Stress relaxation

About: Stress relaxation is a research topic. Over the lifetime, 12959 publications have been published within this topic receiving 270815 citations.

Ag/cu(111) structure revisited through an extended mechanism for stress relaxation

Abstract: The Ag/Cu(111) system can be considered as a model one concerning the atomic structure of one monolayer deposited on a substrate in the case of strong size mismatch. Thus, it has been the subject of many experimental [Auger electron spectroscopy, low-energy electron diffraction and scanning tunneling microscopy (STM)] and theoretical studies, in particular within N-body potentials. Although most results agreed both with the existence of an $n\ifmmode\times\else\texttimes\fi{}n$ superstructure accommodating the size mismatch and with a strong corrugation of the Ag adlayer, the morphologies---derived from STM on the one hand and numerical simulations on the other hand---were not found to be consistent. Here we revisit the previous theoretical study, taking into account different additional mechanisms (Ag and Cu vacancy formation, partial dislocation loop) to relax the interfacial stress. As a result, we obtain that the most efficient relaxation mechanism is the formation of partial dislocation loops in the first Cu substrate layer, requiring the formation of four or five Cu vacancies per unit cell in this plane. This leads to a strong damping of the corrugation in the Cu underlayers, and a perfect agreement is reached between observed and calculated surface morphology.

Power-law-like stress relaxation of block copolymers. Disentanglement regimes

Abstract: We consider stress relaxation in a strongly segregated lamellar mesophase, where block copolymers are in the "brush" state with junction points confined to the interface between the adjacent lamellae and blocks stretched out away from it. If the molecular weight of the blocks is large enough, they entangle with their neighbors as well as with blocks from the opposite brush. The number of entanglements of one particular chain with the opposite brush varies from chain to chain even in the monodisperse system. We demonstrate that this dispersion of the number of entanglements leads to a very broad spectrum of relaxation times and to an effectively power-law-like stress relaxation function log G - (log t)" with CY = 1/2 (a = 2) in the strong (weak) chain-stretching limit. We analyze various disentanglement mechanisms for diblocks and triblocks and the onset of the diffusion of copolymers along the interface. Another relaxation mechanism is due to the displacement of a block across the interface into the "enemy" phase. We conclude that for highly entangled copolymers it will be an important mode of stress relaxation (especially for triblock copolymers). For asymptotically long times the relaxation of stress along the perfectly ordered lamella will be liquidlike. In a system with defects in domain structure the relaxation of stress is controlled by the processes of equilibration of excess density along the layers. For the lamellar mesophase we found G(t) - t 4. 2, while for the cylindrical mesophase, G(t) - t

The large shear strain dynamic behaviour of in-vitro porcine brain tissue and a silicone gel model material.

Abstract: The large strain dynamic behaviour of brain tissue and silicone gel, a brain substitute material used in mechanical head models, was compared. The non-linear shear strain behaviour was characterised using stress relaxation experiments. Brain tissue showed significant shear softening for strains above 1% (approximately 30% softening for shear strains up to 20%) while the time relaxation behaviour was nearly strain independent. Silicone gel behaved as a linear viscoelastic solid for all strains tested (up to 50%) and frequencies up to 461 Hz. As a result, the large strain time dependent behaviour of both materials could be derived for frequencies up to 1000 Hz from small strain oscillatory experiments and application of Time Temperature Superpositioning. It was concluded that silicone gel material parameters are in the same range as those of brain tissue. Nevertheless the brain tissue response will not be captured exactly due to increased viscous damping at high frequencies and the absence of shear softening in the silicone gel. For trend studies and benchmarking of numerical models the gel can be a good model material.