Tangential Separation Research Articles

Modelling of void nucleation and fiber debonding in a composite material

In order to approach the real behaviour of a metal-matrix composite, the existence of a decohesion stress at the reinforcement–matrix interface is taken into account. This modelling is done with a finite-element calculation based on a dynamic explicit formulation that allows for large non-linearities. The debonding behaviour at the interface is represented in terms of a cohesive-zone model that describes decohesion by normal separation as well as tangential separation. After the decohesion, the behaviour at the interface becomes a unilateral contact with friction. A first application is the case of an aluminium metal-matrix composite reinforced by spherical particles of silicon carbide, whilst in a second application, the fiber pull-out test realised on a single-filament polymer composite is modelled.

Fibre debonding and breakage in a whisker-reinforced metal

A numerical cell-model analysis of the development of damage in aluminium reinforced by aligned, short SiC fibres, is given for materials with arrays of transversely aligned fibres, to be compared with the results obtained previously for transversely staggered fibres. The stress distributions in the matrix and fibres are quite different for the different fibre arrangements, and this has a significant effect on failure by fibre breakage or decohesion. Fibre fracture is represented by a critical value of the average tensile stress on a cross-section, while interfacial failure is modelled in terms of a cohesive zone formulation that accounts for decohesion by normal separation as well as by tangential separation. It is shown that highly constrained plastic flow associated with transversely aligned fibres gives rise to early transverse void growth towards coalescence, but the onset of failure requires higher stress levels.

A numerical analysis of fracture and high temperature creep characteristics of brittle matrix composites with discontinuous ductile reinforcements

The role of the material parameters and interface characteristics in the fracture and creep behavior of discontinuous ductile fiber-reinforced brittle matrix composite systems was investigated numerically. To simulate the fracture behavior, the ductile fibers were modelled using a constitutive relationship that accounts for strength degradation resulting from the nucleation and growth of voids. The matrix was assumed to be elastic and fails according to requirements of a stress criterion. The debonding behavior at the fiber interfaces was simulated in terms of a cohesive zone model which describes the decohesion by both normal and tangential separation. Results indicate that for rigid interfaces between the ductile reinforcing phase and the matrix the contribution of the ductile reinforcement to the work-of-fracture value (toughness) of the composite increases with less exhaustion of its work-hardening capacity before the onset of matrix failure. Therefore the failure strength and elastic modulus values of the matrix become important material parameters. In the case of interfacial debonding the load transfer to the discontinuous reinforcements after matrix failure should be somehow maintained for utilization to full capacity of the reinforcement and interfacial behavior. In the creep regime, for rigidly bonded interfaces the creep rate of the composite is not significantly influenced by the material properties and geometric parameters of the ductile reinforcing phase owing to the development of triaxial stress state and constrained deformation in the reinforcement. For debonding interfaces the geometric parameters of the reinforcing phase become important; however, even with very weak interfacial behavior, low composite creep rates can be achieved by suitable selection of the geometric parameters of the ductile reinforcing phase. Significant increases in room temperature fracture toughness can be achieved without extensively sacrificing the creep strength by ductile discontinuous reinforcements.

The role of interfaces and matrix void nucleation mechanism on the ductile fracture process of discontinuous fibre-reinforced composites

The role of fibre morphology, interface failure and void nucleation mechanisms within the matrix on the deformation and fracture behaviour of discontinuous fibre-reinforced composites was numerically investigated. The matrix was modelled using a constitutive relationship that accounts for strength degradation resulting from the nucleation and growth of voids. For the matrix, two materials exhibiting identical strength and ductility but having different void-nucleation mechanisms (stress-controlled and strain-controlled) were considered and fibres were assumed to be elastic. The debonding behaviour at the fibre interfaces was simulated in terms of a cohesive zone model which describes the decohesion by both normal and tangential separation. The results indicate that in the absence of interface failure, for a given fibre morphology the void nucleation in the matrix is the key controlling parameter of the composite strength and ductility, hence, of the fracture toughness. The weak interfacial behaviour between the fibres and the matrix can significantly increase the ductility without sacrificing strength for certain fibre morphology and for certain matrix void-nucleation mechanisms.

