Papers by Vito Tagarielli
Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, 2019
This study investigates the transient loading on rigid, isolated, box-like objects by impinging p... more This study investigates the transient loading on rigid, isolated, box-like objects by impinging pressure waves of variable intensity and time duration. A numerical solver is used to predict the transient flow around the object and the consequent pressure on the object's surface. An analytical model is developed which is capable of predicting the transient loading history on the faces of a box-like object; it was found in good agreement with the numerical predictions. The numerical and analytical models are then used to construct non-dimensional design maps. Different regimes of loading are identified and explored.
Journal of Engineering Materials and Technology
We use three-dimensional printing to manufacture lattices with uniform and graded relative densit... more We use three-dimensional printing to manufacture lattices with uniform and graded relative density, made from a composite parent material comprising a nylon matrix reinforced by short carbon fibers. The elastic–plastic compressive response of these solids is measured up to their densification regime. Data from experiments on the lattices with uniform relative density are used to deduce the dependence of their elastic–plastic homogenized constitutive response on their relative density, in the range 0.2–0.8. These data are used to calibrate finite element (FE) simulations of the compressive response of functionally graded lattices (FGLs), which are found in good agreement with the corresponding measurements, capturing the salient features of the measured stress versus strain responses. This exercise is repeated for two lattice topologies (body-centered cubic and Schwarz-P). The phenomenological constitutive models produced in this study can be used in topology optimization to maximize...
Accidental deflagration of gas clouds poses a persisting safety risk in industries handling flamm... more Accidental deflagration of gas clouds poses a persisting safety risk in industries handling flammable substances. Today, the assessment of structures against accidental loads is an integral part of the design process of modules in, among others, the oil&gas industry. Simplified models to estimate structural loading and response to deflagration incidents are yet scarce, and are mostly extrapolated from models developed for military applications. In this thesis various aspects of the effects of deflagration waves on structures are investigated, and predictive analytical models for the loading and structural response of structures are developed. First, the influence of fluid-structure interaction on pressure wave loading is analysed. Using a combined analytical and numerical approach, the effect is quantified and results are synthesized in the form of non-dimensional maps covering a wide range of possible scenarios. It was found that fluid-structure interaction only plays a minor role ...
<jats:p>Sandwich structures are increasingly used in marine applications where high bending... more <jats:p>Sandwich structures are increasingly used in marine applications where high bending specific stiffness and strength are required.</jats:p> <jats:p>So far, expanded polymeric foams such as Styrene Acrilonitrile (SAN) and Polyvinyl Chloride (PVC) have been proved to be the most suitable core materials for motor and sailing yacht sandwich constructions. ISO standards have accepted these materials and suggested design rules built around their mechanical properties. One of the main drawbacks in the application of sandwich composite structures is the fact that their load carrying ability may be significantly reduced by the presence of a local damage such as core indentation or partial face sheet delamination. Local damages usually take place in dynamic scenarios and they can be caused by indentation due to floating object impacts, interaction with mooring structures and rocks or by high deformations due to slamming loads. It should be noted that such damages usually cannot be observed from outside the sandwich construction and they may cause a serious reduction in load-bearing capacity, jeopardizing the safety of the boat and its crew.</jats:p> <jats:p>ARPRO® is an expanded polypropylene (EPP) foam extensively used in the automotive industry, where it has gained full approval due to its outstanding energy absorption characteristics. Unlike standard polymeric foams, ARPRO® foams possess a hyper-elastic stress versus strain response and recover their initial shape after having undergone very high deformations. For this reason this material is expected to enable the design of sandwich panels with an enhanced damage tolerance. In the case of an impact, the core stores the energy as it is compressed and once the load is released it will return to its original geometry preventing the skin from debonding. SAN or PVC cores would absorb the energy by destroying their cell structure leading in this way to core-skin delamination. On the other hand, ARPRO® has lower shear and compressive moduli which implies lower sandwich structures performances in static scenarios.</jats:p> <jats:p>This paper presents static and dynamic testing aimed at determining the relative mechanical performances of sandwich beams cored both with ARPRO® and SAN foams. Comparison of the static and dynamic response ofthe two materials shows that ARPRO® is a potentially competitive core material for composite sandwich construction.</jats:p>
The thesis describes a new test technique and an associated bespoke apparatus to conduct tensile ... more The thesis describes a new test technique and an associated bespoke apparatus to conduct tensile testing at high strain rates and in environmentally-controlled conditions. The technique is suitable to measure the dynamic tensile response of brittle materials difficult to grip (brittle solids and materials in the form of fibre yarns or tapes) and of ring-shaped specimens. The apparatus is compact in size and shape, and this facilitates testing in a conditioned atmosphere. The method also offers the advantage that identical specimens are used in all tests (across wide ranges of strain rate and temperature). The technique allows the measurement of the tensile response of monolithic poly(methyl methacrylate) (PMMA) and unidirectional composites based on Dyneema® tape, Dyneema® SK75 yarn and Kevlar® 49 yarn. The measurements are conducted in a wide range of strain rate (0.0001-323/s ) and yield valid dynamic stress versus strain histories, including part of the elastic material response....
