Micromechanical modelling of fracture processes in cement composites

Cement composites are the most popular and widely used construction material in the world. Understanding and predicting fracture processes in these materials is scientifically challenging but important for durability assessments and life extension decisions. A recently proposed microstructure-informed site-bond model with elasticbrittle spring bundles is developed further to predict the elastic properties and fracture process of cement paste. It accounts for microstructure characteristics obtained from high resolution X-ray computed microtomography (micro-CT). Volume fraction and size distribution of anhydrous cement grains are used to determine the model length scale and pore-less elasticity. Porosity and pore size distribution are used for tuning elastic and failure properties of individual bonds. The fracture process is simulated by consecutive removal of bonds subject to failure criterion. The stress-strain response and elastic properties of cement paste are obtained. The simulated Young’s modulus and deformation response prior to peak stress agree very well with the experimental data. The proposed model provides an effective tool to simulate micro-cracks initiation, propagation, coalescence and localization

Event-related Potentials reveal differential Brain Regions implicated in Discounting in Two Tasks

The way people make decisions about future benefits – termed discounting - has important implications for both financial planning and health behaviour. Several theories assume that, when delaying gratification, the lower weight given to future benefits (the discount rate) declines exponentially. However there is considerable evidence that it declines hyperbolically with the rate of discount being proportionate to the delay distance. There is relatively little evidence as to whether neural areas mediating time- dependent discounting processes differ according to the nature of the task. The present study investigates the potential neurological mechanisms underpinning domain-specific discounting processes. We present high-density event-related potentials (ERPs) data from a task in which participants were asked to make decisions about financial rewards or their health over short and long time-horizons. Participants (n=17) made a button-press response to their preference for an immediate or delayed gain (in the case of finance) or loss (in the case of health), with the discrepancy in the size of benefits/losses varying between alternatives. Waveform components elicited during the task were similar for both domains and included posterior N1, frontal P2 and posterior P3 components. We provide source dipole evidence that differential brain activation does occur across domains with results suggesting the possible involvement of the right cingulate gyrus and left claustrum for the health domain and the left medial and right superior frontal gyri for the finance domain. However, little evidence for differential activation across time horizons is found.Decision Making, Domain-Specific Discounting, Event-Related Potentials

A lattice-spring model for damage evolution in cement paste

AbstractTo understand better the fracture processes in cement-based materials, it is essential to predict the evolution of damage in cement paste. A recently proposed site-bond model is developed further to take into account the key microstructure data, such as pore size distribution, porosity, and size distribution and volume fraction of anhydrous cement grains obtained from high resolution X- ray tomography. The grains are associated with lattice sites linked by deformable bonds. The bonds are bundles of elastic-brittle springs, resisting normal and shear relative displacements between grains with potential for failure. The model length scale and thence spring constants are controlled by grain statistics. The spring failure properties are controlled by pore statistics. Macroscopic damage develops by a succession of local failures, represented by spring removal. The model is used to simulate the stress-strain response and damage in cement paste under uniaxial tensile loading. The influence of porosity on tensile strength and damage evolution is estimated in a quantitative manner. The predictions of the model are in a very good agreement with the available experimental data

Site-bond modelling of structure-failure relations in quasi-brittle media

AbstractThe non-linear behaviour of quasi-brittle media emerges from distributed micro-cracking. This is analysed conveniently by discrete lattice models. A 3D site-bond model is specialised here for materials with three-phase microstructures: stiff inclusions in a compliant matrix containing pores. The deformation behaviour is based on analytically derived relations between bond properties, length scale and macroscopic elastic constants. The microstructure-model mapping is based on size distributions and volume densities of inclusions and pores, typically obtained through analyses of 3D images. Inclusions data is used to calculate the required length scale. Pores data is used to define the failure behaviour of individual bonds. Applications of the methodology to cement-based materials and nuclear graphite are presented separately in this volume

Lead halide perovskite nanowires stabilized by block copolymers for Langmuir-Blodgett assembly

The rapid development of solar cells based on lead halide perovskites (LHPs) has prompted very active research activities in other closely-related fields. Colloidal nanostructures of such materials display superior optoelectronic properties. Especially, one-dimensional (1D) LHPs nanowires show anisotropic optical properties when they are highly oriented. However, the ionic nature makes them very sensitive to external environment, limiting their large scale practical applications. Here, we introduce an amphiphilic block copolymer, polystyrene-block-poly(4-vinylpyridine) (PS-P4VP), to chemically modify the surface of colloidal CsPbBr3 nanowires. The resulting core-shell nanowires show enhanced photoluminescent emission and good colloidal stability against water. Taking advantage of the stability enhancement, we further applied a modified Langmuir-Blodgett technique to assemble monolayers of highly aligned nanowires, and studied their anisotropic optical properties. [Figure not available: see fulltext.]

Mode couplings in superstructure fiber Bragg gratings

Author name used in this publication: A-Ping ZhangAuthor name used in this publication: Xiao-Ming Tao2001-2002 > Academic research: refereed > Publication in refereed journalVersion of RecordPublishe