186,873 research outputs found
Application of optical measurement techniques to high strain rate deformations in composite materials
No full textTraditionally high strain rate material characterisations have been conducted using strain gauges and/or cross head displacements in servo-hydraulic test machines, and force transducers in split Hopkinson bar experiments. Non-contact full-field techniques for experimental stress/strain analysis have been available for many years and used extensively for structural analysis under static or quasi static loading. These techniques have the advantage that they are non-contact and high resolution, so damage initiation can be captured within the field of view and the material behaviour is not modified by the sensor. In the paper, one such technique known as Digital Image Correlation (DIC) is used to assess the material behaviour by using high-speed digital cameras to capture images from material subject to high strain rate events. The high strain rate loading is achieved using an Instron VHS high speed tensile test machine that allows the applied strain rates to vary from 12.5 s–1 to 125 s–1. Although the strain rates that can be achieved are low in comparison to those achieved with the Hopkinson bar, the test machine provides better optical access and opportunities for illumination of the specimen necessary for the DIC. In the paper, a review of the literature associated with high strain rate testing using servo-hydraulic machines is first provided. Then, an experimental study of the high strain rate behaviour of the both composite material and the resin alone is described. The results from both the DIC and strain gauges are compared and discussed
Thermotoga lettingae sp. nov., a novel thermophilic, methanol-degrading bacterium isolated from a thermophilic anaerobic reactor
No full textA novel, anaerobic, non-spore-forming, mobile, Gram-negative, thermophilic bacterium, strain TMO(T), was isolated from a thermophilic sulfate-reducing bioreactor operated at 65 degrees C with methanol as the sole substrate. The G C content of the DNA of strain TMO(T) was 39.2 molÐThe optimum pH, NaCl concentration, and temperature for growth were 7.0, 1.0°and 65 degrees C, respectively. Strain TMO(T) was able to degrade methanol to CO(2) and H(2) in syntrophic culture with Methanothermobacter thermautotrophicus DeltaH or Thermodesulfovibrio yellowstonii. Thiosulfate, elemental sulfur, Fe(III) and anthraquinone-2,6-disulfonate were able to serve as electron acceptors during methanol degradation. In the presence of thiosulfate or elemental sulfur, methanol was converted to CO(2) and partly to alanine. In pure culture, strain TMO(T) was also able to ferment methanol to acetate, CO(2) and H(2). However, this degradation occurred slower than in syntrophic cultures or in the presence of electron acceptors. Yeast extract was required for growth. Besides growing on methanol, strain TMO(T) grew by fermentation on a variety of carbohydrates including monomeric and oligomeric sugars, starch and xylan. Acetate, alanine, CO(2), H(2), and traces of ethanol, lactate and alpha-aminobutyrate were produced during glucose fermentation. Comparison of 16S rDNA genes revealed that strain TMO(T) is related to Thermotoga subterranea (98€and Thermotoga elfii (98Ž The type strain is TMO(T) (=DSM 14385(T)=ATCC BAA-301(T)). On the basis of the fact that these organisms differ physiologically from strain TMO(T), it is proposed that strain TMO(T) be classified as a new species, within the genus Thermotoga, as Thermotoga lettingae
Epidermal Passive RFID Strain Sensor for Assisted Technologies
No full textAn epidermal passive wireless strain sensor using RFID tags is presented. The tag is intended to detect eyebrow or neck skin stretch where paraplegic patients have the capability to tweak facial muscles. The tag is designed on a Barium Titanate loaded PDMS substrate and is assessed to demonstrate the strain gauge sensitivity and repeatability as a function of skin stretch
Optimized coupler design for slot waveguide ring resonators
No full textThe highly sensitive nature of slot waveguide micro-resonator power couplers is discussed with respect to the resonator geometry and evanescent coupler design. A modified, tri-bar, evanescent field coupler is demonstrated that shows reduced sensitivity to ring radius as compared with conventional couplers, while maintaining a compact beat-length of only a few micrometers. With the measurement of bending losses of curved slot waveguides, critically coupled ring resonators were designed and fabricated, exhibiting a resonance extinction exceeding 22 dB
Intermediate strain rate testing methodologies and full-field optical strain measurement techniques for composite materials characterisation
