3D numerical simulation of the behaviour of a spherical particle suspended in a Newtonian fluid and submitted to a simple shear

The 3D flow around a rigid spherical particle suspended in a Newtonian fluid and submitted to simple shear is numerically studied using Rem3D^® finite element code. The sphere motion is imposed by a sticking contact between the sphere and the fluid. The effect of the particle size as compared with the finite dimension of the shear cell was investigated. The direct calculations show that 3D modelling is necessary to correctly predict the sphere behaviour. The proximity of the particle and the cell walls strongly affects the flow velocities, the sphere motion (increase of the rotation period of the sphere) and the stress field (change of orientation angle and increase of maximal local stresses).     

Crack tip fields in a viscoplastic solid: monotonic and cyclic loading

The asymptotic stress and strain field near the tip of a plane strain Mode I stationary crack in a viscoplastic material are investigated in this work, using a unified viscoplastic model based on Chaboche (Int J Plast 5(3):247–302, 1989). Asymptotic analysis shows that the near tip stress field is governed by the Hutchinson–Rice–Rosengren (HRR) field (Hutchinson in J Mech Phys Solids 16(1):13–31, 1968; Rice and Rosengren in J Mech Phys Solids 16(1):1–12, 1968) with a time dependent amplitude that depends on the loading history. Finite element analysis is carried out for a single edge crack specimen subjected to a constant applied load and a simple class of cyclic loading history. The focus is on small scale creep where the region of inelasticity is small in comparison with typical specimen dimensions. For the case of constant load, the amplitude of the HRR field is found to vanish at long times and the elastic K field dominates. For the case of cyclic loading, we study the effect of stress ratio on inelastic strain and find that the strain accumulated per cycle decreases with stress ratio.     

Monitoring and kinetics study of a single particle on a simple microfluidic chip

A single-channel poly(dimethylsiloxane) microchip was developed for the desorption process monitoring and kinetics studies of a single particle. The microchannel consisting of a narrow section following a relatively wide part enabled particle introduction, transfer, and location. A microinfusion pump was employed to delivery eluting solution at a precise rate. Once the particle contacted with the eluting solution, the solute transferred from particle into eluting solution and would be detected by laser-induced fluorescence or a chemiluminescence detector. Desorption process of a single particle was sensitively monitored. Depending on the desorption curves obtained, kinetics studies were carried out. The sediment desorption process analyses of single resin particles and single active carbon particles were performed.     

Evaluation of stress and strain non-uniformity during cyclic simple shear test using DEM: effect of the horizontal platen asperities

Direct simple shear apparatus has been increasingly used to determine cyclic soil properties. Stress and strain non-uniformity inside the specimen, however, can be considered the main drawback in cyclic simple shear tests. In the current study, 3D discrete element method is employed to evaluate the effect of the shape of the horizontal surface asperities on the stress and strain distribution inside the constant volume specimen in stacked-ring simple shear device during cyclic loading. It was observed that limiting the potential slippage at the interface of the horizontal boundaries and particles, the stress ratio distributes more uniformly inside the specimen. Different types of asperities could cause different patterns as well as different degrees of strain non-uniformity inside the constant volume DSS specimens. The shear and volumetric strain non-uniformity grows with the number of cycles across all of the specimens. Overall, it was concluded that one could assure a uniform stress and strain distribution by preventing the slippage at the interface. However, limiting the degree of non-uniformity across the specimen demands close consideration of boundary condition.     

Scaling laws for dislocation microstructures in monotonic and cyclic deformation of fcc metals

This work reviews and critically discusses the current understanding of two scaling laws, which are ubiquitous in the modeling of monotonic plastic deformation in face-centered cubic metals. A compilation of the available data allows extending the domain of application of these scaling laws to cyclic deformation. The strengthening relation tells that the flow stress is proportional to the square root of the average dislocation density, whereas the similitude relation assumes that the flow stress is inversely proportional to the characteristic wavelength of dislocation patterns. The strengthening relation arises from short-range reactions of non-coplanar segments and applies all through the first three stages of the monotonic stress vs. strain curves. The value of the proportionality coefficient is calculated and simulated in good agreement with the bulk of experimental measurements published since the beginning of the 1960s. The physical origin of what is called similitude is not understood and the related coefficient is not predictable. Its value is determined from a review of the experimental literature. The generalization of these scaling laws to cyclic deformation is carried out on the base of a large collection of experimental results on single and polycrystals of various materials and on different microstructures. Surprisingly, for persistent slip bands (PSBs), both the strengthening and similitude coefficients appear to be more than two times smaller than the corresponding monotonic values, whereas their ratio is the same as in monotonic deformation. The similitude relation is also checked in cell structures and in labyrinth structures. Under low cyclic stresses, the strengthening coefficient is found even lower than in PSBs. A tentative explanation is proposed for the differences observed between cyclic and monotonic deformation. Finally, the influence of cross-slip on the temperature dependence of the saturation stress of PSBs is discussed in some detail. This works takes into account current discussions on the microstructural aspects of cyclic deformation and highlights further work that is required for fully understanding the physical origin of the two scaling laws.     

