A plasticity model for powder compaction processes incorporating particle deformation and rearrangement

This article is available open access through the publisher’s website at the link below. Copyright @ 2008 Elsevier Ltd.This paper develops a mechanistic model of granular materials that can be used with a commercial finite element package (ABAQUS). The model draws on the ideas of critical state soil mechanics and combines them with the theory of envelopes to develop an elasto-plastic model with a non-associated flow rule. The model incorporates both local deformation at the granule contacts, and rearrangement of the granules so that jointly they account for any bulk deformation. The mechanics of the model closely reflect the physicality of the material behaviour and the model parameters are closely linked (although not simplistically identical) to the characteristics of the granules. This not only gives an insight into the material behaviour, but also enables the model to be used to facilitate design of the material, its processing properties and, hence, component development. The model is used to simulate drained triaxial tests, settlement of a powder in a bin, and some examples of die pressing. Simulations are compared with experimental data and with predictions obtained using other models

The acquisition of spatial data from archival photographs and their application to geomorphology

This thesis discusses the development and application of an analytical photogrammetric technique which enables accurate spatial data, of known quality, to be derived from archival photographs. Such a facility represents an important advancement, particularly for geomorphologists, because the effects of geomorphological process can be assessed quantitatively and directly by comparing spatial data derived from photographs at different epochs. Sources of archival photographs of England are identified and the type, quantity, range and age of each major collection is discussed. Existing methods of deriving spatial data from photographs are reviewed and illustrated by previous research, with particular emphasis upon the limitations associated with each method. The technique that was developed is based upon a self calibrating bundle adjustment and both the functional and stochastic models suitable for successful restitution of archival photographs were established. Five computer programs were developed and the algorithms associated with each are given. These programs are run sequentially and assist in rapid restitution of archival photography and to derive measures of data quality. The technique is applied successfully to a forty year old sequence of archival photographs, obtained from a variety of sources, of the Black Ven landslide, Dorset, England. Spatial data was derived from five photographic epochs, at approximately 10 year intervals, using an analytical plotter. A secondary aim of the research was to extend existing techniques and devise new methods of processing these spatial data, for geomorphological purposes. Several techniques were found to be especially valuable including: the production of morpho-genetic maps; DTI's of difference; evolutionary models; animated sequences and distributions of slope angle. The latter has shown that the evolutionary model of 'dynamic equilibrium is valid for the Black Ven landslides. All aspects of data quality are examined, particularly the functional model used in the self-calibrating bundle adjustment. This least squares estimating procedure is found to be perfectly adequate for the successful restitution of archival photographs

A time-dependent model for improved biogalvanic tissue characterisation

Measurement of the passive electrical resistance of biological tissues through biogalvanic characterisation has been proposed as a simple means of distinguishing healthy from diseased tissue. This method has the potential to provide valuable real-time information when integrated into surgical tools. Characterised tissue resistance values have been shown to be particularly sensitive to external load switching direction and rate, bringing into question the stability and efficacy of the technique. These errors are due to transient variations observed in measurement data that are not accounted for in current electrical models. The presented research proposes the addition of a time-dependent element to the characterisation model to account for losses associated with this transient behaviour. Influence of switching rate has been examined, with the inclusion of transient elements improving the repeatability of the characterised tissue resistance. Application of this model to repeat biogalvanic measurements on a single ex vivo human colon tissue sample with healthy and cancerous (adenocarcinoma) regions showed a statistically significant difference (p 0.05) between tissue types was found when measurements were subjected to the current model, suggesting that the proposed model may allow for improved biogalvanic tissue characterisation

An algorithm for simulating the Ising model on a type-II quantum computer

Presented here is an algorithm for a type-II quantum computer which simulates the Ising model in one and two dimensions. It is equivalent to the Metropolis Monte-Carlo method and takes advantage of quantum superposition for random number generation. This algorithm does not require the ensemble of states to be measured at the end of each iteration, as is required for other type-II algorithms. Only the binary result is measured at each node which means this algorithm could be implemented using a range of different quantum computing architectures. The Ising model provides an example of how cellular automata rules can be formulated to be run on a type-II quantum computer.Comment: 14 pages, 11 figures. Accepted for publication in Computer Physics Communication

Guidelines on the use of Structure from Motion Photogrammetry in Geomorphic Research

As a topographic modelling technique, structure-from-motion (SfM) photogrammetry combines the utility of digital photogrammetry with a flexibility and ease of use derived from multi-view computer vision methods. In conjunction with the rapidly increasing availability of imagery, particularly from unmanned aerial vehicles, SfM photogrammetry represents a powerful tool for geomorphological research. However, to fully realize this potential, its application must be carefully underpinned by photogrammetric considerations, surveys should be reported in sufficient detail to be repeatable (if practical) and results appropriately assessed to understand fully the potential errors involved. To deliver these goals, robust survey and reporting must be supported through (i) using appropriate survey design, (ii) applying suitable statistics to identify systematic error (bias) and to estimate precision within results, and (iii) propagating uncertainty estimates into the final data products

