Stress relaxation

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

Continuum Microviscoelasticity Model for Aging Basic Creep of Early-Age Concrete

Abstract: We propose a micromechanics model for aging basic creep of early-age concrete. Therefore, we formulate viscoelastic boundary value problems on two representative volume elements, one related to cement paste (composed of cement, water, hydrates, and air), and one related to concrete (composed of cement paste and aggregates). Homogenization of the “nonaging” elastic and viscoelastic properties of the material’s contituents involves the transformation of the aforementioned viscoelastic boundary value problems to the Laplace-Carson (LC) domain. There, formally elastic, classical self-consistent and Mori-Tanaka solutions are employed, leading to pointwisely defined LC-transformed tensorial creep and relaxation functions. Subsequently, the latter are back-transformed, by means of the Gaver-Wynn-Rho algorithm, into the time domain. Temporal derivatives of corresponding homogenized creep and relaxation tensors, evaluated for the current maturation state of the material (in terms of current volume fractions of cem...

Comprehensive Study of the Complex Dynamics of Forward Bias-Induced Threshold Voltage Drifts in GaN Based MIS-HEMTs by Stress/Recovery Experiments

Abstract: The transient recovery characteristics of the threshold voltage drift (ΔVth) of GaN-based HEMTs with a SiO2 gate dielectric induced by forward gate bias stress are systematically and comprehensively investigated for stress times from 100 ns to 10 ks, recovery times from 4 μs to 10 ks, and stress biases from 1 to 7 V. The measured recovery data are analyzed using the concept of capture emission time maps. It is shown that the observed data cannot be explained by simple first-order defect kinetics. It is revealed that the recovery curves for constant stress times scale with the stress bias. Furthermore, the shape of the recovery curves changes from concave to convex with increasing stress time, independent of the stress bias. For short stress times and low stress bias, a dominant rate limiting effect of the III/N barrier layer is proposed. Defect-related physical processes with a broad distribution of characteristic time constants are discussed to explain the logarithmic time dependency of ΔVth stress and recovery, at which the role of the Coulomb feedback effect, complex defects, and spatially distributed defects are considered.

An elasto-plastic self-consistent model with hardening based on dislocation density, twinning and de-twinning: Application to strain path changes in HCP metals

Abstract: In this work, we develop a polycrystal mean-field constitutive model based on an elastic–plastic self-consistent (EPSC) framework. In this model, we incorporate recently developed subgrain models for dislocation density evolution with thermally activated slip, twin activation via statistical stress fluctuations, reoriented twin domains within the grain and associated stress relaxation, twin boundary hardening, and de-twinning. The model is applied to a systematic set of strain path change tests on pure beryllium (Be). Under the applied deformation conditions, Be deforms by multiple slip modes and deformation twinning and thereby provides a challenging test for model validation. With a single set of material parameters, determined using the flow-stress vs. strain responses during monotonic testing, the model predicts well the evolution of texture, lattice strains, and twinning. With further analysis, we demonstrate the significant influence of internal residual stresses on (1) the flow stress drop when reloading from one path to another, (2) deformation twin activation, (3) de-twinning during a reversal strain path change, and (4) the formation of additional twin variants during a cross-loading sequence. The model presented here can, in principle, be applied to other metals, deforming by multiple slip and twinning modes under a wide range of temperature, strain rate, and strain path conditions.

Surface ripples, crosshatch pattern, and dislocation formation: Cooperating mechanisms in lattice mismatch relaxation

Abstract: We study the interplay of elastic and plastic strain relaxation of SiGe/Si(001). We show that the formation of crosshatch patterns is the result of a strain relaxation process that essentially consists of four subsequent stages: (i) elastic strain relaxation by surface ripple formation; (ii) nucleation of dislocations at the rim of the substrate followed by dislocation glide and deposition of a misfit dislocation at the interface; (iii) a locally enhanced growth rate at the strain relaxed surface above the misfit dislocations that results in ridge formation. These ridges then form a crosshatch pattern that relax strain elastically. (iv) Preferred nucleation and multiplication of dislocations in the troughs of the crosshatch pattern due to strain concentration. The preferred formation of dislocations again results in locally enhanced growth rates in the trough and thus leads to smoothing of the growth surface.

The Free Energy Function of the Doi‐Edwards Theory: Analysis of the Instabilities in Stress Relaxation

Abstract: The molecular model used by Doi and Edwards in developing their rheological theory admits a simple form for the free energy of deformation, at least when the independent alignment approximation is used. A general expression is derived, as well as the specific form which applies to an instantaneous shear deformation. The latter is used to analyze the possibility that instabilities, or “deformational phase separations,” arise in the course of stress relaxation experiments. The results of the analysis are compared with recent experimental results by Osaki and Kurata and by Vrentas and Graessley which show an anomalous relaxation behavior.