Post-forming monotonic and cyclic behavior in a HSLA steel sheet after large deformations by in-plane compression

The effects of large prestrains (18–40%), produced by in-plane compression, on the asymmetry and the anisotropy of the stress response and on the fatigue life are investigated under fully reversed axial strain for a 345 MPa yield strength V–N high strength low alloy steel sheet. After prestraining, the hysteresis loops are asymmetric and the stress response is anisotropic, i.e., the response differs in directions parallel and perpendicular to that of the compressive prestrain. To understand the cyclic flow stress asymmetry, monotonic tension and compression tests were conducted in these two directions after prestraining. It is shown that the loop asymmetry is related to the Bauschinger effect after prestraining. Two cyclic stress strain curves, one corresponding to the tension side of the hysteresis loops and the other to the compression side, are defined to accurately describe the post-prestraining behavior. The amount of strengthening gained by prestraining is partially retained after cycling. Prestraining increases the fatigue life at low strain amplitudes but decreases it at high strain amplitudes.     

Cohesive crack modelling of a simple concrete: Experimental and numerical results

Fracture tests were performed on six types of simple concrete made with two types of mortar matrix w/c = 0.32 and w/c = 0.42, two types of spherical aggregates (strong aggregates that debonded during concrete fracture, and weak aggregates, able to break), and two kinds of matrix-aggregate interface (weak and strong).The tensile strength, fracture energy and elasticity modulus of the six types of concrete were measured. These results are intended to serve as an experimental benchmark for checking numerical models of concrete fracture and for providing certain hints to better understand the mechanical behaviour of concrete.A bilinear softening function was used to model the fracture of concrete. The influence of the type of matrix, aggregate, and interface strength on the parameters of the softening curve are discussed: particularly, the fracture energy, the cohesive strength and the critical crack opening.     

Damage in biocomposites: Stiffness evolution of aligned plant fibre composites during monotonic and cyclic fatigue loading

The non-linear stress–strain behaviour of plant fibre composites is well-known in the scientific community. Yet, the important consequences of this, in terms of the evolution of stiffness as a function of applied strain and cycles to failure, are not well-studied in literature. This is despite the fact that stiffness degradation is a well-accepted indicator of damage in a composite material, and is regularly used as a component failure criterion. This article systematically explores the evolution of stiffness of various aligned plant fibre composites, subjected to (i) monotonic loading, (ii) low-cycle, repeated progressive loading, and (iii) fatigue loading. The evolution in stiffness in plant fibre composites is found to be complex: structural changes in the elementary fibre cell wall and damage development in the composite have often competing effects on stress–strain behaviour. Indeed, the evolution in stiffness of plant fibre composites is found to be unlike that typically observed in traditional composites, and therefore needs to be taken into account in the design of structural components.     

Experimental and numerical study on the behavior of concrete subjected to biaxial tension and shear

In this article an experimental and numerical study on the behavior of concrete subjected to biaxial loading is outlined. For this purpose the unique biaxial machine available at the Stevin Laboratory was used. Two load-paths were pursued, namely axial tension at constant shear and proportional tension/shear. The recently developed lattice model was used to simulate the two load-paths. The remarkable feature of the lattice model is its ability to simulate curved overlapping cracks which resemble the experimental findings.     

Asymptotic and numerical analysis of a simple model for blade coating

Motivated by the industrial process of blade coating, the two-dimensional flow of a thin film of Newtonian fluid on a horizontal substrate moving parallel to itself with constant speed under a fixed blade of finite length in which the flows upstream and downstream of the blade are coupled via the flow under the blade is analysed. A combination of asymptotic and numerical methods is used to investigate the number and nature of the steady solutions that exist. Specifically, it is found that in the presence of gravity there is always at least one, and (depending on the parameter values) possibly as many as three, steady solutions, and that when multiple solutions occur they are identical under and downstream of the blade, but differ upstream of it. The stability of these solutions is investigated, and their asymptotic behaviour in the limits of large and small flux and weak and strong gravity effects, respectively, determined.     

