Surface instabilities in shock loaded granular media

© 2017 Elsevier Ltd The initiation and growth of instabilities in granular materials loaded by air shock waves are investigated via shock-tube experiments and numerical calculations. Three types of granular media, dry sand, water-saturated sand and a granular solid comprising PTFE spheres were experimentally investigated by air shock loading slugs of these materials in a transparent shock tube. Under all shock pressures considered here, the free-standing dry sand slugs remained stable while the shock loaded surface of the water-saturated sand slug became unstable resulting in mixing of the shocked air and the granular material. By contrast, the PTFE slugs were stable at low pressures but displayed instabilities similar to the water-saturated sand slugs at higher shock pressures. The distal surfaces of the slugs remained stable under all conditions considered here. Eulerian fluid/solid interaction calculations, with the granular material modelled as a Drucker–Prager solid, reproduced the onset of the instabilities as seen in the experiments to a high level of accuracy. These calculations showed that the shock pressures to initiate instabilities increased with increasing material friction and decreasing yield strain. Moreover, the high Atwood number for this problem implied that fluid/solid interaction effects were small, and the initiation of the instability is adequately captured by directly applying a pressure on the slug surface. Lagrangian calculations with the directly applied pressures demonstrated that the instability was caused by spatial pressure gradients created by initial surface perturbations. Surface instabilities are also shown to exist in shock loaded rear-supported granular slugs: these experiments and calculations are used to infer the velocity that free-standing slugs need to acquire to initiate instabilities on their front surfaces. The results presented here, while in an idealised one-dimensional setting, provide physical understanding of the conditions required to initiate instabilities in a range of situations involving the explosive dispersion of particles.he work was supported by the Defense Advanced Projects Agency under grant number W91CRB-11-1-0005 (Program manager, Dr. J. Goldwasser)

Cohesive detachment of an elastic pillar from a dissimilar substrate

The adhesion of micron-scale surfaces due to intermolecular interactions is a subject of intense interest spanning electronics, biomechanics and the application of soft materials to engineering devices. The degree of adhesion is sensitive to the diameter of micro-pillars in addition to the degree of elastic mismatch between pillar and substrate. Adhesion-strength-controlled detachment of an elastic circular cylinder from a dissimilar substrate is predicted using a Dugdale-type of analysis, with a cohesive zone of uniform tensile strength emanating from the interface corner. Detachment initiates when the opening of the cohesive zone attains a critical value, giving way to crack formation. When the cohesive zone size at crack initiation is small compared to the pillar diameter, the initiation of detachment can be expressed in terms of a critical value H c of the corner stress intensity. The estimated pull-off force is somewhat sensitive to the choice of stick/slip boundary condition used on the cohesive zone, especially when the substrate material is much stiffer than the pillar material. The analysis can be used to predict the sensitivity of detachment force to the size of pillar and to the degree of elastic mismatch between pillar and substrate.NAF is grateful for financial support in the form of an ERC MULTILAT Grant 669764, and to the US ONR (N62909-14-1-N232, project manager, Dr. Dave Shifler). NAF and RMcM acknowledge support from the Alexander von Humboldt Foundation in the form of their Forschungspreise, which enabled them to undertake research at INM-Leibniz Institute for New Materials, Saarbrücken. EA acknowledges funding from the European Research Council under the European Union's Seventh Framework Programme (FP/2007-2013)/ERC Grant agreement no. 340929

The indentation response of Nickel nano double gyroid lattices

The indentation response of Nickel nano double gyroid films has been measured using a Berkovich nanoindenter and the effective mechanical properties of the Ni double gyroid lattices inferred via a multi-scale finite element analysis. The 1μm thick double gyroid films were manufactured by block copolymer self-assembly followed by electrodeposition of the Ni resulting in two interpenetrating single gyroids of opposite chirality, an overall relative density of 38% and a cell size of about 45 nm. The measured hardness was ∼0.6 GPa with no discernable indentation size effect. A multi-scale finite element (FE) analysis revealed that the uniaxial compressive strength is approximately equal to the hardness for this compressible lattice. Thus, the 38% relative density Ni double gyroid has a strength equal to or greater than the strongest fully dense bulk Ni alloys. The FE calculations revealed that this was a consequence of that fact that the Ni in the 13 nm gyroid struts was essentially dislocation free and had a strength of about 5.7 GPa, i.e. approaching the theoretical strength value of Ni. The measurements and calculations reported here suggest that in spite of the nano gyroids having a bending-dominated topology they attain strengths higher than those reported for stretching-dominated micron scale lattice materials made via 3D printing. We thus argue that relatively fast and easy self-assembly processes are a competitive alternative to 3D printing manufacture methods for making high strength lattice materials

The effect of matrix shear strength on the out-of-plane compressive strength of CFRP cross-ply laminates