The thesis explores the role of microstructural heterogeneity in solids on their macroscopic mech... more The thesis explores the role of microstructural heterogeneity in solids on their macroscopic mechanical response and develops a stochastic modelling framework which accounts for the effects of such heterogeneity. First, the response of unidirectional 3D-printed PLA was investigated, and compared to that of injection-moulded PLA, to highlight the role of microstructural heterogeneity. The heterogeneous 3D-printed material is orthotropic and characterised by a strong tension-compression asymmetry. The material is tougher when loaded in the extrusion direction than in the transverse direction. The response of this unidirectional 3D-printed heterogeneous material was compared to that of its homogeneous counterpart (injection-moulded PLA), showing that manufacturing by 3D-printing imposed higher microstructural heterogeneity improves toughness. A new damage measurement technique was then developed, with the inspiration from Continuous Stiffness Measurement (CSM) and studies on damage mea...
The use of nanomaterials enables the development of new materials with tailored properties and co... more The use of nanomaterials enables the development of new materials with tailored properties and coupled responses in different physical domains. In particular, the superlative combination of properties displayed by carbon nanotubes makes them an extremely interesting filler in polymer composites. The progress of industrial applications is, however, limited by the quality of predictions of their behaviour. This work develops multiscale simulation methods capable of predicting the electromechanical response of carbon nanotube polymer composites. Finite element simulations able to capture the main mechanisms responsible for the conductivity of carbon nanotube polymer composites and their sensitivity to deformation are presented. The electrical contact between nanotubes is represented by a new element which accounts for quantum tunnelling effects and its sensitivity to deformation. Monte Carlo analyses allow for retrieving the mechanical, electrical and coupled response of these materials. These simulations are used to successfully train an artificial neural network that predicts the bulk conductivity of these composites at negligible computational cost. Similarly, predictive multiscale models of the multiaxial electro-mechanical response are developed, enabling the simulation of the strain-sensing response of components of arbitrary shape subject to a non-uniform, multiaxial strain field. Savings in computational time of more than six orders of magnitude are obtained. The effects of non-uniform concentration of carbon nanotubes on the bulk conductivity are explored using realistic three-dimensional representative volume elements. These are generated from measured two-dimensional concentration maps and a feature size parameter. The models are used to predict the influence of inter-nanotube distance on the bulk conductivity, compared with the effects of applying an electric field during the curing process. This work develops a series of modelling techniques that contribute to a better understanding of the electromechanical response of carbon nanotube composites. These methods may support the design and optimisation of applications in conductive and self-sensing structures.
Royal Society Open Science, 2017
Experiments were conducted on an aqueous growth medium containing cultures of Escherichia coli ( ... more Experiments were conducted on an aqueous growth medium containing cultures of Escherichia coli ( E. coli ) XL1-Blue, to investigate, in a single experiment, the effect of two types of dynamic mechanical loading on cellular integrity. A bespoke shock tube was used to subject separate portions of a planktonic bacterial culture to two different loading sequences: (i) shock compression followed by cavitation, and (ii) shock compression followed by spray. The apparatus allows the generation of an adjustable loading shock wave of magnitude up to 300 MPa in a sterile laboratory environment. Cultures of E. coli were tested with this apparatus and the spread-plate technique was used to measure the survivability after mechanical loading. The loading sequence (ii) gave higher mortality than (i), suggesting that the bacteria are more vulnerable to shear deformation and cavitation than to hydrostatic compression. We present the results of preliminary experiments and suggestions for further exper...
