Tim Newson

Application of structural analysis to the mechanical behaviour of the cornea

Structural engineering analysis tools have been used to improve the understanding of the biomechanical behaviour of the cornea. The research is a multi-disciplinary collaboration between structural engineers, mathematical and numerical analysts, ophthalmologists and clinicians. Mathematical shell analysis and nonlinear finite-element modelling have been used in conjunction with laboratory experiments to study the behaviour of the cornea under different loading states and to provide improved predictions of the mechanical response to disease and injury. The initial study involved laboratory tests and mathematical back analysis to determine the corneal material properties and topography. These data were then used to facilitate the construction of accurate finite-element models that are able to reliably trace the performance of cornea upon exposure to disease, injury or elevated intra-ocular pressure. The models are being adapted to study the response to keratoconus (a disease causing loss of corneal tissue) and to tonometry procedures, which are used to measure the intra-ocular pressure. This paper introduces these efforts as examples of the application of structural engineering analysis tools and shows their potential in the field of corneal biomechanics.

Experimental investigation of dynamic effects in capillary pressure: Grain size dependency and upscaling

[1] The macroscopic flow equations used to predict two-phase flow typically utilizes a capillary pressure-saturation relationship determined under equilibrium conditions. Theoretical reasoning, experimental evidence, and numerical modeling results have indicated that when one fluid phase replaces another fluid, this relationship may not be unique but may depend on the rate at which the phase saturations change in response to changes in phase pressures. This nonuniqueness likely depends on a variety of factors including soil-fluid properties and possibly physical scale. To quantify this dependency experimentally, direct measurements of equilibrium and dynamic capillary pressure-saturation relationships were developed for two Ottawa sands with different grain sizes using a 20 cm long column. A number of replicate air-water experiments were conducted to facilitate statistical comparison of capillary pressure-saturation relationships. Water and air pressures and phase saturations were measured at three different vertical locations in the sand column under different desaturation rates (1) to measure local capillary pressure-saturation relationships (static and dynamic); (2) to quantify the dynamic coefficient T , a measure of the magnitude of observed dynamic effects, as a function of water saturation for different grain sizes and desaturation rates; (3) to investigate the importance of grain size on measured dynamic effects; and (4) to assess the importance of sample scale on the magnitude of dynamic effects in capillary pressure. A comparison of the static and dynamic P c -S w relationships showed that at a given water saturation, capillary pressure measured under transient water drainage conditions is statistically larger than capillary pressure measured under equilibrium or static conditions, consistent with thermodynamic theory. The dynamic coefficient T , used in the expression relating the static and dynamic capillary pressures to the desaturation rate was dependant on porous media mean grain size but not on the desaturation rate. Results also suggest that the magnitude of the dynamic coefficient did not increase with the increased averaging volume considered in this study, as has been reported in the literature. This work suggests that dynamic effects in capillary pressure should be included in numerical models used to predict multiphase flow in systems when saturations change rapidly, particularly in fine-grained soil systems (e.g., CO 2 sequestration, enhanced oil recovery, air sparging for remediation).

Mathematical modeling of the biomechanics of the lamina cribrosa under elevated intraocular pressures.

Comprehensive understanding of the biomechanical performance of the lamina cribrosa (LC) and the optic nerve head is central to understanding the role of elevated intraocular pressures (IOP) in chronic open angle glaucoma. In this paper, six closed-from mathematical models based on different idealizations of the LC are developed and compared. This approach is used to create further understanding of the biomechanical behavior by identifying the LC features and properties that have a significant effect on its performance under elevated IOP. The models developed are based on thin circular plate and membrane theories, and consider influences such as in-plane pretension caused by scleral expansion and large deflections. Comparing the results of the six models against a full ocular globe finite element model suggests the significance of the in-plane pretension and the importance of assuming that the sclera provides the LC with a clamped edge. The model that provided the most accurate representation of the finite element model was also used to predict the behavior of a number of LC experimental tests presented in the literature. In addition to the deflections under elevated IOP, the model predictions include the distributions of stress and strain, which are shown to be compatible with the progression of visual field loss experienced in glaucoma.

Measurement of evaporation from saline tailings storages

In the arid regions of Australia, fine-grained slurried wastes produced during gold mining are typically disposed of in large storages using subaerial deposition. Management of the storage is often aimed at maximising the evaporative drying that occurs, thereby maximising the density achieved. Ore processing is conducted using groundwater that can have salinities that approach solution saturation. Precipitation of salts on the tailings surfaces during evaporation leads to the development of thin salt crusts that can significantly reduce the rate of evaporation. Quantification of the actual rate of evaporation from the drying tailings surfaces is important for assessing disposal strategies. Different methods of estimating evaporation were employed at mine sites in Western Australia (WA) to provide more information on the drying behaviour of saline tailings. These techniques are described and typical results from a saline tailings storage are presented.