No full textTwo optical full-field strain measurement techniques, Digital Image Correlation and the Grid Method, are applied to characterise the strain-rate dependent constitutive behaviour of composite materials. Optical strain measurement techniques based on full-field images are well established for material characterisation in the quasi-static strain rate region, however in this work they are developed to study the material behaviour at intermediate strain rates, which is relatively unexplored. For this purpose a testing methodology that combines high speed imaging and the use of a high speed test machine is devised. The overall goal is to extract composite materials constitutive parameters to be used in the modelling of strain rate dependent behaviour. Particularly the strain rate dependence of the stiffness of glass and carbon fibre reinforced epoxy materials is investigated. A characterisation procedure based on off-axis specimens with oblique end-tabs is developed and applied to the study of the shear behaviour of a carbon/epoxy composite material.The research in the PhD programme constitutes an essential first step for more profitable applications of full-field measurement techniques to high speed testing. Full-field data acquired with the experimental methodology devised here can be used to investigate non linear material behaviours. Furthermore this experimental methodology, applied to specimens that generate non uniform strain fields, can produce strain maps useful for the application of the Virtual Fields Method. This will lead to a reduction of the experiments needed to characterise materials
Discontinuous modelling of strain localisation and failure
No full textThe computational simulation of failure in solids poses many challenges. A proper understanding of how structures respond under loading, both before and past the peak load, is important for safe and economical constructions. This requires numerical models for failure which are both faithful to the physical reality and mathematically well founded. A serious computational issue is that of objectivity with respect to the spatial discretisation of a problem. This requires that upon refinement of the spatial discretisation of a problem, a unique, physically meaningful result is approached. One approach to ensure objectivity with respect to spatial discretisation when simulating failure in solids is to allow displacement discontinuities in the solution. In this work, different techniques, of varying complexity, are developed to simulate displacement discontinuities which are independent of the spatial discretisation using finite elements. The different techniques are then critically evaluated. The first model examined involves adding only the effect of a displacement discontinuity to a finite element as an incompatible strain mode. This allows a traction–separation relationship to be applied at an interface and can be implemented simply in a standard finite element code. It is however shown that this type of model can be cast in an equivalent continuum format, a form which is known to be sensitive to the spatial discretisation. The second approach developed involves the addition of the Heaviside function to the underlying finite element interpolation basis. This method is based on the partition of unity concept, and allows the Heaviside function to be added locally to a finite element mesh to simulate a propagating displacement discontinuity. The approach is formulated for geometrically linear, geometrically nonlinear, quasi-static and dynamic problems. It is shown to be completely independent of the spatial discretisation. The partition of unity-based model is used also to simulate failure using a regularised strain softening model. When a critical level of inelastic deformation is reached, a displacement discontinuity is inserted. This model is better suited to modelling the entire failure process than a continuum or discontinuous model alone. Through numerical examples, it is shown that the inclusion of a displacement discontinuity during the failure process can lead to a different failure mode than for a continuum-only model
Full-field strain measurement and identification of composites moduli at high strain rate with the virtual fields method
No full textThe present paper deals with full-field strain measurement on glass/epoxy composite tensile specimens submitted to high strain rate loading through a split Hopkinson pressure bar (SHPB) device and with the identification of their mechanical properties. First, the adopted methodology is presented: the device, including an Ultra-High Speed camera, and the experimental procedure to obtain relevant displacement maps are described. The different full-field results including displacement, strain and acceleration maps for two mechanical tests are then addressed. The last part of the paper deals with an original procedure to identify stiffnesses on this dynamic case only using the actual strain and acceleration maps (without the applied force) by using the Virtual Fields Method. The results provide very promising values of Young’s modulus and Poisson’s ratio on a quasi-isotropic glass-epoxy laminate. The load reconstructed from the moduli and strains compares favourably with that from the reading
Full surface strain measurement using shearography
No full textShearography is a full-field optical technique usually used for the
determination of surface strain. Correlation of interferometric speckle patterns
recorded before and after the object deformation yields fringes sensitive to
displacement gradient, from which the surface strain can be calculated. A full
analysis of the surface strain requires the measurement of six displacement