© 2018 Elsevier Ltd The failure mechanism of ‘indirect tension’ is explored for cross-ply IM7/8552 carbon fibre/epoxy laminates subjected to quasi-static, out-of-plane compressive loading. The sensitivity of compressive response to strain rate and to the state of cure is measured, motivated by the hypothesis that the out-of-plane compressive strength is sensitive to the matrix shear strength. A pressure-sensitive film is placed between specimen and loading platen, and reveals that a shear lag zone of reduced compressive traction exists at the periphery of the specimen, giving rise to a size effect in compressive strength. The width of the shear lag zone reduces with increasing shear strength of the matrix. The laminates fail by the indirect tension mechanism: out-of-plane compressive loading generates tension in the fibre direction for each ply and ultimately induces fibre tensile failure. Finite element (FE) simulations and an analytical model are developed to account for the effect of matrix shear strength, specimen geometry, and strain rate on the out-of-plane compressive strength. Both the FE simulations and the analytical model suggest a recipe for increasing the through-thickness compressive strength

Physical hydrodynamic propulsion model study for creeping viscous flow through a ciliated porous tube

The present investigation focuses on a mathematical study of creeping viscous flow induced by metachronal wave propagation in a horizontal ciliated tube containing porous media. Creeping flow limitations are imposed i.e. inertial forces are small compared with viscous forces and therefore very low Reynolds number (Re<<1) is taken into account. The wavelength of metachronal wave is also considered as very large for cilia movement. The physical problem is linearized and exact solutions are developed for the differential equation problem. Mathematica software is used to compute and illustrate numerical results. The influence of slip parameter and Darcy number on velocity profile, pressure gradient and trapping of bolus are discussed with the aid of graphs. It is found that with increasing magnitude of slip parameter the trapped bolus inside the streamlines increases in size. The study is relevant to biological propulsion of medical micro-machines in drug delivery

Fluid-structure interaction of three-dimensional magnetic artificial cilia

A numerical model is developed to analyse the interaction of artificial cilia with the surrounding fluid in a three-dimensional setting in the limit of vanishing fluid inertia forces. The cilia are modelled using finite shell elements and the fluid is modelled using a boundary element approach. The coupling between both models is performed by imposing no-slip boundary conditions on the surface of the cilia. The performance of the model is verified using various reference problems available in the literature. The model is used to simulate the fluid flow due to magnetically actuated artificial cilia. The results show that narrow and closely spaced cilia create the largest flow, that metachronal waves along the width of the cilia create a significant flow in the direction of the cilia width and that the recovery stroke in the case of the out-of-plane actuation of the cilia strongly depends on the cilia width. © 2012 Cambridge University Press

Failure and toughness of bio-inspired composites: Insights from phase field modelling

Using a phase field model we explore crack propagation in bio-inspired composites in which the mineral and organic phases are arranged in a layered fashion. We show how the crack paths can be drastically altered by varying the elastic modulus mismatch between the organic and mineral layers, and by changing the thickness of the organic layer. Depending on the modulus mismatch and the thickness of the organic layer, the crack can either propagate straight, can branch inside organic layer or can get deflected along the interface, leading to delamination. The mechanism that governs the crack trajectories are analysed in terms of energy distribution near the crack tip. The critical energy release rate of the composite is also analysed as a function of the thickness of the organic layer and the modulus mismatch. A considerable enhancement is achieved when the ratio of the elastic modulus of the organic to mineral phase is less than 0.2. In such cases, for a given modulus mismatch, the critical energy release rate attains a maximum only for an optimal thickness of the organic phase. The origin of the optimal thickness is also investigated. © 2014 Elsevier B.V. All rights reserved

Swimming dynamics of bidirectional artificial flagella

We study magnetic artificial flagella whose swimming speed and direction can be controlled using light and magnetic field as external triggers. The dependence of the swimming velocity on the system parameters (e.g., length, stiffness, fluid viscosity, and magnetic field) is explored using a computational framework in which the magnetostatic, fluid dynamic, and solid mechanics equations are solved simultaneously. A dimensionless analysis is carried out to obtain an optimal combination of system parameters for which the swimming velocity is maximal. The swimming direction reversal is addressed by incorporating photoresponsive materials, which in the photoactuated state can mimic natural mastigonemes. © 2013 American Physical Society

Numerical modelling of chirality-Induced bi-Directional swimming of artificial flagella

Biomimetic micro-swimmers can be used for various medical applications, such as targeted drug delivery and micro-object (e.g. biological cells) manipulation, in lab-on-a-chip devices. Bacteria swim using a bundle of flagella (flexible hair-like structures) that form a rotating cork-screw of chiral shape. To mimic bacterial swimming, we employ a computational approach to design a bacterial (chirality-induced) swimmer whose chiral shape and rotational velocity can be controlled by an external magnetic field. In our model, we numerically solve the coupled governing equations that describe the system dynamics (i.e. solid mechanics, fluid dynamics and magnetostatics). We explore the swimming response as a function of the characteristic dimensionless parameters and put special emphasis on controlling the swimming direction. Our results provide fundamental physical insight on the chirality-induced propulsion, and it provides guidelines for the design of magnetic bi-directional micro-swimmers. © 2013 The Author(s) Published by the Royal Society. All rights reserved