G Vekinis

R-curve behaviour of Al2O3 ceramics

Abstract The fracture micromechanics and underlying physical processes of fracture in Al2O3-based ceramic specimens have been studied as a function of grain size by instrumented in situ dynamic scanning electron microscopy (SEM) using the double torsion technique. The toughness is found to increase with grain size. Crack bridging is found to extend over hundreds of grain diameters behind the crack tip, resulting in R-curve behaviour. Evidence is amassed which points to frictional energy dissipation, rather than distrubuted microcracking or crack-closure due to elastic ligaments, as the dominant contribution to toughening. The friction occurs at grains which bridge the crack faces and are pulled out as the faces separate. Restraining stresses, which constrain the bridging grains in their sockets, are believed to be the result of both grain morphology and the thermal expansion anisotropy of the material. Simple modelling indicates that only a few percent of the grains need be involved in the frictional process to account for the toughening. The conclusion is supported by hysteresis measurements.

Plaster of Paris as a model material for brittle porous solids

Plaster of Paris is a brittle, porous solid, easy to shape, which has potential as a model material for the study of brittle, porous, solids such as ceramics, rocks and cement. This paper describes the mechanical properties of plaster of Paris — modulus, strength, fracture toughness, etc. — as a function of porosity. The material is then used to study the initiation and propagation of cracks in compression, as a function of porosity, stress state and stress concentration.

The compressive failure of alumina containing controlled distributions of flaws

Abstract Alumina samples containing a controlled number and size of crack-line flaws were tested in simple compression. Crack extension and linkage were observed during loading by in situ scanning electron microscopy. Youngs modulus, the stress for the initiation of crack growth and that for final failure were recorded. The observations, the most complete on a real ceramic system (as opposed to model materials like PMMA) confirm mechanisms which have been proposed for compressive fracture of brittle solids.

Direct observation of the fracture of CAS-Glass/SiC composites

The fracture behaviour of a CAS-glass/SiC-fibre-reinforced composite was observed by dynamic in situ scanning electron microscopy (SEM). In a companion paper [1], tests on common delamination geometries are described and the basis of micromechanics models is critically evaluated. Flexure geometries and also the unnotched tensile response of ceramicmatrix composites (CMCs) have received considerable attention, both theoretically and experimentally. The effect of through-thickness notches on the tensile fracture of CMCs has however been relatively neglected. Previous work on polymer-matrix composites demonstrates the strong influence of subcritical damage on the fracture behaviour. In this paper we examine failure of notched CAS-glass/SiC composites in tension, under static- and fatigue-loading conditions, using a combination of in situ and conventional test methods. The subcritical damage which forms is compared with that in polymer-matrix composites, and the consequences for prediction of the notched strength are discussed.

The micromechanisms of fracture of alumina and a ceramic-based fibre composite: Modelling the failure processes

Abstract The development of structural ceramics and ceramic composites often relies on assumptions about their cracking and fracture characteristics under load. It is most important to understand the nature of the cracking processes and the interaction between neighbouring cracks. In other words, a clear picture of the dynamic micromechanisms of cracking and the accumulation of damage is essential to the development of sound physical models to explain measurements of the strength and toughness of these materials.