Showing papers on "Slip line field published in 1993"

Fracture and slip of interfaces in cementitious composites. i: characteristics

Abstract: A new and relatively simple constitutive theory that describes fracture and slip of an interface in cementitious composites such as concrete is presented. The constitutive model is developed and formulated in analogy to the theory of incremental plasticity. Adhesion, debonding and mobilized friction mechanisms are considered in the characterization of the interface behavior. The debonding mechanism in the presence of a combination of normal and shear stress, i.e., the degradation of tensile and shear strengths, is monitored via a work‐softening rule that entails tensile crack opening as well as tangential slip. As a result, a fracture energy release‐based plasticity model is obtained. The material parameters are physically transparent and are conveniently calibrated from characteristic response functions that can be observed experimentally in pure tension and in pure shear. Analytical predictions are compared with experimental results for the case when constant normal traction is applied while the slip di...

Non-Schmid effects and localized plastic flow in intermetallic alloys

Abstract: A general strain rate dependent crystallographic slip theory which incorporates both non-Schmid effects and thermal deformation is presented. The theory is applied to the description of deformation in intermetallic alloys such as Ni 3 Al. For Ni 3 Al, as an example, it is shown that the approach embodies descriptions of stress state dependent yielding as observed experimentally as well as described by existing models such as that of Paidar, Pope and Vitek ( Acta Metall., 32 (1984)435). Finite element calculations of crystals deforming on only one slip system demonstrate that Asaro and Rice's ( J. Mech. Phys. Solids, 25 (1977) 309) criterion for bifurcation is a necessary condition for the formation of shear bands in crystals undergoing slip on only one slip system. Geometric effects are shown, however, to play an important role in the development of such localized shear bands. Strain rate sensitivity can delay significantly the formation of the localization, and lattice rotations relative to the surrounding lattice inside the shear bands are found to be quite small. This is in contrast to the case in multiple slip where lattice rotations play an important role in the localization process. In multiple slip the criteria for localized plastic flow are found to be of the sort described by Asaro ( Acta Metall., 27 (1979) 445; Mech. Mater., 4 (1985) 343), although localization generally occurs much sooner in the deformation process as a result of deviations from Schmid's rule.

A Slip Model for Linear Polymers Based on Adhesive Failure

Abstract: Eyring's theory of liquid viscosity was adapted to the special case of polymer molecules at a solid interface. The resulting model gives the slip velocity as a function of wall shear stres...

Discrete micromechanics of elastoplastic crystals

Abstract: The prediction of elastoplastic behaviour of single crystals is a basic problem which is central to the prediction of the overall behaviour of the crystal aggregate. It is well known that quasi-static and isothermal plastic deformation in single crystals arises almost solely from slip on specific crystallographic planes, and that this process occurs when the resolved shear stress on a critical slip system reaches a certain maximum value. In this paper we use small-deformation multisurface plasticity theory to phrase properly the problem of crystal slips at infinitesimal increments

Wall slip in the extrusion of linear polyolefins

Abstract: A molecular model, based on the concept of the activation rate theory, is developed for slip of a polymer as it flows through a die. The model correctly predicts the dependence of the critical shear stress for the onset of slip and the slip velocity of high molecular weight, linear polyethylene resins on temperature, pressure, wall shear stress, and molecular weight. It also predicts the increased slip which occurs when die surfaces are coated with a fluoropolymer having a low surface tension. It is not applicable at and above the critical shear stress for the onset of gross melt fracture.

Role of the secondary slip system in a computer simulation model of the plastic behaviour of single crystals

Abstract: A computer simulation model is proposed for investigating the plastic behaviour of single crystals oriented for single slip. To match transmission electron microscopy results our model consists of a dislocation system of parallel straight edge dislocations with a maximum of two different Burgers vectors, yielding essentially a two-dimensional problem. The velocity of the dislocations is taken to be proportional to the local shear stress, and generation of new dislocations is allowed. On the basis of the continuum theory of dislocations a special equation is set up to simulate a tensile deformation experiment with constant rate. An AMT DAP computer was used for the computations. In the first simulations, dislocation motion was allowed only in one slip system. This relatively simple case is enough to reproduce the properties of stage I of the deformation of single crystals. During the deformation the dislocations tend to arrange in sheets parallel to the slip direction. There is no sign of cell formation, but a long-range fluctuation appears in the stress field of the dislocations. In a couple of simulations we introduce two slip systems with 45° and 105° angles to the tensile axis, whereupon after stage I the stress starts to increase much more rapidly as the dislocations start to form walls and cells.

