Modelling the forming mechanics of engineering fabrics using a mutually constrained pantographic beam and membrane mesh

A method of combining 1-d and 2-d structural finite elements to capture the fundamental mechanical properties of engineering fabrics subject to finite strains is introduced. A mutually constrained pantographic beam and membrane mesh is presented and simple homogenisation theory is developed to relate the macro-scale properties of the mesh to the properties of the elements within the mesh. The theory shows that each of the macro-scale properties of the mesh can be independently controlled. An investigation into the performance of the technique is conducted using tensile, cantilever bending and uniaxial bias extension shear simulations. The simulations are first used to verify the accuracy of the homogenisation theory and then used to demonstrate the ability of the modelling approach in accurately predicting the shear force, shear kinematics and out-of-plane wrinkling behaviour of engineering fabrics

Correction to ‘Evaluation of normalisation methods for uniaxial bias extension tests on engineering fabrics’

This short communication is intended to correct certain erroneous conclusions drawn in the recently published paper by Härtel and Harrison (2014). The investigation of Härtel and Harrison (2014) was intended to evaluate the performance of normalisation methods for the uniaxial bias extension test. Predictions of two published theories for rate-independent behaviour were examined. New conclusions drawn in light of the error found in Härtel and Harrison (2014) are presented and corrected results are provided

Magnetic induction measurements and identification of the permeability of magneto-rheological elastomers using finite element simulations

The isotropic and anisotropic magnetic permeability of Magneto-Rheological Elastomers (MREs) are identified using a simple inverse modelling approach. This involves measuring the magnetic flux density and attractive force occurring between magnets, when MRE specimens are placed in between the magnets. Tests were conducted using isotropic MREs with 10% to 40% and for anisotropic MREs with 10% to 30%, particle volume concentration. Magnetic permeabilities were then identified through inverse modelling, by simulating the system using commercially available multi-physics finite element software. As expected, the effective permeability of isotropic MREs was found to be scalar-valued; increasing with increasing particle volume concentration (from about 1.6 at 10% to 3.7 at 30% particle volume concentration). The magnetic permeability of transversely isotropic MRE was itself found to be transversely isotropic, with permeabilities in the direction of particle chain alignment from 1.6 at 10% to 4.45 at 30%, which is up to 1.07 to 1.25 times higher than in the transverse directions. Results of this investigation are demonstrated to show good agreement with those reported in the literature

Evaluation of normalisation methods for uniaxial bias extension tests on engineering fabrics

An investigation has been conducted to evaluate the performance of normalisation methods for the uniaxial bias extension test. The predictions of two published theories for rate-independent behaviour are examined and a third heuristic method is proposed. Using hypothetical test data, the predictions of the two rate-independent theories are shown to be equivalent for specimen dimensions of high aspect ratio; in this case the predictions can be well-represented using the simple heuristic formula. The predictions diverge for specimens of low aspect ratio, when specimens are sheared to very high shear angles. In order to examine the significance of this divergence on real data, results from tests on several different engineering fabrics are normalised. Differences in the predictions of the normalisation methods are observed and the question of the significance of these differences is discussed. The paper also examines the applicability of rate-independent theory for normalising rate-dependent materials

Equi-biaxial tension tests on magneto-rheological elastomers

A bespoke test rig has been designed to facilitate testing of magneto-rheological (MR) elastomers (MREs) under equi-biaxial tension using a standard universal test machine. Tests were performed up to 10% strain on both isotropic and anisotropic MREs with and without the application of an external magnetic field. Assumptions regarding the material's response were used to analyse stress–strain results in the two stretching directions. The assumptions have been verified previously by uniaxial tension tests and by simulations of the magnetic flux distribution performed using a commercial multi-physics finite element software. The MR effect, which is defined as the increase in tangent modulus at a given strain, has been studied versus engineering strain. The latter was measured optically in the experiments using a digital image correlation system. Relative MR effects up to 74% were found when the particle alignment of anisotropic MREs was oriented parallel to an applied magnetic induction of just 67.5 mT

Directed Energy Interception of Satellites

High power Earth and orbital-based directed energy (DE) systems pose a potential hazard to Earth orbiting spacecraft. The use of very high power, large aperture DE systems to propel spacecraft is being pursued as the only known, feasible method to achieve relativistic flight in our NASA Starlight and Breakthrough Starshot programs. In addition, other beamed power mission scenarios, such as orbital debris removal and our NASA program using DE for powering high performance ion engine missions, pose similar concerns. It is critical to quantify the probability and rates of interception of the DE beam with the approximately 2000 active Earth orbiting spacecraft. We have modeled the interception of the beam with satellites by using their orbital parameters and computing the likelihood of interception for many of the scenarios of the proposed systems we are working on. We are able to simulate both the absolute interception as well as the distance and angle from the beam to the spacecraft, and have modeled a number of scenarios to obtain general probabilities. We have established that the probability of beam interception of any active satellite, including its orbital position uncertainty, during any of the proposed mission scenarios is low ($\approx10^{-4}$). The outcome of this work gives us the ability to predict when to energize the beam without intercept, as well as the capability to turn off the DE as needed for extended mission scenarios. As additional satellites are launched, our work can be readily extended to accommodate them. Our work can also be used to predict interception of astronomical adaptive optics guide-star lasers as well as more general laser use.Comment: 47 pages, 8 figure

Towards comprehensive characterisation and modelling of the forming and wrinkling mechanics of engineering fabrics

Through a combination of direct measurement and inverse modelling, a route to characterising the main mechanical forming properties of engineering fabric is demonstrated. The process involves just two experimental tests, a cantilever bending test and a modified version of the uniaxial bias extension test. The mechanical forming properties of a twill weave carbon fabric have been determined, including estimates of the in-plane bending stiffness and the torsional stiffness of a sheared fabric. As a result of measuring and incorporating all the main mechanical properties of the fabric in forming simulations (tensile, shear, out-of-plane bending, in-plane bending & torsion), the specimen size-dependent shear kinematics and wrinkling response measured in experiments, is faithfully reproduced in simulations of the uniaxial bias extension (UBE) test

Contact model for elastoplastic analysis of half-space indentation by a spherical impactor

This paper presents a new contact model for analysis of post-yield indentation of a half-space target by a spherical indenter. Unlike other existing models, the elastoplastic regime of the present model was modelled using two distinct force–indentation relationships based on experimentally and theoretically established indentation characteristics of the elastoplastic regime. The constants in the model were derived from continuity conditions and indention theory. Simulations of the present model show good prediction of experimental data. Also, an approach for determining the maximum contact force and indentation of an elastoplastic half-space from the impact conditions has been proposed

Postmodernism confronts planning: Some thoughts on an appropriate response

During the past few decades a new and profoundly destabilising movement has threatened to overturn the great intellectual and artistic traditions in­herited from the nineteenth century Enlightenment. This movement, com­monly referred to as postmodernism, confronts development planners in South Africa with stimulating but uncomfortable challenges

Research on fully distributed data processing systems

Issued as Quarterly progress reports, nos. 1-11, and Project report, Project no. G-36-64