Numerical Investigation of the Inclined Pullout Behavior of Anchors Embedded in Clay

Two-dimensional plane strain finite element analysis has been used to simulate the inclined pullout behavior of strip anchors embedded in cohesive soil. Previous studies by other researchers were mainly concerned with plate anchors subjected to loads perpendicular to their longest axis and applied through the centre of mass. This paper investigates the behavior of vertical anchors subjected to pullout forces applied at various inclinations with respect to the longest anchor axis, and applied at the anchor top and through the centre of mass. The effects on the pullout behavior of embedment depth, overburden pressure, soil–anchor interface strength, anchor thickness, rate of clay strength increase, anchor inclination, load inclination and soil disturbance due to anchor installation were all studied. Anchor capacity is shown to increase with load inclination angle for anchors loaded through the centre of mass; greater effects are found for higher embedments. The results also show that anchor capacity improves at a decreasing rate with higher rates of increase of soil shear strength with depth. In addition, the capacity of vertically loaded anchors is shown to approximately double when the soil–anchor interface condition changes from fully separated to fully bonded. Similarly, disturbed clay strengths adjacent to the anchor following installation cause a significant reduction in anchor capacity. The results showed a significant effect of the point of load application for anchors inclined and normally loaded. The effects of other parameters, such as anchor thickness, were found to be less significant.

Structural Response of a Commercial Wind Turbine to Various Stopping Events

Increases in forced curtailments at commercial wind farms have triggered a need to investigate turbine behaviour during stopping events. A 2.3 MW commercial horizontal axis wind turbine instrumented with a fiber Bragg grating strain array was subjected to the three most common types of stopping sequences performed by the turbine to measure the supporting towers structural response. An overview of each considered stop type is included identifying their individual mechanical processes as well as their specific triggers. The along-wind and transverse strain measurements at varying elevations of the tower are presented and discussed. Distinguishable response characteristics for each stop type are identified at stop initiation, during rotor deceleration, and following event completion. Results obtained are subsequently compared to the tower strain reactions as a result of typical power production as well as pure yawing of the nacelle. Furthermore, the strain signal has been subjected to a discrete wavelet transform identifying variations in the signal frequency content, triggered by the stopping events.

A framework for modeling ocular drug transport and flow through the eye using micro-CT

This study uses micro-computed tomography (micro-CT) imaging for assessment of concentration and transport mechanisms of ocular drug surrogates following intravitreal injection. Injections of an iodinated contrast agent were administered to enucleated porcine eyes prior to scanning over 192 min. Image analysis was performed using signal profiles and regions of interest that corresponded to specific iodine concentrations. Diffusion coefficients of the injected iodine solutions were calculated using nonlinear regression analysis with a diffusion model. There was a predominantly diffusive component in the movement of the contrast to the back of the eye in the horizontal (sagittal & coronal) directions, with ultimate retinal fate observed after 120 min. The diffusion coefficients were found to have a mean of 4.87 × 10(-4) mm(2) s(-1) and standard deviation of 8.39 × 10(-5) mm(2) s(-1) for 150 mg ml(-1) iodine concentration and 6.13 × 10(-4) ± 1.83 × 10(-4) mm(2) s(-1) for 37.5 mg ml(-1) concentration. However, it should be noted that these coefficients were time dependent and were found to decay as the diffusion front interacted with the retinal wall. A real-time, accurate, non-invasive method of tracking a bolus and its concentration is achieved using a high spatial resolution and fast scanning speed micro-CT system.

Validation of a non-associated critical state model

Abstract A non-associated constitutive critical state model is proposed. The yield surface is that of Modified Cam Clay, whilst the plastic potential is an empirical function. The yield and plastic potential surfaces in the octahedral plane vary from circular at low stress ratios, to the Matsuoka-Nakai surface at failure. Assessment of the model has been by comparison with laboratory tests on soft clay. Further validation has been by predicting centrifuge model behaviour using a modified form of the CRISP finite element program. Comparisons of the numerical analyses, using the proposed model and Modified Cam Clay, show improved correlations with the experimental data. ©

Numerical Modeling of Soil and Surface Foundation Pressure Effects on Buried Box Culvert Behavior

AbstractBox culverts can be subjected to considerable induced earth pressure from overlaying structures and foundations. Therefore, the effect of surface foundations on soil pressures around box culverts needs to be properly investigated and incorporated into the analysis and design of culverts. In this study, the effect of a surface foundation on the response of box culverts was investigated experimentally and numerically. A series of centrifuge tests were performed to evaluate the additional soil pressures around box culverts installed in sand due to adjacent surface foundations. The experimental results were used to calibrate and verify a two-dimensional (2D) numerical model developed using computer software. This model was then used for a parametric study to investigate the effect of the relative foundation location on various aspects of the culvert response. The soil pressures, culvert bending moments, and soil-culvert interaction factors were all considered for different soil depths and surface foun...

Behavior of Laterally Loaded Footings with Enlarged Shear Keys on Drained Soils

Numerical investigations have been conducted to study the feasibility of a novel enlarged shear-key footing (ESF) foundation system for supporting laterally loaded structures. The behavior of the proposed system was examined under various vertical, horizontal and moment load combinations considering drained loading conditions. Extensive finite element analysis using the software package PLAXIS 2D was performed to investigate the behavior of the ESF foundation system. The objectives of this study are as follows: (a) to examine the feasibility of the ESF foundation system to support laterally loaded structures, (b) to provide guidance for use of ESF for different soil conditions, while taking into account the influence of the relative geometry of the constituent elements of the ‘hybrid’ system (i.e. the key length and footing width). The study indicated that the interaction between the structural elements of the hybrid system increased the strength and stiffness of the foundation system and mobilized high lateral load and rocking resistances, even for low displacements. The system resistance was found to rely on its geometry, especially the key length-to-footing width ratio.