Catherine O'Sullivan

Variations due to analysis technique in intracellular pH measurements in simulated and in vivo 31P MR spectra of the human brain

To investigate variation in pH generated by different analysis techniques and to find the most robust method, 31P MR brain spectra were acquired in vivo. Three different methods were used to measure the chemical shift of inorganic phosphate (Pi) relative to phosphocreatine (PCr).

Use of elastic stability analysis to explain the stress-dependent nature of soil strength

The peak and critical state strengths of sands are linearly related to the stress level, just as the frictional resistance to sliding along an interface is related to the normal force. The analogy with frictional sliding has led to the use of a ‘friction angle’ to describe the relationship between strength and stress for soils. The term ‘friction angle’ implies that the underlying mechanism is frictional resistance at the particle contacts. However, experiments and discrete element simulations indicate that the material friction angle is not simply related to the friction angle at the particle contacts. Experiments and particle-scale simulations of model sands have also revealed the presence of strong force chains, aligned with the major principal stress. Buckling of these strong force chains has been proposed as an alternative to the frictional-sliding failure mechanism. Here, using an idealized abstraction of a strong force chain, the resistance is shown to be linearly proportional to the magnitude of the lateral forces supporting the force chain. Considering a triaxial stress state, and drawing an analogy between the lateral forces and the confining pressure in a triaxial test, a linear relationship between stress level and strength is seen to emerge from the failure-by-buckling hypothesis.

Study on the Deformation of Loose Sand under Cyclic Loading by DEM Simulation

The extensive use of granular materials as roadbeds accelerates the study of sand behavior under cyclic loading. While several studies have been done on sand deformation under cyclic loading, little information is available on the explanations for this behavior. By discrete element method (DEM) simulation, this paper offered a preliminary insight into particles interaction of loose sand which controls the material’s stress-strain behavior. A 2-D ‘sample’ of 896 ‘quartz’ disks, with diameters of 0.20, 0.25 and 0.30mm, was produced by self-gravity sediment. Then it was K0 loaded to t’ = 127 kPa, at which point, the sample started to be cyclically loaded for 5000 cycles when t’ kept constant. Cyclic amplitudes varied from 0.02 to0.04 t’. Increasing strain accumulation with increased number of cycles was observed. For smaller cyclic amplitude, strain accumulation increased smoothly, but for larger one, strain accumulation increased erratically after a large number of cycles. The coordination number variation during cycling load was considered to be responsible for this phenomenon. Induced anisotropy was observed, and it could be explained by deviatoric fabric analysis

Experimental validation of particle-based discrete element methods

As a consequence of its particulate nature, soil exhibits a highly complex response to applied loads and deformations. Traditionally, geotechnical engineers have coupled continuum numerical analysis tools (such as the finite element method) with complex constitutive models to analyze soil response. This approach does not explicitly consider the particle-scale interactions underlying the macro-scale response observed in the laboratory and field. With increasing computational speeds, particle-based discrete element methods are becoming popular amongst geotechnical engineers in both research and practice. On a practical level discrete element methods are particularly useful for studying finite deformation problems, while from a more theoretical perspective they can be used to create virtual laboratories where the micro-mechanics of soil response can be analyzed in detail. This paper describes a series of validation studies that were performed to confirm that, despite their inherent simplifications, discrete element methods can accurately capture the macro-scale response of granular materials. It is shown that, once validated, these methods can provide useful information to explain the complex response exhibited by granular materials in conventional laboratory tests.

Calculating strain in 3D DEM simulations

Particulate DEM allows us to simulate and evaluate in detail the evolution of localizations in particulate material, whether bonded/cemented or unbonded. DEM simulations generate a wealth of particle scale data including particle displacements, velocities and contact forces. Traditionally in geomechanics we understand material response based upon a continuum mechanics framework that considers stress and strain. There is little debate as to how to calculate stress from DEM simulation results, however there is no consensus on how to calculate strain. Most of the methods proposed in the literature to calculate strain have considered the overall response of an assembly of grains, rather than the local in homogeneities that are associated with shear band evolution. This paper outlines the challenges associated with quantifying strain based upon DEM simulation results and demonstrates that a local wavelet based homogenization approach as proposed by may have advantages over triangulation based linear interpolation.

Numerical study of dendrite coherency during equiaxed solidification by the Discrete Element Method

Equiaxed solidification of Al alloys has been simulated by a continuum model in 2D, producing morphology variations from near-globular to highly-branched dendritic. The resulting microstructures were taken as initial samples to perform direct-shear simulations using the Discrete Element Method (DEM) and study the dendrite coherency point. Crystal rearrangement in response to direct-shear is analysed in different grain morphologies with a focus on force transmission and crystal translations and rotations. The simulations show that the coherency point decreases significantly as the morphology becomes more dendritic. Significant rotation was observed around the shear plane, leading to local dilation. The modelling results reproduce the key trends reported in prior experiments on the effect of grain size and morphology on dendrite coherency, and suggest that the coherency point depends on both the internal fraction of liquid within the crystal envelopes and also on the shape of envelopes.