Materials Science and Engineering: A, 2017
Tension, compression, three-point bending and indentation experiments are conducted on high purit... more Tension, compression, three-point bending and indentation experiments are conducted on high purity Indium at room temperature and low strain rates. The material displays a ductile viscoplastic response, found to be size-independent in tension and compression. Simple analytical models are constructed to aid interpretation of the test results and detection of a size effect in bending and indentation, associated to a length-scale of order 50-100 µm.
Composite Structures, 2017
We perform Monte Carlo analyses of the anisotropic viscoelastic response of random RVEs represent... more We perform Monte Carlo analyses of the anisotropic viscoelastic response of random RVEs representing the microstructure of UD fibre composites. Both fibres and matrix are taken as isotropic viscoelastic solids; the fibres have different shapes including circular, elliptical, Reauleaux and star-shaped; they are separated from the matrix by interphase regions of different mechanical properties and thicknesses. The analyses allow determining the sensitivity of the transversely isotropic, viscoelastic response of UD composites to fibre volume fraction, fibre shape, interphase volume fraction and interphase properties.
Journal of the Mechanical Behavior of Biomedical Materials, 2016
Titanium foams of relative density in the range 0.35-0.50 are tested in quasi-static compression,... more Titanium foams of relative density in the range 0.35-0.50 are tested in quasi-static compression, tension and shear. The response is ductile in compression but brittle, and weaker, in shear and tension. Virtual foam microstructures are generated by an algorithm based on Voronoi tessellation of three-dimensional space, capable of reproducing the measured size distribution of the pores in the foam. Finite Element (FE) simulations are conducted to explore the mechanical response of the material, by analysing the elasto-plastic response of a statistical volume element (SVE). The simulations correctly predict the ductile compressive response and its dependence on relative density.
Composite Structures, 2015
Finite element calculations are performed to model failure of a carbon/epoxy composite laminate l... more Finite element calculations are performed to model failure of a carbon/epoxy composite laminate loaded in tension in the through-thickness direction, and to predict the dependence of failure loads upon specimen size. The spatial variability of the inter-laminar strength is modelled by introducing different types of discrete random fields of material tensile strength. Fracture processes are modelled using the cohesive segment method, within the extended finite element framework of Abaqus Standard. Monte Carlo Simulation are conducted on different realisations of the random fields; the predicted responses are compared to previously published measurements and to reference FE simulations, in which the material strength is taken as uniform and equal to the measured average. The comparison shows that the modelling approach presented here provides more accurate predictions of the structural failure loads and their dependence on size, as well as capturing the failure modes observed in experiments.
International Journal of Solids and Structures, 2012
The mechanical response of a widely used electro-active polymer (EAP) material, and the sensitivi... more The mechanical response of a widely used electro-active polymer (EAP) material, and the sensitivity of this response to imposed strain rate, are measured by performing tension and compression tests at constant strain rate as well as relaxation experiments. Simple actuators are constructed and used to determine the sensitivity of the material's relative dielectric permittivity to excitation frequency and imposed equibiaxial strain. Actuators are subjected to both a monotonic, slowly increasing electrical excitation and to a more rapid oscillating electric field; the differences in the resulting actuation strains reveal the importance of the material's viscosity in its performance as an actuator.
International Journal of Impact Engineering, 2015
Quasi-static and dynamic experiments are conducted to characterise the mechanical response of a s... more Quasi-static and dynamic experiments are conducted to characterise the mechanical response of a syntactic foam comprising hollow glass microballoons in a polyurethane matrix. Stress versus strain histories are measured in uniaxial tension and compression as well as in pure shear, at strain rates ranging from 4 10 to 3-1 10 s , via non-standard experimental techniques; quasi-static in-situ tests are conducted to visualise the deformation mechanisms in tension and compression. The material displays a pronounced sensitivity to the imposed strain rate and relatively high tensile and shear ductility at both low and high strain rates. A tension/compression asymmetry is displayed in quasi-static tests but is lost at high rates of strain.
