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This paper provides micromechanical bases to explain the time-dependent stress corrosion behaviour of high-strength prestressing steel wires. To this end, two eutectoid steels in the form of hot rolled bar and cold drawn wire were subjected to slow strain rate tests in aqueous environments in corrosive conditions corresponding to localized anodic dissolution and hydrogen assisted cracking. While a tensile crack in the hot rolled bar always propagates in mode I, in the cold drawn wire an initially mode I crack deviates significantly from its normal mode I growth plane and approaches the wire axis or cold drawing direction, thus producing a mixed mode propagation. In hydrogen assisted cracking the deviation happens just after the fatigue precrack, whereas in localized anodic dissolution the material is able to undergo mode I cracking before the deflection takes place. Therefore, a different time-dependent behaviour is observed in both steels and even in the same steel in distinct environmental conditions. An explanation of such behaviour can be found in the pearlitic microstructure of the steels. This microstructural arrangement is randomly oriented in the case of the hot rolled bar and markedly oriented in the wire axis direction in the case of the cold drawn wire. Thus both materials behave as composites at the microstructural level and their plated structure (oriented or not) would explain the different time-dependent behaviour in a corrosive environment.

Relaxation of residual stresses in metallic materials is caused by diffusion of inhomogeneously distributed solved alloying elements and by microstructural mechanisms like rearrangements of dislocation structure and precipitation processes, respectively. Both processes are often linked with each other. Therefore, the description and the modelling of stress relaxation on a microstructural basis is very complicated. The aim of this study was to demonstrate some models for stress relaxation in metallic materials. The paper presents the relaxation of tensile loading stresses in Ni-based and Co-based superalloys at high temperatures. In stress relaxation tests the microstructure development was connected with a rearrangement of the dislocation structures and by a decreasing dislocation density. The stress relaxation in carbide strengthened superalloys is a typical case of dislocation relaxation in the presence of precipitations. By including the determined microstructure parameters, the effective stress model and also a constitutive model yields to qualitatively correct predictions of the tensile and relaxation behaviour.

Bamboo with high specific strength is a renewable biomaterial. Studying the rheological properties of bamboo is helpful to improve the performance and quality of bamboo products. In this paper, four-element Burgers model was applied to describe the creep behavior of moso bamboo (Phyllostachs pubescens) in compression perpendicular to grain under hot-pressing process. The relationship between creep components and experimental factors (temperature, moisture content and stress level) was investigated. More importantly, four rheological parameters in Burgers model were also determined at different temperatures, moisture contents and stress levels. And the effect of experimental factors on rheological parameters was quantitatively explored. The results showed that, when compressive stress was below the yield limit, the amount of three components of creep was proportional to experimental factors, but the increase in temperature and moisture content could reduce the proportion of elastic deformation, and improve the proportion of viscoelastic deformation and viscous deformation. Besides, rheological parameters were insensitive to stress level when temperature and moisture content remained unchanged. But they were greatly affected by temperature and moisture content, presenting a linear inverse proportion to them.

The long-term compression–shear–seepage coupling of rock mass is a cause of many engineering geological disasters. This study aimed to explore the creep characteristics of rock mass under different seepage conditions. Based on the shear-creep–seepage test results of shale, the shear-creep–seepage model considering damage was constructed using a series connection of the elastomer (H), a nonlinear viscoelastic body with nonlinear function $\lambda $ (NVEP), a viscoplastic body with seepage switch $S$ (VPB), and a viscoelastic–plastic body considering damage (VEPB). The variation law of the model parameters was analyzed, and the results showed that the model effectively described the entire change process of rock-creep characteristics, notably the deformation law of the accelerated-creep stage. The correlation coefficient $R^{2}$ was greater than 0.98, and the fitting curve was highly consistent with the experimental data. Furthermore, the greater the seepage-water pressure, the smaller the shear stress applied in the corresponding test of each stage, and the greater the cumulative shear strain of each stage. Moreover, the seepage-water pressure had a damaging effect on the mechanical strength of the rock samples. The parameter values $k_{1}$ and $\lambda $ were negatively correlated with seepage-water pressure and shear stress, whereas the parameter values $k_{2}$ and $\eta _{1}$ were negatively correlated with seepage-water pressure and positively correlated with shear stress. The results of this study can provide theoretical support for the research and analysis of rock-mass engineering stability under long-term seepage conditions.

The shear and bulk relaxation moduli required to characterize a homogeneous, isotropic, and linearly viscoelastic material were determined using a confined compression experiment and by introducing a new iterative scheme that accounts for the fact that the hoop and radial strains are not step functions. In addition, the coefficients of thermal and hygral expansion of the epoxy being considered were determined along with its diffusive behavior. Fickian diffusion of moisture was confirmed by coupling radial diffusion in an epoxy disk with optical interference measurements of the out-of-plane displacements. These properties are essential components of the modified free-volume model of nonlinear viscoelasticity established by Popelar and Liechti (J. Eng. Mater. Technol. 119:205–210, 1997, Mech. Time-Depend. Mater. 7(2):89–141, 2003). For the nonlinear component of the model, its distortional parameters were evaluated from Arcan pure shear test results at one strain rate and temperature/humidity state. The nonlinear viscoelasticity model was then used to predict the shear stress-strain response under other conditions. The dilatational parameters were extracted from uniaxial tensile test results at one strain rate and temperature humidity state and predictions under other conditions compared favorably with the results from experiments. This exercise adds to a growing list of glassy polymers whose nonlinear stress-strain behavior can be modeled by this modified free-volume model.

This paper intend to describe the entire creep process of a salt rock under triaxial loading, especially the creep characteristics in the accelerated creep stage, by replacing the Newtonian dashpot in the Maxwell model with the variable-order fractional derivative component and extending it from one to three dimensions. The experimental data were obtained from the creep of salt rock under multistage loading for approximately five months to examine the applicability of the new model. The fitting results show that the new model can accurately describe the creep behavior of the salt rock and can fully reflect the accelerative rheological property of the salt rock. Compared with the traditional nonlinear rheological model, the new model has fewer parameters and increased accuracy of the fitting results, and is anticipated to have an increased application area.

Dispersion-hardened aluminum materials of pure aluminum with extremely fine oxide and carbide dispersions and very fine grain sizes were creep-deformed under compressive loadings between 573 and 773 K. The creep behavior of the investigated materials is influenced by time, temperature, stress level and microstructure. An increasing content of dispersions causes increasing threshold stresses σthand resistances against creep. The Norton plots of the minimum creep rate $$\dot \varepsilon _{\min } $$ versus stress σ are characterized by extremely high stress exponents n. On the basis of the threshold concept it is demonstrated that the same diffusion process dominates in the dispersion-hardened aluminum materials as in pure aluminum. Their true stress exponents n*as the slopes of the best fit lines of the $$\log \dot \varepsilon _{\min } {\text{ }}versus{\text{ }}\log (\sigma - \sigma _{{\text{th}}} )$$ are close to 5. The threshold stress decreases considerably with increasing temperature due to the thermally activated recovery of long-range internal back stresses of quasi-planar dislocation structures on the grain boundaries.