Experimental studies of density segregation in the 3D rotating cylinder and the absence of banding

We experimentally investigated 3D biparticulate systems that segregate solely due to density differences in the 3D horizontal rotating drum geometry and compare these to systems which segregate due to size differences. Radial segregation was observed in all systems studied after a few drum rotations. Size induced axial segregation (banding) was observed, as expected. However, contrary to what has sometimes been reported, we found that density differences alone did not induce axial segregation for density ratios up to 4.9.     

Experimental and numerical study of granular medium-rough wall interface friction

Wall roughness plays a crucial role in granular medium - rough wall interface friction. In this study, an experimental device has been designed to study the influence of boundary conditions, more specifically wall roughness, on the behavior of sheared granular medium. The study is based on use of an analog model, and consists of simulating roughness by means of notches and grains in the medium by monodisperse beads and on use of a numerical model based on the discrete element method. The test protocol entails displacing at fixed speed notched rods under confined granular medium. Movement of the beads layer near the rods as well as friction of the beads against the rods are both studied herein. Results indicate that the parameter controlling friction at the granular medium - rough wall interface is primarily the depth of beads embedment in surface asperities. The objective of the associated numerical modeling is to supplement the experimental results.     

Powder flow in an automated uniaxial tester and an annular shear cell: a study of pharmaceutical excipients and analytical data comparison

Background: An automated version of uniaxial powder flow testing has recently been developed and there is a need for experimental data from pharmaceutical powders.

Purpose: To compare the novel testing method with an annular shear cell using different pharmaceutical excipients. A particular aim was to gain an improved understanding of potential differences in the obtained flow results.

Methods: Nine excipients were studied with both flow testers at different consolidation levels. Unconfined yield strengths were determined at similar major consolidation stresses. Finally, an anisotropic stress factor was calculated and the fractal character of the powders was assessed by means of image analysis in a rotating drum.

Results: Data correlated generally well; however, the unconfined yield strength from uniaxial testing resulted mostly in lower values compared to annular shear cell testing. Differences were specific for the given excipients and mannitol demonstrated the highest discrepancy of measured flow parameters. The differences were first discussed by considering wall friction, anisotropy of forces, brittleness as well as the fractal nature of the powder surface. This heterogeneity of the powder as well as the anisotropy of forces was also found to be important for the relative flow index.

Conclusions: The automated uniaxial method demonstrated faster and more reproducible flow testing as compared to an annular shear cell. Therefore, the new method has a high potential in pharmaceutics for example in the quality-control of powders.     

Experimental and numerical investigation of the influence of imperfect bonding on the strength of NCF double-lap shear joints

The influence of imperfect bonding, owing to partial lack of adhesive, on the strength of composite non-crimp fabric (NCF) double-lap shear (DLS) joints was experimentally and numerically investigated. Fabrics were layered and compacted using a thermoplastic veil while infiltration of the preforms was done using the vacuum assisted process. Paste adhesive bonding was carried out by implementing the novel insertion squeeze flow process. Quality of adhesive bonding was tested using X-ray imaging and ultrasonic C-scan inspection. The tensile lap shear strength of the DLS joints was determined experimentally. Digital macrographs revealed that the specimens failed due to shear failure of the adhesive (debonding) and fracture of the composite boundary layer. As a second approach, a mesomechanical model based on the FE method and the (homogenized) progressive failure analysis method was developed. In the model, the areas without adhesion, as detected by the C-scans, were included. Numerical simulations of failure initiation and progression at the NCF joint and the adhesive indicated that it is possible to predict the strength and failure mechanisms of the imperfect bonded DLS joints.     

Regimes of segregation and mixing in combined size and density granular systems: an experimental study

Granular segregation in a rotating tumbler occurs due to differences in either particle size or density, which are often varied individually while the other is held constant. Both cases present theoretical challenges; even more challenging, however, is the case where density and size segregation may compete or reinforce each other. The number of studies addressing this situation is small. Here we present an experimental study of how the combination of size and density of the granular material affects mixing and segregation. Digital images are obtained of experiments performed in a half-filled quasi-2D circular tumbler using a bi-disperse mixture of equal volumes of different sizes of steel and glass beads. For particle size and density combinations where percolation and buoyancy both contribute to segregation, either radial streaks or a “classical” core can occur, depending on the particle size ratio. For particle combinations where percolation and buoyancy oppose one another, there is a transition between a core composed of denser beads to a core composed of smaller beads. Mixing can be achieved instead of segregation if the denser beads are also bigger and if the ratio of particle size is greater than the ratio of particle density. Temporal evolution of these segregated patterns is quantified in terms of a “segregation index” (based on the area of the segregated pattern) and a “shape index” (based on the area and perimeter of the segregated pattern).     