Experimental estimation of wall slip for filled rubber compounds

Abstract: The phenomenon of wall slip during flow of rubber compounds through capillaries is investigated for a typical styrene-butadiene elastomer with carbon black. It was found that at low temperature (110°C) the dependencies of slip velocity V c on shear stress are described by the power law but, additionally, V c depends on radius of a channel. At high temperatures there is a critical shear stress below which sliding is absent. Sliding appears only at higher shear stresses where, again, V c depends on shear stress and the radius of a channel.

Non-linear creep caused by slip plane formation

Abstract: The minute compression failures of the cell walls known as slip planes are quantified with respect to their occurrence in the compression side of beams subjected to mechano-sorptive creep. It is shown that slip planes occur at values of stress/strain lower than previously assumed. The slip plane intensity proves to be linearly related to creep and non-linearly related to stress. A permanent increase, ΔS, of longitudinal shrinkage /swelling results from slip plane formation, and this parameter therefore serves as a substitute for slip plane intensity, which is not easily measured. Slip plane intensity is considered a measure of accumulated damage, and the substitute parameter ΔS is used to quantify the accumulated damage of structural timber beams. The load level experienced by structural timber beams subjected to mechano-sorptive creep is shown to be reflected in the accumulated damage as assessed by ΔS.

Dynamic crack tip fields at steady growth and vanishing strain-hardening

Abstract: Solutions for propagating mode III cracks in elastic perfectly-plastic solids are obtained as the limit solutions for infinitesimal crack tip speed and infinitesimal linear strain-hardening. The solution for a perfectly plastic material by Chitaley and McClintock (1971, J. Mech. Phys. Solids 19, 147) is not obtained. A point above or below the crack plane will experience first a centred-fan slip line field, followed by elastic unloading and finally a plastic sector where reloading occurs at constant stress. A turning-point and a boundary layer develop immediately before elastic unloading occurs. This takes place at 32.8° in the quasi-static limit and at angles up to about 38° if inertia is considered. This should be compared with 19.7° for perfect plasticity. One key result is that even a very small crack tip speed affects the neartip conditions appreciably. Another noteworthy result is that stress rates change continuously through a fast wave, which in the limit will form a jump. The result is a discontinuous stress rate that may not be assumed a priori.

The Incipient Deformation Analysis for Plane Strain Open-Die Forging Processes with V-shaped Dies Using the Force Balance Method

Abstract: Force balance method is employed to predict forging information such as forging load, tool pressure and normal stress at the surface of tangential velocity discontinuity. The incipient stages of deformation for the plane strain forging of rectangular billets in V-shaped dies of different semi-angles are analysed. To construct an approximate model for the analysis of deformation by the force balance method in the incipient deformation stages, slip-line field is used. When the deformation mode by slip-line method is the same as that by force balance method, the slip-line method and the force balance method give identical solutions. The effects of die angle, coefficient of friction, billet geometries and deforma- tion characteristics are also investigated. In order to verify the validity of force balance analysis, the rigid-plastic finite element simulation for the various forgig parameters are performed and performed and find to be in good agreement.

The problem of parametric integration to the slip line field in the compression of thick workpiece

Abstract: An integration depending on a parameter to the compression of a thick workpiece has been obtained. For the conventional prevailing numerical formula $$\int {\phi _1 dy = \sum\limits_{i = 1}^n {\phi _i \cdot \Delta } } y$$ a definite functional relationship between ϕ and y is found. Therefore a paremetric integration can be used to get an analytical solution. Take the slip line field for l/h=0.121 as an example, the analytical solution is basically the same as the prevailing numerical one. It is justified theoretically that for the slip line field a parametric integration is perfectly possible for a satisfactory analytical solution.