Guidelines on the use of Structure from Motion Photogrammetry in Geomorphic Research

As a topographic modelling technique, structure-from-motion (SfM) photogrammetry combines the utility of digital photogrammetry with a flexibility and ease of use derived from multi-view computer vision methods. In conjunction with the rapidly increasing availability of imagery, particularly from unmanned aerial vehicles, SfM photogrammetry represents a powerful tool for geomorphological research. However, to fully realize this potential, its application must be carefully underpinned by photogrammetric considerations, surveys should be reported in sufficient detail to be repeatable (if practical) and results appropriately assessed to understand fully the potential errors involved. To deliver these goals, robust survey and reporting must be supported through (i) using appropriate survey design, (ii) applying suitable statistics to identify systematic error (bias) and to estimate precision within results, and (iii) propagating uncertainty estimates into the final data products

Assisted Magnetic Soft Continuum Robot Navigation via Rotating Magnetic Fields

Innovative robotic catheters that are soft, flexible, and controlled by magnets have the potential to revolutionize minimally invasive surgical procedures in critical areas such as the lungs, brain and pancreas, which currently pose significant safe access challenges using existing technology. These shape forming millimetre-scale magnetic soft continuum robots (MSCRs) can be designed to be highly dexterous in order to access regions of the anatomy otherwise deemed inaccessible. However, due to their soft and slender nature, MSCRs are prone to buckling under compressive loads during insertion. In this study we demonstrate buckling free insertion of high aspect ratio (80 mm long by 2 mm diameter) MSCRs into narrow, tortuous lumens enabled by coupling a specific lengthwise magnetic profile with exposure to a rotating magnetic field (RMF). We present design, finite element modelling (FEM) of the motion, fabrication and actuation of three different MSCRs. These robots are cast from NdFeB doped silicone polymer to obtain 2 mm and 3 mm diameter catheters. These are magnetized in a predefined profile such that when the catheters are placed in an RMF, a serpentine motion is generated. Experiments were conducted to quantify the behaviour of these soft catheters navigating through a soft phantom that mimicked narrow tortuous lumens such as the pancreas and bile ducts. Oscillating actuation increased the inserted depth reached by the MSCR in a tortuous channel and even enabled squeezing through a 1 mm diameter opening via shape morphing. The experiments showed that an RMF reduced the required insertion forces by almost 45% and increased the distance inserted in a fixed time frame by 3 times

The impact of electrode resistance on the biogalvanic characterisation technique

Measurement of a tissue-specific electrical resistance may offer a discriminatory metric for evaluation of tissue health during cancer surgery. With a move toward minimally-invasive procedures, applicable contact sensing modalities must be scalable, fast and robust. A passive resistance characterisation method utilising a biogalvanic cell as an intrinsic power source has been proposed as a potentially suitable solution. Previous work has evaluated this system with results showing effective discrimination of tissue type and damage (through electroporation). However, aspects of the biogalvanic cell have been found to influence the characterisation performance, and are not currently accounted for within the system model. In particular, the electrode and salt-bridge resistance are not independently determined, leading to over-predictions of tissue resistivity.This paper describes a more comprehensive model and characterisation scheme, with electrode parameters and salt-bridge resistivity being evaluated independently. In a generalised form, the presented model illustrates how the relative resistive contributions from the electrodes and medium relate to the existing characterisation method efficacy. We also describe experiments with physiologically relevant salt solutions (1.71, 17.1, 154 mM), used for validation and comparison. The presented model shows improved performance over the current biogalvanic measurement technique at the median conductivity. Both the proposed and extant system models become unable to predict conductivity accurately at high conductivity due to the dominance of the electrodes. The characterisation techniques have also been applied to data collected on freshly excised human colon tissue (healthy and cancerous). The findings suggest that the resistance of the cell under the test conditions is electrode dominated, leading to erroneous tissue resistance determination. Measurement optimisation strategies and the surgical applicability of the biogalvanic technique are discussed in light of these findings

Effect of quantum fluctuations on structural phase transitions in SrTiO_3 and BaTiO_3

Using path-integral Monte Carol simulations and an ab initio effective Hamiltonian, we study the effects of quantum fluctuations on structural phase transitions in the cubic perovskite compounds SrTiO3 and BaTiO3. We find quantum fluctuations affect ferroelectric (FE) transitions more strongly than antiferrodistortive (AFD) ones, even though the effective mass of a single FE local mode is larger. For SrTiO3 we find that the quantum fluctuations suppress the FE transition completely, and reduce the AFD transition temperature from 130K to 110K. For BaTiO3, quantum fluctuations do not affect the order of the transition, but do reduce the transition temperature by 35-50 K. The implications of the calculations are discussed.Comment: Revtex (preprint style, 14 pages) + 2 postscript figures. A version in two-column article style with embedded figures is available at http://electron.rutgers.edu/~dhv/preprints/index.html#wz_qs