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.     

Experimental and numerical investigations into the effect of an interference fit on the fatigue life of double shear lap joints

In this research the effect of bolt interference fit on the fatigue life of lap joints in double shear was investigated by conducting experimental fatigue tests and also analytically by FE simulation. In the experimental part, fatigue tests were carried out on specimens made from aluminium alloy 2024-T3 plates joined together as double lap joints and secured using bolts having fits ranging from zero clearance to different levels of interference. The results demonstrate how the failure is affected using different levels of interference fit. In the numerical study, 3-D FE models were used to simulate the different pin in hole fits considered and the results have been used to help explain the trends which were observed in the experimentally obtained S–N curve behaviour.     

Basic mechanical behaviors and mechanics of shear banding in BMGs

In this paper, some basic mechanical behaviors of bulk metallic glasses (BMGs) were discussed. It can be found from the discussions that the mechanical behaviors of BMGs are mainly due to the formation and operation of shear bands in BMGs. Furthermore, the relevant mechanics of shear banding were investigated in the paper. The theoretical analysis of deformation coupling thermal softening and free volume creation softening demonstrates that the free volume creation and thermal softening can jointly promote the formation of shear bands in BMGs, and the observed post mortem shear band width looks more like that governed by free volume creation.     

Reversible nature of shear bands in copper single crystals subjected to iterative shear of ECAP in forward and reverse directions

Copper single crystals were subjected to equal-channel angular pressing for two passes via the routes A and C, in order to examine the effect of iterative shear in forward and reverse directions on the development of shear bands in a crystallographic aspect. Shear bands were clearly revealed metallographically after one pass, which accompanies splitting of distinct crystallographic orientations. These shear bands remained after the second pass via route A, where shear was given in a forward direction with regard to the previous shear. Micro-indentation tests show that the shear bands were harder than the matrix, and both the shear bands and the matrix became harder progressively by the second pass. In route C, where the second shear is given in the parallel plane, but in the reverse direction with regard to the previous shear, most of these shear bands were less visible in metallographic and EBSD observations. Besides, the distribution of microhardness became homogeneous across the traces of shear bands and the matrix. It is suggested that the shear bands were dissolved by merging with the matrix by diffusion of the geometrically necessary dislocations (GND) delineating the shear bands and the matrix.     

Self-organization behaviors of shear bands in 7075 T73 and annealed aluminum alloy

The self-organization behaviors of multiple adiabatic shear bands (ASBs) in the 7075 T73 aluminum alloy were investigated by means of the thick-walled cylinder (TWC) technique. Shear bands first nucleate at the inner boundary of the aluminum alloy tube and propagate along the maximum shear stress direction in the spiral trajectory. On the cross section of the specimen, shear bands distribute either in the clockwise or the anticlockwise direction. The number of ASBs in the clockwise direction is roughly twice that in the anticlockwise direction. However, the 7075 annealed alloy does not generate any shear band under the same experimental conditions.Numerical simulation with coupled thermo-mechanical analysis was carried out to investigate the evolution mechanism of adiabatic shear bands. Both uniform and non-uniform finite element models were created. The simulation results of the non-uniform model are in better agreement with those of the experiment. In the non-uniform case, the spacing between ASBs is larger than that of the uniform model, and most of the ASBs prefer to propagate in the clockwise direction. For the first time, two types of particles (second phase), hard particles and soft particles, are separately introduced into the metal matrix in the non-uniform model to simulate their effects on the self-organization of ASBs. The soft particles reduce the time required for ASBs nucleation. Stress collapse first occurs at the region where the soft particles are located and most of the ASBs pass through these soft particles. However, ASBs propagate along the paths that are adjacent to the hard particles instead of passing through them. As experimental observations, there is no shear band nucleating in the annealed alloy in simulation. Under the same conditions, the energy barrier for the formation of ASBs in the annealed aluminum alloy is about 2.5 times larger than that in the T73 alloy, which means that the adiabatic shearing is less likely to nucleate in the annealed alloy. This is consistent with the experimental and numerical simulation results.     