Model studies of fibre breakage and debonding in a metal reinforced by short fibres

F or a metal reinforced by short, brittle fibres the development of damage by fibre cracking or decohesion is studied, taking into account the interaction between these two types of damage. The metal matrix composite, containing a periodic array of aligned fibres, is represented in terms of a cell model, and solutions for the stress and strain fields are determined numerically. Failure at the interface is modelled in terms of a cohesive zone model that accounts for decohesion by normal separation as well as by tangential separation, whereas fibre fracture is simply represented by a critical value of the average tensile stress on a cross-section. The effect of various material parameters and of the macroscopic stress state on the two types of damage is investigated, together with the subsequent mode of void growth.

Prandtl–Batchelor flow past a flat plate at normal incidence in a channel–inviscid analysis

A calculation is made of the steady profile adopted by a touching pair of vortex regions with equal and opposite vorticity in a bounded uniform stream. A family of possible solutions is deduced, depending upon the magnitude of a (non-dimensionalized) vorticity parameter. A similar calculation is carried out incorporating a flat plate normal to the stream at the upstream end of the vortex configuration. The requirement of tangential separation at the plate tip selects a unique value of the vorticity. It is found that, as the width of the plate is reduced in relation to that of the channel, the vortex profile asymptotically approaches one member of the above mentioned family. The asymptotic form of the flow in the vicinity of the plate is deduced for this case and compared with a previous calculation.

The role of fiber morphology on the creep behavior of discontinuous-fiber-reinforced composites

The role of the geometrical parameters of the fibers and effects of fiber distribution in the matrix on the creep deformation behavior of 20% discontinuous-fiber-reinforced composite were numerically investigated, including the debonding and fiber pull-out mechanisms. During the analyses, it is assumed that the matrix deforms with a power-law creep relationship and the fibers are rigid. The debonding behavior at the interface was stimulated in terms of a cohesive zone model which describes the decohesion by both normal and tangential separation. The results indicate that, for a given fiber geometry and distribution parameters in the matrix, an increase in the interfacial strength or an increase in the work of separation per unit area of interface reduces the creep rates. For even very weak interface characteristics, it is possible to achieve lower creep rates and delayed interface failure by suitable selection of the geometrical parameters of fibers and by controlling the distribution of fibers in the matrix. The role of applied stress direction and formation of a more ductile phase between the fibers and the matrix in the overall creep behavior were also elucidated.

Effect of ductile particle debonding during crack bridging in ceramics

The increased fracture toughness due to a dispersed ductile phase in a brittle material is studied. For a ductile particle intercepted by a brittle matrix crack the large elastic-plastic deformations during crack bridging are analyzed numerically, and the work of deformation is determined. Debonding at the particle-matrix interface is represented in terms of a cohesive zone model that describes decohesion by normal separation as well as decohesion by tangential separation. The particle deformations are analyzed for various levels of strain hardening and for different interface strengths. It is found that, compared to perfectly bonded particles, the toughening effect of the particles is significantly increased if partial debonding takes place during bridging. Thus, a limited amount of debonding is advantageous, but the computations also show that too low bond strength gives complete debonding, which adds only a little toughness to that of the brittle matrix material.

Effect of thermally induced residual stresses on the failure of a whisker-reinforced metal

Residual stresses in whisker-reinforced metal-matrix composites are computed, and their influence on deformation and failure mechanisms is analysed. The composite material containing a periodic array of aligned fibres is represented in terms of a unit cell model, and the analyses are carried out numerically. The residual stresses, arising from thermal expansion mismatch during cooling from the processing temperature, give compressive stresses on the matrix-fibre interface, and the effect of these stresses on debonding and on friction during fibre pull-out is analysed. Thermoplastic constitutive relations are used for the matrix material, and a cohesive zone model that accounts for decohesion by normal separation as well as by tangential separation is used to model failure at the interface. The development of fibre stresses during deformation of the composite is computed, and the possibility of fibre breakage prior to debonding is discussed.