gradient components, using three illumination directions and two directions of
applied shear. Additionally shearography may be used to measure surface slope by
correlation of interferograms obtained before and after a source displacement to
yield fringes sensitive to surface slope. Integration of the slope yields the
object shape. In this paper shearography is used to measure the six components
of displacement gradient of a gas main pipe under pressure, the surface slope of
the pipe and the shape of the pipe. The object slope and shape are used to
correct the displacement gradient measurements for variation in sensitivity
vector across the object surface and for sensitivity variations due to the
dependence of the applied shear upon the local slope of the object surface. A
coordinate transformation, incorporating the object shape information, is used
to obtain the in-plane and out-of-plane displacement gradients relative to the
local profile of the surface
A continuous/discontinuous Galerkin formulation for a strain gradient-dependent damage model
No full textThe numerical solution of strain gradient-dependent continuum problems has been hindered by continuity demands on the basis functions. The presence of terms in constitutive models that involve gradients of the strain field means that the continuity of standard finite element shape functions is insufficient. Despite a resurgence of research interest in strain gradient continuum models to represent micro-mechanical effects, a sound, effective and simple framework for the numerical solution of strain gradient-dependent problems is lacking. Here, a formulation is presented which allows the use of finite element shape functions for the solution of a prototype strain gradient-dependent damage model. The formulation is examined in two dimensions for the simulation of crack propagation. Particular attention is paid to the application of non-standard boundary conditions
Modelling of behaviour of metals at high strain rates
No full textThe aim of the work presented in this thesis was to produce the improvement of the existing simulation tools used for the analysis of materials and structures, which are dynamically loaded and subjected to the different levels of temperatures and strain rates. The main objective of this work was development of tools for modelling of strain rate and temperature dependant behaviour of aluminium alloys, typical for aerospace structures with pronounced orthotropic properties, and their implementation in computer codes. Explicit finite element code DYNA3D has been chosen as numerical test-bed for implementation of new material models. Constitutive model with an orthotropic yield criterion, damage growth and failure mechanism has been developed and implemented into DYNA3D.
Second important aspect of this work was development of relatively simple experimental methods for characterization of engineering materials, and extensive experimental work has been undertaken. Tensile test has been used for the characterisation of two aluminium alloys, at different levels of the strain rates and temperatures, and for three different orientations of materials. The results from these tests allowed derivation of material constants for constitutive models and lead to a better understanding of aluminium alloy behaviour. Procedures for derivation of parameters for temperature and strain rate dependant strength models were developed and parameters for constitutive relations were derived on the basis of uniaxial tensile tests. Taylor cylinder impact test was used as a validation experiment. This test was used to validate the implementation, and accuracy of material model in computer code. At the end of each incremental development, validation of the constitutive material model has been performed through numerical simulations of Taylor cylinder impact test, where simulation results have been compared with the experimental post-test geometries in terms of major and minor side profiles and impact-interface footprints. Plate impact test has been used to determine the material properties at high strain rate, and to investigate damage evolution in impact-loaded material.
Initially the material model has been designed as a temperature and strain rate dependant strength model in a simple isotopic form, which then has been tested and verified against the experimental results. Coupling of the Hill’s orthotropic yield criterion with isotropic, temperature and strain rate dependant, hardening material model, has been chosen to suit the orthotropic behaviour. Method for calibration of orthotropic yield criterion has been developed and parameters have been identified for the orthotropic model under the associated flow rule assumption and in case of plane stress on the basis of tensile and cylinder impact tests. The complexity of the model has been further increased through coupling of hardening model with orthotropic yield criterion including damage evolution and failure criteria. The constitutive model was developed within the general framework of continuum thermodynamics for irreversible processes, and plate impact test and tensile tests have been used for determination of parameters for damage part of the new material model.Airbus U
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