the ceramic front plate and to deduce which are the main mechanical properties involved in the ballistic performance of armour ceramics.

At: ENSAIT (Ecole Nationale Supérieure des Arts et Industries Textiles); International audience; Armour-Piercing (AP) projectiles constitute a major issue when designing military armours for personal and vehicle protection. Bi-layer shieldings are made of a ceramic front plate and a ductile composite/metal backplate to stop, erode and fragment the projectile. The bullet’s core, made of tungsten or strong steel alloy, defines the penetration capability of the projectile so its characterisation is crucial for both understanding failure modes occurring in the core and performing a numerical simulation of impact. Nevertheless, such simulations require constitutive models for which the identification of parameters is often sensitive and even dependent on the specimen itself. In this work, a high-strength steel core extracted from a 7.62 x 39 mm API-BZ projectile is tested in quasi-static loading conditions at room and high temperatures and at dynamic loading rates in order to determine parameters used in the Johnson-Cook plasticity and failure models. Three geometries are investigated: a dog-bone specimen, a Shear-Compression Specimen (SCS) and a modified SCS. The equivalent von Mises stress and the equivalent plastic strain in the sample are deduced so the Johnson-Cook model parameters can be obtained. Results show that, while flow stress resistance is similar for the three specimens, the ductility and failure strains vary to some extent, thus changing some of the parameters used in the Johnson-Cook model. This question raises the concern of the appropriate choice of sample geometry to identify the model parameters.

Some ceramic grades, such as silicon carbide (SiC) or alumina (Al2O3), are used as ballistic materials thanks to their excellent mechanical performances, such as their hardness, while being light, where weight gain is a major issue for the design of military equipment for personal and vehicle protection. Since the Vietnam War, ceramics have been largely used and integrated as front face in bilayer shielding to stop the threat of AP (Armour Piercing)-type projectiles during a ballistic impact. Nevertheless, the projectile leads to an intense damage in the ceramic due to, amongst other phenomena, a dynamic tensile loading that manifests by multiple cracking, called fragmentation, particularly unfavourable for the integrity of the ballistic protection and its capacity to deal with a second impact. In order to develop a more performing shielding material, it is essential to understand the link between the microstructure of ceramics, the damage generated under impact and their ballistic performances.This thesis seeks to better understand the dynamic fragmentation phenomenon generated at high strain rates in high fracture-toughness ceramics, including a bio-inspired alumina material mimicking nacre microstructure. This artificial nacre is, a priori, more crack resistant than conventional ceramics as it is characterised by a high static fracture-toughness due to its specific “Brick-and-Mortar” (or BM) microstructure reproduced in the material called here MAINa., Certaines nuances de céramiques, comme les carbures de silicium (SiC) ou les alumines (Al2O3), sont utilisées comme matériaux balistiques en raison de leurs excellentes performances mécaniques, notamment leur dureté, tout en étant légères, là où le gain de masse est un enjeu majeur pour la conception d’équipements militaires de protection individuelle ou de véhicule. Depuis la guerre du Viêt Nam, les céramiques ont été largement utilisées et intégrées comme face avant de blindage bicouche pour stopper la menace des projectiles de type AP (Armour-Piercing) durant un impact balistique. Néanmoins, le projectile provoque un endommagement intense dans la céramique en raison, notamment, d’un chargement de traction dynamique qui se manifeste par une fissuration multiple, appelée fragmentation, particulièrement défavorable à l’intégrité de la protection balistique et à sa capacité à faire face à un second impact. Afin de développer un matériau de blindage plus performant, il est essentiel de comprendre le lien entre la microstructure des céramiques, l’endommagement généré sous impact et leurs performances balistiques.Cette thèse cherche à mieux comprendre le phénomène de fragmentation dynamique généré à hautes vitesses de déformation dans des céramiques à forte ténacité, incluant un matériau aluminé bio-inspiré de la nacre. Cette nacre artificielle est, a priori, plus réfractaire aux fissures que les céramiques conventionnelles car elle se caractérise par une haute ténacité statique en raison d’une microstructure spécifique de type « Brique-Mortier » (ou BM) reproduite dans le matériau appelé ici MAINa.

International audience