Exploring the role of shear in oblique impacts: A comparison of experimental and numerical results for planar targets

Oblique impacts produce asymmetric damage patterns due to asymmetric, directed shock waves; these patterns are seen for both laboratory and planetary scale craters [1] and [2]. Previous laboratory and computational studies of impact-induced damaged have focused mainly on tensile failure following hypervelocity impacts. Though extension plays a significant role in impact-induced damage, it is widely accepted that shear failure also occurs during hypervelocity impacts. Shear failure occurs over a variety of scales both during and after impacts [3], [4] and [5]. Here we examine this process in more detail for oblique impacts. Experiments not only provide a general view of small-scale processes (including damage patterns in their final form), but also can be difficult to relate to larger impacts with confidence, even though similarities can be documented [e.g. 1]. Detailed computer models provide complementary information. Although they detail underlying processes during crater formation, they do not always contain adequate constitutive models, thereby requiring simplifying assumptions. A comprehensive model taking into account deformation following failure of rocks is still unavailable, which limits conclusions based solely on numerical simulations. Consequently, a combination of models and experiments must be used. Impact experiments into planar polymethylmethacrylate (PMMA) targets at small scale are examined in an attempt to constrain the sequence, location and style of failure. Two- and three-dimensional CTH models (with identical conditions to the experiments) were computed using a variety of failure criteria in order to determine the parameter set that best matches the experimental results. High-speed imaging recorded the sequence and location of failure within various PMMA targets, which was then compared with results from theoretical models. The CTH models provide critical details about specific failure style and indicate only minimal failure due to extension following the impact except for tensile failure at the base of the block. Instead, shear failure dominates below the crater. While the CTH hydrocode models generally match the extent of the damaged region, some differences remain. Projectile properties (density, composition, size) for impacts with the same kinetic energy affect the extent, style, and growth of damage in a given target. This includes differences in degree of uprange damage, subarcuate fractures, and sub-parallel failure planes. Comparisons between experiment and hydrocode results reveal that projectile failure (even at hypervelocity) contributes to the observed differences.     

Temperature measurement under convection and segregation in a vibrated bed of powder: A numerical study

In numerically simulated vibrated beds of powder, we measure temperature under convection by the generalized Einstein's relation. The spatial temperature distribution turns out to be quite uniform except for the boundary layers. In addition to this, temperature remains uniform even if segregation occurs. This suggests the possibility that there exists some thermal equilibrium state even in a vibrated bed of powder. This finding may lead to a unified view of the dynamic steady state of granular matter.     

Experimental and numerical investigation of the debonding interface between an old concrete and an overlay

The paper focuses on debonding propagation along an interface, notably on the major influence of the interlocking between the two faces of the debonding interface. The aim of the study is to obtain the data necessary for relevant and efficient debonding modelling. The work associates experiment and simulation with the purpose of quantifying the interlocking along the interface. The overlay material investigated was a fibre reinforced mortar (FRM). Direct tension tests of notched FRM specimens were firstly conducted to obtain the tensile strength and the residual normal stress—crack width relationship. Its Young's modulus was determined from compression tests. The substrate-overlay interface was investigated by direct tension tests and flexure tests performed on composite substrate-overlay specimens. The direct tension tests provided the interface tensile strength and the relationship between debonding-opening and residual normal tensile stress. Three point flexural static tests informed on the structural behaviour of the interface. The debonding interface propagation was monitored using a video-microscope with a maximum enlargement of ×175. Using the identified and quantified parameters, modelling of the above mentioned static tests was carried out by the finite elements method using CAST3M code developed in France by CEA (Centre for Atomic Energy). The comparison of modelling and experiment results shows a good coherence and proves the important role of interlocking on the debonding mechanism.     

