Flow-induced residual stresses that arise during the injection moulding of amorphous thermoplastic polymers are calculated in both the filling and post-filling stage. To achieve this a compressible version of the Leonov model is employed. Two techniques are investigated. First a direct approach is used where the pressure problem is formulated using the viscoelastic material model. Secondly, generalized Newtonian material behaviour is assumed, and the resulting flow kinematics is used to calculate normal stresses employing the compressible Leonov model. The latter technique gives comparable results, while reducing the computational cost significantly. 相似文献
Both flow- and thermally-induced residual stresses which arise during the injection molding of amorphous thermoplastic polymers are calculated in the filling and post-filling stage. To achieve this, a compressible version of the Leonov model is employed. Two techniques to calculate flow-induced residual stresses are investigated. First, a direct approach is developed where the pressure problem is formulated using the viscoelastic material model. Second, generalized Newtonian material behavior is assumed in formulating the pressure problem, and the resulting flow kinematics is used to calculate normal stresses employing the compressible Leonov model. The latter technique gives comparable results, while reducing the computational cost significantly. 相似文献
A Eulerian approach is applied to the finite element modelling of neo-Hookean rubber material. Two major problems are encountered. The first problem is the construction of an algorithm to calculate stresses in the rubber material from velocities instead of displacements. This problem is solved with an algorithm based on the definition of the velocity gradient. The second problem is the convection of stresses through the finite element mesh. This problem is solved by adapting the so-called Taylor-Galerkin technique. Solutions for both problems are implemented in a finite element program and their validity is shown by test problems. Results of these implementations are compared with results obtained by a standard Lagrangian approach finite element package and good agreement has been found. 相似文献
The stress-optical rule relates birefringence to stress. Consequently, measurement of flow birefringence provides a non-intrusive
technique of measuring stresses in complex flows. In this investigation we explore the use of an axisymmetric geometry to
create a uniaxial elongational flow in polymer melts. In axisymmetric flows both birefringence and orientation angle change
continuously along the path of the propagating light. The cumulative influence of the material's optical properties along
the light's integrated path makes determination of local birefringence in the melt impossible. One can nevertheless use birefringence
measurements to compare with predictions from computer simulations as a means of evaluating the constitutive equations for
the stress. More specifically, in this investigation we compare the light intensity transmitted through the experimental set-up
vs entry position, with the theoretically calculated transmitted intensity distribution as a means of comparing experiment
and simulation. The main complication in our experiments is the use of a flow cell that necessarily consists of materials
of different refractive indices. This introduces refraction and reflection effects that must be modeled before experimental
results can be correctly interpreted. We describe how these effects are taken into account and test the accuracy of predictions
against experiments. In addition, the high temperatures required to investigate polymer melts mean that a further complication
is introduced by thermal stresses present in the flow cell glass. We describe how these thermal-stresses are also incorporated
in the simulations. Finally, we present some preliminary results and evaluate the success of the overall method.
Received: 2 April 2001 Accepted: 27 August 2001 相似文献
Analytical solutions are derived for one-dimensional consolidation, free swelling and electrical loading of a saturated charged porous medium. The governing equations describe infinitesimal deformations of linear elastic isotropic charged porous media saturated with a mono-valent ionic solution. From the governing equations a coupled diffusion equation in state space notation is derived for the electro-chemical potentials, which is decoupled introducing a set of normal parameters, being a linear combination of the eigenvectors of the diffusivity matrix. The magnitude of the eigenvalues of the diffusivity matrix correspond to the time scales for Darcy flow, diffusion of ionic constituents and diffusion of electrical potential. 相似文献
In recent years, damage directly due to tissue deformation has gained interest in deep pressure ulcer aetiology research.
It has been shown that deformation causes muscle cell damage, though the pathway is unclear. Mechanically induced skeletal
muscle damage has often been associated with an increased intracellular Ca2+ concentration, e.g. in eccentric exercise (Allen et al., J Physiol 567(3):723–735, 2005). Therefore, the hypothesis was that compression leads to membrane disruptions, causing an increased Ca2+-influx, eventually leading to Ca2+ overload and cell death. Monolayers of differentiated C2C12 myocytes, stained with a calcium-sensitive probe (fluo-4), were
individually subjected to compression while monitoring the fluo-4 intensity. Approximately 50% of the cells exhibited brief
calcium transients in response to compression, while the rest did not react. However, all cells demonstrated a prolonged Ca2+ up-regulation upon necrosis, which induced similar up-regulations in some of the surrounding cells. Population heterogeneity
is a possible explanation for the observed differences in response, and it might also become important in tissue damage development.
It did not become clear however whether Ca2+-influxes were the initiators of damage. 相似文献
In situ vascular tissue engineering has been proposed as a promising approach to fulfill the need for small‐diameter blood vessel substitutes. The approach comprises the use of a cell‐free instructive scaffold to guide and control cell recruitment, differentiation, and tissue formation at the locus of implantation. Here we review the design parameters for such scaffolds, with special emphasis on differentiation of recruited ECFCs into the different lineages that constitute the vessel wall. Next to defining the target properties of the vessel, we concentrate on the target cell source, the ECFCs, and on the environmental control of the fate of these cells within the scaffold. The prospects of the approach are discussed in the light of current technical and biological hurdles.