Study On Failure Behavior Of Ceramics Subjected To Plate Impact Loading

Ceramics are widely applied in national defense engineering and military science asarmor against kinetic energy penetrators with their good physical and mechanicalcapabilities of high compressive strength, remarkable hardness, strong temperatureresistance and low density. The dynamic response and failure behaviors of brittleceramics under impulsive loading are closely related to its impact resistance, havebecome an important area of research in the impact dynamics field. So far the study onthe impact compression failure of ceramics has mostly focused on the experimentalwork and phenomenological understanding. And, there still has been no universallyagreed model to describe the physical mechanism and evolution of impact compressionfailure. Thus the study on the impact compression failure behaviors can provide thetheoretical basis for the design and optimization of ceramic protective armor, which hasimportant academic significance and application value.Recent researches on the dynamic response and failure behaviors in shocked ceramicsat home and abroad were reviewed and open questions were analyzed in this thesis. TheA95alumina ceramic was adopted to be the main research object, and the experimental,theoretical researches and numerical simulation on wave motion and dynamic failurebehaviors of ceramics have been carried out in the dissertation. The impact failuremechanism and evolution of ceramics have been also discussed and analyzed in detail.The principal work and conclusions in this dissertation are as follows:1. The failure mode, dynamic constitutive model, experimental techniques and testingmethods of shocked ceramics were reviewed on the related literatures. The physicalmechanism of the formation and propagation of failure waves in shocked glasses andceramics were investigated specially.2. A series of plate impact experiments have been designed and conducted on aluminaspecimens with different thicknesses through the light gas gun. The rear free surfacevelocity histories of shocked alumina were monitored by VISAR and the recoveredspecimens were observed by scanning electron microscope in order to research theinelastic response and failure characteristics in shocked alumina. The results showedthat the alumina specimen failed catastrophically under impact loading in theseexperiments, and the principal failure mode was due to the evolution of microcrackswith little plastic deformation. 3. The original definition of Hugoniot Elastic Limit was reviewed in thedissertation. Based on this idea and associated with the failure characteristics of theceramics, the Mohr-Coulomb failure criterion was adopted to establish the relationshipHugoniot Elastic Limit and the shear strength of ceramics. The effects of impactintensity, strain rate and material microstructure on Hugoniot Elastic Limit of aluminawere discussed with the plate impact experimental results.4. The modified stress wave theory, which demonstrates that a volumetric wave and arotational wave as well as a deviatoric wave propagate in linear elastic solids, wasresearched in this dissertation. And the properties of wave motion equations,kinematical behaviors and corresponding stress state of these three stress waves wereanalyzed in detail. Considering the plate impact compression condition, rotationaldeformation of the solids is constrained. So only a volumetric wave and a deviatoricwave can propagate in the medium. The propagation and surface effect of these twostress waves have been researched in the plate impact experiment.5. Based on the modified stress wave theory and mesoscopic heterogeneous ofceramics, it is pointed out that the formation of failure wave in brittle material underimpulsive loading requires two main factors:1) presence of mesoscopic heterogeneityin ceramics, namely containing the original microdefects such as microcracks,microvoids.2) presence of sufficient deviatoric strain energy under shock loading,namely the intensity of deviatoric waves must exceed the failure threshold of materials.The mechanism of transformation of energy was discussed associate with themesoscopic physical mechanism of propagation of failure waves. It is considered thatthe failure propagation process converts the deviatoric strain energy in the intactmaterial ahead of the failure front to the volumetric potential energy in the comminutedmaterial behind the front, resulting in the change in the types of the governingdifferential equations.6. The failure wave should be described by a diffusion equation instead of a waveequation through the analysis of transition between governing equations ahead of andbehind the failure front. Base on this idea, a nonlinear diffusion equation and thecorresponding constitutive relation were built up to describe the evolution of failure inshocked brittle materials. Finally numerical modeling propagation of failure waves inglasses and alumina under plate impact loading were carried out according topropagating features of failure waves.