Stress relaxation

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

Stress relaxation of a soda lime silicate glass below the glass transition temperature

Abstract: Stress relaxation is an important effect in the ion-exchange procedure of glasses, as it controls the stress profile and the strength. Creep and stress relaxation tests have been performed to study the viscoelastic behavior of soda-lime silicate glass at typical ion-exchange temperatures. The experimental data of these tests can be fitted well by the Burger model and a comparison between the viscosity data from both tests was made. The strain and temperature dependences of the stress relaxation process were studied and the glass exhibited a non-linear viscoelastic behavior and an anomalous temperature dependence. In addition, it was found there is a relationship between the glass density and the stress relaxation behavior.

Mechanisms controlling creep of single phase metals and alloys.

Abstract: The physical mechanisms that control creep of single phase metals and alloys are reviewed Attention is given to both steady state and non-steady state properties The evolution of dislocation substructure in the course of primary creep is followed for both pure metals and solid solution alloys in an effort to demonstrate its dominant role in creep It is shown that the subgrain size and dislocation density scale in a natural way with the applied stress The long range internal back stresses that are responsible for creep anelasticity are shown to be determined not only by the scale of the substructure but also by the bias of the applied stress Creep transients following load changes are also described and interpreted in terms of the nature and properties of the creep substructure In addition, the characteristics of steady state creep are described by referring to the extensive phenomenological evidence which supports the concept that steady state creep at high temperatures is controlled by lattice self diffusion It is suggested that the diffusion controlled recovery and thermally activated glide approaches to creep should be complimentary rather than competitive In general, dislocation motion can occur either as a result of thermal activation past obstacles or as a consequence of obstacle recovery We argue that the inherently inhomogeneous dislocation substructures that form during creep produce inhomogeneous stresses that enhance the rate of obstacle recovery in the “hard” regions of the creep substructure and inhibit thermally activated glide in the “soft” regions This “composite” model of creep allows us to explain both steady state and non-steady state creep properties in a semiquantitative way

Creep and fracture of a vitreous-bonded aluminium oxide

Abstract: Creep and creep-rupture behaviour of a commercial grade of glass-bonded, 96% aluminum oxide was characterized as a function of temperature and applied stress. The creep data were fitted to the classical empirical relation usually used to describe this phenomenon. The apparent activation enthalpy, ΔH = 926 kJ mol−1, and the stress exponent,n = 4.8, lie at the high end of the range reported for two-phase materials, primarily as a result of structural modifications that occur during creep. A stress-modified Monkman-Grant relationship was fitted to the creep-rupture data to give a stress exponent of −4.2. None of the available theories of creep rupture provided a satisfactory description of the present set of data. Analytical electron microscopy was used to characterize the composition and structure of this material. In the as-received material the intergranular phase was a glass of nearly uniform composition. During high-temperature exposure, devitrification of the glass resulted in the formation of various crystalline phases within the intergranular region of the material. Devitrification depended on both the proximity to the surface, where it was most pronounced, and on the state of stress. In this regard, flexural creep samples exhibited extensive crystallization within the tensile region of the flexural specimens, but little crystallization within the compressive cross-section. From the composition of the retained glass, estimates of the viscosity of the glass at the grain boundaries were made and used, in combination with microstructural information, to compare the creep behaviour with available theories of creep. The results of this paper are consistent with percolation and solution precipitation mechanisms of creep deformation. By contrast, cavitation did not seem to play a major role in the creep deformation process.

Fluorescence studies of confinement in polymer films and nanocomposites: Glass transition temperature, plasticizer effects, and sensitivity to stress relaxation and local polarity

Abstract: Confinement effects in polystyrene and poly(methyl methacrylate) films and nanocomposites are studied by fluorescence. The ability to employ an intensive measurable, the excited-state fluorescence lifetime, in defining the glass transition temperature, Tg, of polymers is demonstrated and compared to the use of an extensive measurable, fluorescence intensity. In addition, intrinsic fluorescence from the phenyl groups in polystyrene is used to determine the Tg-confinement effect in films as thin as ~15 nm. The decrease in Tg with decreasing film thickness (below ∼60 nm) agrees well with results obtained by extrinsic pyrene fluorescence. Dye label fluorescence is used to quantify the enhancement in Tg observed with decreasing thickness (below ~90 nm) in poly(methyl methacrylate) films; addition of 2–4 wt% dioctyl phthalate plasticizer reduces or eliminates the Tg-confinement effect in films down to 20 nm thickness. Intrinsic polystyrene fluorescence, which is sensitive to local conformation, is used to quantify the time scales (some tens of minutes) associated with stress relaxation in thin and ultrathin spin-coated films at Tg + 10 K. Finally, the shape of the fluorescence spectrum of pyrene doped at trace levels in polystyrene films and polystyrene-silica nanocomposites is used to determine effects of confinement on microenvironment polarity.