CRACK BRIDGING AND TRAPPING IN BOROSILICATE MATRIX COMPOSITES WITH DISTRIBUTED METAL PARTICLES

Abstract One of the most efficient ways to improve the fracture toughness of ceramics is to reinforce them by large volume fraction of bonded metal particles (e.g., Krstic [1]; Evans and McMeekin

Understanding the edge crack phenomenon in ceramic laminates

Layered ceramic materials (also referred to as “ceramic laminates”) are becoming one of the mostpromising areas of materials technology aiming to improve the brittle behavior of bulk ceramics. The utilizationof tailored compressive residual stresses acting as physical barriers to crack propagation has already succeededin many ceramic systems. Relatively thick compressive layers located below the surface have proven veryeffective to enhance the fracture resistance and provide a minimum strength for the material. However, internal compressive stresses result in out-of plane stresses at the free surfaces, what can cause cracking of thecompressive layer, forming the so-called edge cracks. Experimental observations have shown that edge cracking may be associated with the magnitude of the compressive stresses and with the thickness of the compressive layer. However, an understanding of the parameters related to the onset and extension of such edge cracks in the compressive layers is still lacking. In this work, a 2D parametric finite element model has been developed to predict the onset and propagation of an edge crack in ceramic laminates using a coupled stress-energy criterion. This approach states that a crack is originated when both stress and energy criteria are fulfilled simultaneously. Several designs with different residual stresses and a given thickness in the compressive layers have been computed. The results predict the existence of a lower bound, below no edge crack will be observed, and an upper bound, beyond which the onset of an edge crack would lead to the complete fracture of the layer.&nbsp

Quantification of increasing fracture toughness of glass matrix reinforced by alumina platelets composite

A borosilicate glass matrix composite containing alumina platelets was considered to investigate toughening mechanisms and crack tip behaviour in dispersion reinforced brittle matrix composites. Fracture toughness was determined applying the chevron notched specimen technique and fractographic analysis was employed to reveal the active toughening mechanisms with increasing content of reinforcement. A roughness-induced shielding effect has been quantified to prove the relation between fracture toughness and fracture surface roughness. Theoretical calculations of the fracture toughness enhancement based on a modified crack deflection model developed by Faber and Evans, combined with the influence of the increase in Young’s modulus, were found to be in good agreement with experimental data. The crack deflection model was further extended to capture a synergy between crack deflection and the contribution of residual stresses to toughening in the investigated composites

ADIABATIC SHEAR IN POROUS MEDIA

On propose un modèle avancé pour décrire la déformation de cisaillement qui se produit lors de la déformation à grande vitesse d'un solide poreux ductile réalisée par un simple chargement en cisaillement plan auquel est superposée une pression hydrodynamique. Afin de décrire un matériau thermoplastique rigide et poreux, on utilise la théorie de Gurson sur la plasticité en expansion. L'apparition d'un cisaillement adiabatique est interprétée en terme d'instabilités mathématiques dans les équations différentielles appropriées. On démontre l'influence substantielle de la porosité sur le durcissement et sur l'adoucissement thermique.An advanced model of shear deformation occuring in high strain rate deformation of ductile porous solids is proposed under simple planar shear loading superposed by hydrostatic pressure. To describe porous - rigid thermoplastic material the Gurson theory of dilatant plasticity is applied and appearance of an adiabatic shear is interpreted in terms of mathematical instabilities in the underlying differential equations. The substantial influence of porosity on strain hardening and thermal softening effects is demonstrated

Understanding the edge crack phenomenon in ceramic laminates

Layered ceramic materials (also referred to as “ceramic laminates”) are becoming one of the most promising areas of materials technology aiming to improve the brittle behavior of bulk ceramics. The utilization of tailored compressive residual stresses acting as physical barriers to crack propagation has already succeeded in many ceramic systems. Relatively thick compressive layers located below the surface have proven very effective to enhance the fracture resistance and provide a minimum strength for the material. However, internal compressive stresses result in out-of plane stresses at the free surfaces, what can cause cracking of the compressive layer, forming the so-called edge cracks. Experimental observations have shown that edge cracking may be associated with the magnitude of the compressive stresses and with the thickness of the compressive layer. However, an understanding of the parameters related to the onset and extension of such edge cracks in the compressive layers is still lacking. In this work, a 2D parametric finite element model has been developed to predict the onset and propagation of an edge crack in ceramic laminates using a coupled stress-energy criterion. This approach states that a crack is originated when both stress and energy criteria are fulfilled simultaneously. Several designs with different residual stresses and a given thickness in the compressive layers have been computed. The results predict the existence of a lower bound, below no edge crack will be observed, and an upper bound, beyond which the onset of an edge crack would lead to the complete fracture of the laye

Micromechanics of initiation of craze growth in glassy polymers under the effect of environment

Translated from Czech (Kovove Materialy 1996 (4) p. 217-229)SIGLEAvailable from British Library Document Supply Centre-DSC:9023.190(8423)T / BLDSC - British Library Document Supply CentreGBUnited Kingdo

Numerical simulation of plastic deformation and the accompanying acoustic emission signal

26.00; Translated from Czech. (Kovove Mater. 1989 v. 27(2) p. 228-239)SIGLEAvailable from British Library Document Supply Centre- DSC:9023.19(VR-Trans--4452)T / BLDSC - British Library Document Supply CentreGBUnited Kingdo