Experimental and numerical simulation of the flow in a driving module with an annular and linear double-slot nozzle

This paper presents the results of numerical and experimental studies of the flow and comparison of propulsion performance characteristics of a model of a jet engine exhaust system equipped with an annular or (equivalent in gas consumption) linear double-slot nozzle with an inner cavity and circular segment deflector in the axial section. Calculations performed for the annular nozzle and double-slot nozzle corresponding to it in geometric parameters demonstrate that a flow similar to the flow in nozzles with a central body is formed in the exhaust system. According to the data obtained, the initial turn of the flow takes place in the oblique shock wave. In the double-slot nozzle, the final turn of the flow in the direction of the thrust vector occurs in a configuration of four shock waves positioned downwards in the flow; in the annular nozzle, it is in the intense barrel shock wave. It was established that the exhaust system with an annular of the linear doubleslot nozzle develops a thrust and specific impulse that exceed the corresponding values for the sonic nozzle equivalent in gas consumption by almost a factor of 2.     

Microstructural characterization and evolution mechanism of adiabatic shear band in a near beta-Ti alloy

The adiabatic shear band (ASB) was obtained by split Hopkinson pressure bar (SHPB) technique in the hat-shaped specimen of a near beta-Ti alloy. The microstructure and the phase transformation within the ASB were investigated by means of TEM. The results show that the elongated subgrains with the width of 0.2-0.4 μm have been observed in the shear band boundary, while the microstructure inside the ASB consists of fine equiaxed subgrains that are three orders of magnitude smaller than the grains in the matrix. The β → ω_(althermal) phase transformation has been observed in the ASB, and further analysis indicates that the shear band offers thermodynamic and kinetic conditions for the ω_(althermal) phase formation and the high alloying of this alloy is another essential factor for this transformation to take place. The thermo-mechanical history during the shear localization is calculated. The rotational dynamic recrystallization (RDR) mechanism is used to explain the microstructure evolution mechanism in the shear band. Kinetic calculations indicate that the recrystallized fine subgrains are formed during the deformation and do not undergo significant growth by grain boundary migration after deformation.     

Effects of fiber misalignment and transverse shear modulus on localization and shear crippling instability in thick imperfect cross-ply rings under external pressure

The effect of transverse shear modulus on the compressive response of a thick plane strain cross-ply ring (very long cylindrical shell) weakened by the presence of a modal imperfection is investigated. The present study is primarily motivated to obtain the hitherto unavailable results pertaining to the effect of reduced transverse shear modulus, , of a lamina weakened by the presence of randomly distributed fiber misalignments. A simple expression for the reduced transverse shear modulus, , of a layer material is derived in terms of the average fiber misalignment angle. A fully nonlinear finite element analysis, that employs a cylindrically curved 16-node layer-element and is based on the assumption of layer-wise linear displacement distribution through thickness (LLDT), is utilized in the analysis of the afore-mentioned cross-ply ring. The interaction of a micro-structural defect in the form of initial fiber misalignments with its macro-structural counterpart represented by a modal imperfection is a key to understanding this meso-structural level phenomenon. Hitherto unavailable numerical results pertaining to the influence of this effect on the localization of buckling patterns and the ensuing shear crippling instability are also presented.     

Experimental investigation of transient nonlinear phenomena in an annular thermoacoustic prime-mover: observation of a double-threshold effect

Thermoacoustic engines have been subjected to numerous studies for the past 10 years. Our current research is focused on the transient regime in an annular thermoacoustic prime-mover. It appears that several nonlinear phenomena can play a role in the amplification and saturation regimes. Indeed, acoustically induced conductivity, forced convection due to acoustic streaming, minor loss phenomenon, and saturation due to harmonic generation can be quoted among the others. The experiments presented here show for the first time a double-threshold phenomenon during the amplification regime. The first threshold, which corresponds to the setting of the thermoacoustic instability, is followed by a saturation regime. Then after a time delay, without any changes in the control parameters, a second threshold corresponding to an additional amplification has been observed.     

Experimental and numerical investigation of fracture in double-edge notched steel plates

Double-edge notched (DENT) steel plates were pulled until complete fracture and several experimental observations were made (using profilometry and scanning electron microscopy). The essential work of fracture (EWF) model was found to be well verified. Numerical simulations—up to the maximum load only—of some experiments were performed using the finite element method (FEM), and incorporating geometric and material non-linearities (large deformation elasto-plasticity). Some experimental measurements were compared with the corresponding numerical computations and excellent agreement was found.