Titanium foams of relative density ranging from 0.3 to 0.9 were produced by titanium powder sinte... more Titanium foams of relative density ranging from 0.3 to 0.9 were produced by titanium powder sintering procedures and tested in uniaxial compression at strain rates ranging from 0.01 to 2,000 s −1. The material microstructure was examined by X-ray tomography and Scanning Electron Microscopy (SEM) observations. The foams investigated are strain rate sensitive, with both the yield stress and the strain hardening increasing with applied strain rate, and the strain rate sensitivity is more pronounced in foams of lower relative density. Finite element simulations were conducted modelling explicitly the material's microstructure at the micron level, via a 3D Voronoi tessellation. Low and high strain rate simulations were conducted in order to predict the material's compressive response, employing both rate-dependant and rate-independent constitutive models. Results from numerical analyses suggest that the primary source of rate sensitivity is represented by the intrinsic sensitivity of the foam's parent material.
ABSTRACT Instrumented nano -indentation techniques are presently under investigation since they r... more ABSTRACT Instrumented nano -indentation techniques are presently under investigation since they represent a new and promising tool in experimental analysis. In particular, they are suitable for investigating the mechanical propertie s of thin films and multi-layer structures, such as TBC (Thermal Barrier Coatings). This paper starts with a brief description of the test and of related problems. Then, both analytical approaches and recent numerical studies are presented, in order to exp lain how to extract mechanical properties from test results. Many aspects and details of the experimental procedure are discussed, in order to help the experimentalists in their work. Experimental results of nano -indentation tests on a bond coat material of a TBC are also presented and critically discussed.
Lethal spring traps are widely used for killing small mammals in the UK. Many require government ... more Lethal spring traps are widely used for killing small mammals in the UK. Many require government approval, based primarily on humaneness. However, mole traps and break-back traps for rats and mice are exempt; those available vary widely in price and apparent quality. The EU is considering implementing a Trapping Directive that would alter UK legislation, and a recent report advised the EU that trapping legislation should cover all trapped species and encourage improvement of traps. Mechanical trap performance is often used as an indicator of welfare impact. We examined the mechanical evidence for scope to improve the welfare standards of rat, mouse and mole spring traps. We measured mechanical performance among a range of rat, mouse and mole traps. Impact momentum values varied 6-8 fold, and clamping force values 4-5.5 fold, among traps for killing each species. There was considerable overlap in the performance of rat and mouse traps. Trap-opening angle and spring type were related ...
Explosion Blast Response of Composites
Scientific Reports, 2021
We propose and implement a computational procedure to establish data-driven surrogate constitutiv... more We propose and implement a computational procedure to establish data-driven surrogate constitutive models for heterogeneous materials. We study the multiaxial response of non-linear n-phase composites via Finite Element (FE) simulations and computational homogenisation. Pseudo-random, multiaxial, non-proportional histories of macroscopic strain are imposed on volume elements of n-phase composites, subject to periodic boundary conditions, and the corresponding histories of macroscopic stresses and plastically dissipated energy are recorded. The recorded data is used to train surrogate, phenomenological constitutive models based on neural networks (NNs), and the accuracy of these models is assessed and discussed. We analyse heterogeneous composites with hyperelastic, viscoelastic or elastic–plastic local constitutive descriptions. In each of these three cases, we propose and assess optimal choices of inputs and outputs for the surrogate models and strategies for their training. We fin...
Volume 9: Oil and Gas Applications; Supercritical CO2 Power Cycles; Wind Energy, 2019
In this study we present the application of numerical and analytical models to predict the transi... more In this study we present the application of numerical and analytical models to predict the transient loading of structures by impinging pressure and shock waves in air, which have been recently developed by the authors. Non-dimensional design maps are provided which yield predictions of the maximum loads on structures as a function of the problem parameters. Practical example applications, with reference to typical structures used in turbomachinery packages, are presented. These examples demonstrate the superiority of the new modelling techniques to current industrial design guidelines which are mostly extrapolated from simplified methods developed for shock waves. Finally, conclusions are drawn regarding the nature of the loading exerted on the structure in different regimes of problem parameters.
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Papers by Vito Tagarielli