Cellular migration patterns in the developing mouse cerebral cortex

The migration patterns of embryonic mouse cortical cells were investigated using a replication-incompetent retrovirus vector (BAG). The lateral ventricles of embryonic day 12 mouse embryos were infected with BAG and brains were harvested 2, 3, 4 and 6 days after infection. The location and morphology of all infected cortical cells were recorded from serial sections of entire brains, which were then reconstructed in three dimensions. Examination of the distribution of labelled cells revealed that there were migration patterns characteristic of each medial-lateral domain of the cortex. In the medial and dorsal areas, migration was often radial, although tangential spread increased with survival time, in large part due to ramification of cells in the intermediate zone. In the dorsolateral and lateral areas of the cortex, radial migration was generally not observed. Rather, variable extents of tangential migration occurred, and often resulted in wide separation of cells in the cortical plate. Almost all of the cellular dispersion occurred in the intermediate zone, although a modest degree of dispersion also occurred within the cortical plate itself. Most dispersion occurred in the mediolateral plane, with relatively little dispersion along the anteroposterior axis. Though characteristic migration patterns could be defined, wide variability in the extents of radial migration and tangential separation of cells was seen. The patterns of migration paralleled the distribution of radial glial fibers in all areas, and are most likely a reflection of the role of this network in supporting the migration of cortical neurons. The extent and variability of cellular dispersion supports a lineage-independent mechanism of cortical column ontogenesis.

Effect of fibre debonding in a whisker-reinforced metal

For a whisker-reinforced metal matrix composite, failure by decohesion of the fibre-matrix interface is analysed. The unit-cell model applied for the numerical analyses represents a periodic array of aligned fibres, where neighbouring fibres are somewhat shifted relative to one another. The debonding behaviour at the interface is represented in terms of a cohesive zone model that describes decohesion by normal separation as well as decohesion by tangential separation. The predicted failure initiates by void formation near the sharp edges at the fibre ends, and subsequently fibre pull-out is described. The effect of friction during pull-out is included in the model.

Tangential Flow Cell Separation from Mammalian Cell Culture

Abstract Tangential flow cell separation from fermenter-derived mammalian cell culture has been studied. The process is governed by transmembrane pressure and interactions of carry-over cells with the membrane. Membrane regeneration and comparison with other cell separation systems were discussed. Higher throughput per unit area over conventional dead end filter was demonstrated.

Ring shake in some hardwood species: The individual tree approach

AbstractThe formation of ring shake, a tangential separation of xylem tissue, is being studied in selected red oaks, white oaks, and black walnut. Previous studies have indicated that it is frequently necessary to make detailed analyses throughout one tree in order to evaluate correctly possible stages of development of this defect for the sample under investigation. In the present study, anatomical and chemical analyses both along the shake zone and for a normal zone within a black walnut tree are discussed. Anatomical investigations formed the basis for selection of xylem subdivisions for subsequent chemical analyses. The ring shake was tangentially confined to a single annual increment. This increment was radially subdivided into eight consecutive portions from earlywood through latewood. Each portion was used for determinations of specific gravity, extractives (ether, alcohol, hot water), methoxyl, and certain inorganics. Similar analyses were made on a “control” increment. The possible development of ring shake within the sample tree is presented in terms of resultant chemical and anatomical data.

A wake source model for bluff body potential flow

A theory is presented for two-dimensional incompressible potential flow external to a symmetrical bluff body and its wake. The desired flow-separation points are made the critical points of a conformal transformation to a complex plane in which surface sources in the wake create stagnation conditions at the critical points. The stagnation streamlines then transform to tangential separation streamlines in the physical plane, with separation at the desired pressure. The position and strength of the sources are determined by the requirements of separation position and pressure coefficient. The flow inside the separation streamlines is ignored and base pressure is assumed constant at the separation value. Features of the theoretical model include a finite wake width, a pressure distribution on the separation streamlines decreasing asymptotically towards the free stream value at infinity and a simple analytic expression for the pressure distribution on the body. Comparisons of the theory with experimental data and with other theories are presented for the normal plate, the circular cylinder, the 90° wedge, and the elliptical cylinder. Although simpler to apply than the other theories, the present theory produces at least as good agreement with the experimental data.