Deformation and fracture of materials are usually considered in terms of brittle and ductile modes. However, there is an intermediate class between these limits that is termed quasi-brittle where there is measurable inelastic deformation prior to final graceful ‘brittle’ failure. In previous work, we presented deformation and fracture data obtained tests conducted using a model material containing different known amounts of up to about 30% surrogate porosity. The model material is a hemi-hydrate gypsum plaster with additions of expanded polystyrene beads in the size range 1.5–2.0 mm dia. Use of gypsum plaster and artificial “pores” enabled creating a simple brittle model material with isolated pores randomly distributed and their size and geometry controlled. Herein, a comparison between two ways of generating microstructures for computer modelling of deformation and fracture of the model material is presented. The predictions for the computer model have been based upon two microstructure inputs (i) a synthetic 3-D pore model and (ii) measurements of porosity obtained from computed X-ray tomography images of the test material. Predictions of the elastic modulus and tensile strength for the different amount of porosity are compared between the two computer models and the experimental flexural data. The predicted deformation and fracture characteristics that describe the observed quasi- brittle response measured for the model material are discussed.@en

Commercial graphites are used for a wide range of applications. For example, Gilsocarbon graphite is used within the reactor core of Advanced Gas Cooled Reactors (UK) as a moderator. In service, the mechanical properties of the graphite are changed as a result of neutron irradiation induced defects and porosity arising from radiolytic oxidation. In this paper, we discuss measurements undertaken of mechanical properties at the micro-length-scale for virgin and irradiated material. These data provide the necessary inputs to an experimentally-informed model that predicts the deformation and fracture properties of Gilsocarbon graphite at the centimetre-length-scale. The results provide an improved understanding of how the mechanical properties and fracture characteristics of this type of graphite change as a result of exposure to the service environment.@en

In this work, porosity-property relationships of quasi-brittle materials are explored through a combined experimental and numerical approach. In the experimental part, hemihyrate gypsum plaster powder (CaSO 4 ⋅1/2H 2 O CaSO4⋅1/2H2O) and expanded spherical polystyrene beads (1.5–2.0 mm dia.) have been mixed to form a model material with controlled additions of porosity. The expanded polystyrene beads represent pores within the bulk due to their light weight and low strength compared with plaster. Varying the addition of infill allows the production of a material with different percentages of porosity: 0, 10, 20, 30 and 31 vol%. The size and location of these pores have been characterised by 3D X-ray computed tomography. Beams of the size of 20×20×150 20×20×150 mm were cast and loaded under four-point bending to obtain the mechanical characteristics of each porosity level. The elastic modulus and flexural strength are found to decrease with increased porosity. Fractography studies have been undertaken to identify the role of the pores on the fracture path. Based on the known porosity, a 3D model of each microstructure has been built and the deformation and fracture was computed using a lattice-based multi-scale finite element model. This model predicted similar trends as the experimental results and was able to quantify the fractured sites. The results from this model material experimental data and the lattice model predictions are discussed with respect to the role of porosity on the deformation and fracture of quasi-brittle materials.@en

Two linked models have been developed to explore the relationship between the amount of porosity arising in service from both radiolytic oxidation and fast neutron damage that influences both the strength and the force-displacement (load-displacement) behaviour and crack propagation in pile grade A graphite used as a nuclear reactor moderator material. Firstly models of the microstructure of the porous graphite for both unirradiated and irradiated graphite are created. These form the input for the second stage, simulating fracture in lattice-type finite element models, which predicts force (load)-displacement and crack propagation paths. Microstructures comprising aligned filler particles, typical of needle coke, in a porous matrix have been explored. The purpose was to isolate the contributions of filler particles and porosity to fracture strength and crack paths and consider their implications for the overall failure of reactor core graphite.@en

In quasibrittle materials the addition of specific microstructural features such as porosity can lead to departure from linear elastic behaviour prior to maximum force, followed by graceful failure. A simple but extreme example is reticulated vitreous carbon foam, with its opencell structure of brittle ligaments connected in a threedimensional array. Tensile testing has been made on foams with various pore and ligament dimensions to provide a measure of forcedisplacement, combined with acoustic monitoring, and evaluation of the associated elastic moduli and fracture strengths. These tests provide insights into the mechanisms of quasibrittle failure. The results are explored by comparing with predictions from a microstructurebased finite beam element model. Inputs to the model are the elastic modulus and fracture strength of the individual ligaments of the foam measured at the micro lengthscale. Results are discussed with respect to the energy of fracture, the distribution and progression of fracture for individual ligaments.@en