Stress relaxation

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

Dynamics of a Glassy Polymer Nanocomposite during Active Deformation

Abstract: We have examined the response of a polymer and a polymer nanocomposite glass to creep and constant strain rate deformations using Monte Carlo and molecular dynamics simulations. We find that nanoparticles stiffen the polymer glass, as evidenced by an increase in the initial slope of the stress−strain curve and a suppression of the creep response. In contrast to previous reports, we also find that, during deformation, the effective relaxation time or mobility of the material is only qualitatively characterized by the instantaneous strain rate. Constant strain rate and constant stress deformations have different effects on the material’s position on its energy landscape, and neither a mechanical variable, such as the stress or strain rate, nor a thermodynamic variable, such as the material’s position on its energy lanscape, is uniquely indicative of the relaxation times in the material.

DEM simulations of thermally activated creep in soils

Abstract: Discrete element modelling (DEM) has been used to simulate creep in assemblies of spherical grains possessing an interfacial coefficient of friction that varies with sliding velocity according to rate process theory. Soil stiffness is represented by a pair of values of linear spring stiffness normal and tangential to each inter-granular contact, and the limiting coefficient of contact friction is described as varying linearly with the logarithm of sliding velocity. DEM simulations of an assembly of 3451 spheres reproduce a number of significant phenomena including: creep rate as a function of the mobilisation of deviatoric stress; initially linear decay of creep strain rate with time plotted on log-log axes and with a slope m in the range −0·8 to −1; and ultimate creep failure in triaxial simulations at high deviatoric stress ratios. Creep-induced failure is shown to occur at a unique axial strain for a given state of initial packing, and to be linked with dilatancy. The numerical results are compared qua...

Residual stress state and hardness depth in electric discharge machining: de-ionized water as dielectric liquid

Abstract: Procedures and results of experimental work to measure residual stresses and hardness depth in electric discharge machined surfaces are presented. Layer removal method is used to express the residual stress profile as a function of depth caused by a die sinking type EDM. Thin stressed layers are removed from machined samples by electrochemical machining. Corresponding deformations due to stress relaxation are recorded for each removal to determine the stress profile from elasticity theory. The relational dependence of the machining parameters with residual stresses is obtained and a semi-empirical model is proposed for plastic mold steel for de-ionized water as dielectric liquid. These stresses are found to be increasing rapidly with respect to depth, attaining to its maximum value, around the yield strength, and then fall rapidly to compressive residual stresses in the core of the material since the stresses within plastically deformed layers are equilibrated with elastic stresses.

Deformation behavior of plasma-sprayed thick thermal barrier coatings

Abstract: A fundamental deformation study of several candidate diesel engine coating materials, independent of a substrate, has been conducted. Both plasma-sprayed 8%Y2O3–ZrO2 and CaTiO3 material specimens were subjected to various isothermal mechanical tests from room temperature to 800 °C to understand their basic constitutive behavior. In this work, it was found that all of the coating materials showed significant irreversible deformation behavior during cyclic loading. In addition, both time- and temperature-dependent behavior were observed through stress relaxation and strain-rate sensitivity tests. This inelastic behavior was seen to occur under all conditions, including tests at room temperature. In a diesel engine, this irreversible deformation behavior will give rise to detrimental tensile stresses when subjected to in-service compressive strain cycles. SEM observations suggest that the combined closing of pre-existing microcracks and sliding along microcrack surfaces results in the deformation behavior observed. In addition, a glassy phase found in the microstructure is thought to aid these microstructural deformation processes at high temperature.

What Controls the Structure and the Linear and Nonlinear Rheological Properties of Dense, Dynamic Supramolecular Polymer Networks?

Abstract: We investigated a series of telechelic polyisobutylenes, previously shown to exhibit self-healing, by means of small-angle X-ray scattering and rheology. All samples form a dense, dynamic network of interconnected micelles resulting from aggregation of the functional groups and leading to viscoelastic behavior. The dynamic character of this network manifests itself in the appearance of terminal flow at long time scales. While the elastic properties are distinctly molecular weight dependent, the terminal relaxation time is controlled by the functional end groups. The yielding properties under large deformation during startup shear experiments can be understood by a model of stress activation of the dynamic bonds. Stress relaxation experiments help to separate the nonlinear response into two contributions: a fast collapse of the network and a slow relaxation, happening on the time scale of the terminal relaxation. The latter is also known to control self-healing of the collapsed structure.