Transverse properties of a Ti-6-4 matrix/SiC fibre-reinforced composite under monotonic and cyclic loading

The transverse response of a Ti-6-4/SM1140+ fibre-reinforced composite to both monotonic and cyclic loading has been investigated. Five distinct regions were found in the monotonic stress versus strain curve: (I) elastic deformation of the composite, (II) failure of the fibre/matrix interfaces, (III) elastic deformation of the remaining matrix ligaments, (IV) yielding of the matrix ligaments, and (V) gross plastic deformation, which ultimately leads to specimen failure. The stresses at which interface debonding, matrix yield and final failure occurred rose with increased displacement rate. Stressing to levels above the interface failure stress caused significant damage and limited (0.025%) plastic deformation. A non-linear stress-strain response was observed on unloading/reloading, because the presence within the specimen of constrained holes (containing debonded fibres) resulted in non-homogeneous elastic straining of the matrix. The transverse low-cycle fatigue lives of Ti-6-4/SM1140+composite specimens were strongly dependent on maximum stress for values up to the interfacial failure stress, but less so for maximum stresses greater than 260–265 MPa, where full fibre/matrix debonding had occurred. Fatigue life was also dependent on the uniformity of fibre spacings within the composite.     

Studies of mode I delamination in monotonic and fatigue loading using FBG wavelength multiplexing and numerical analysis

Bridging by intact fibers in composite materials is one of the most important toughening mechanisms. However, a direct experimental assessment of its contribution is not easy to achieve. In this work a semi-experimental method is proposed to quantify its contribution to fracture of unidirectional carbon fiber/epoxy double cantilever beam (DCB) specimens in mode I delamination under monotonic and 1 Hz fatigue loads. In each specimen, an embedded optical fiber with an array of eight wavelength-multiplexed fiber Bragg gratings is used to measure local strains close to the crack plane. The measured strain distribution serves in an inverse identification procedure to determine the tractions in the bridging zone in monotonic and fatigue loads. These tractions are used to calculate the energy release rate (ERR) associated with bridging fibers. The results indicate that the ERR due to bridging is about 40% higher in fatigue. The bridging tractions are further included in a cohesive element model which allows to predict precisely the complete load displacement curve of monotonic DCB tests. Using the principle of superposition and the identified tractions, the total stress intensity factor (SIF) is calculated. The results show that the SIF, at initiation, is very close to the one calculated at crack propagation and bridging by intact fibers is responsible for the entire increase in toughness seen in the DCB specimens used herein.     

Effect of particle size and boundary conditions on the shear stress in an annular shear cell

The effect of particle size and boundary geometry in granular shear flows is investigated. The measured shear stress of glass spheres in an annular shear cell experiment is reported. In order to explore the particle size effect, the experiments are run using four different particle diameters, d = 2, 3, 4, and 5 mm. It is found that the shear stress follows the Bagnold scaling with respect to the apparent shear rate, but deviates from it with respect to particle size. For high solids concentration the results deviate qualitatively from the kinetic theory for bounded granular shear flows, where the non-dimensional shear stress measured with large particles exceeds that measured for small particles by as much as one order of magnitude. The effect of the boundary geometry is explored by using three different boundary types; type 1 employs aluminum radial half-cylinders, type 2 employs aluminum hemispheres arranged in a polar hexagonal closed packed configuration, and type 3 employs sandpaper. It is shown that the geometry of the boundary has an insignificant effect on dilute flows of small particles. For denser flows and/or larger particles the difference is evident. The sandpaper boundary, which is different from the aluminum ones both in geometry and in its material properties, yields the lowest stress. These results imply that in granular materials-structure interaction, the structure’s properties are just as important as the properties of the granular material. Their interaction may also depend on the relative size between the structure and the grain size.     

Monotonic and cyclic behaviour of isolated FRP anchors loaded in shear

Using anchors made of fibre reinforced polymers (FRP) is an increasingly accepted method to delay the delamination of FRP sheets from the concrete surface and to enhance the capacity of FRP strengthened concrete structures. For many applications, FRP anchors are primarily loaded in shear. When used for seismic retrofitting schemes, the anchors are subjected to cyclic loads which may lead to premature fatigue failure. To date, however, shear strength of FRP anchors has experienced much less attention than their tension resistance. This paper documents tests on isolated FRP anchors which were conducted to determine the seismic shear capacity of FRP anchors and to propose design rules. To this end, a test setup was developed which allows direct and reverse loading of FRP anchors.     

Application of a split-Hopkinson tension bar in a mutual assessment of experimental tests and numerical predictions

This paper shows how experimental test results from a split-Hopkinson tension bar (SHTB) and numerical simulations of the test set-up can be used for mutual verification. Firstly, a SHTB where the tension stress wave is generated by pre-stretching a part of the incident bar is briefly presented. This SHTB is used to carry out tensile tests of four aluminium alloys at high rates of strain, while tests at low to medium strain rates were performed in a servo-hydraulic tensile test machine. Using the test results, the parameters of an anisotropic thermoelastic-thermoviscoplastic constitutive relation and a one-parameter fracture criterion are identified for the materials at hand. Subsequently, the material model is used in explicit finite element analyses of the SHTB tests, including the entire experimental set-up and the stress wave propagation during the test. The numerical predictions were found to represent the observed behaviour in the experimental tests fairly well.