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[en] We have studied the dynamics of propagation in one dimension of a magnetic domain wall (DW) in the presence of an exchange bias field in nanowires patterned in two-dimensional ultra-thin perpendicularly magnetized ferromagnetic–antiferromagnetic (F–AF) films. The DW velocity v(H) is strongly reduced when the DW propagates against the exchange bias field, and its field dependence is consistent with a creep process with a critical exponent μ = 1/4. The increase in energy barrier responsible for the reduction of v(H) cannot be explained by the introduction of a macroscopic exchange bias field. Instead it arises from the AF–F interface that provides an extra-source of random pinning. (paper)
[en] The P-NID (Parametric, Numerical Isothermal Datum) method of extrapolating creep rupture data has been applied to the four large datasets recently analysed by the European Creep Collaborative Committee in order to re-evaluate its own recommended procedure. It is demonstrated from an analysis of these and other datasets that the P-NID method provides a very reliable basis for extrapolation. - Highlights: • Comprehensive description of P-NID approach to rupture extrapolation. • Demonstration of modelling accuracy for four large rupture datasets by P-NID method. • Demonstration of extrapolation accuracy from models of time-restricted datasets. • Comparison of P-NID models and ECCC models for the same data.
[en] Understanding the time-dependent mechanical behavior of rocks is important from various aspects and different scales such as predicting reservoir subsidence due to depletion or proppant embedment. Instead of using the conventional creep tests, nano-dynamic mechanical analysis (nano-DMA) was applied in this study to quantify the displacement and mechanical changes in shale samples over its creep time at a very fine scale. The results showed that the minerals with various mechanical properties exhibit different creep behavior. It was found that under the same constant load and time conditions, the creep displacement of hard minerals would be smaller than those that are softer. On the contrary, the changes in mechanical properties (storage modulus, loss modulus, complex modulus and hardness) of hard minerals are larger than soft minerals. The results from curve fitting showed that the changes in creep displacement, storage modulus, complex modulus and hardness over creep time follow a logarithmic function. We further analyzed the mechanical changes in every single phase during the creep time based on the deconvolution method to realize each phase’s response independently. Two distinct mechanical phases can be derived from the deconvolution histograms. As the creep time increases, the volume percentage of the hard mechanical phase decreases, while this shows an increase for soft phases. The results suggest that nano-DMA can be a strong advocate to study the creep behavior of rocks with complex mineralogy.
[en] A common format is proposed which relates creep deformation equations to the equation formats found for elastic and plastic deformation. This common format gives a reproducible form for writing the relationships between load, deformation and geometry factors. The importance of this common format is that the equation forms used in elastic and plastic deformation can be used to develop the equations needed for creep deformation, thus making the development of some of the relationships used for the analysis of cracked body geometries in creep much simpler. The evaluation of a common forms of equations between creep deformation and elastic and plastic deformation is conducted experimentally and numerically in this paper. A compact specimen geometry is used as the model for both evaluations. The implications of this common format in developing relationships are also briefly discussed
[en] Creep behaviour of polycrystalline silver at 900 deg. C depends primarily on the purity of the Ag. Specimens of 99.999% purity dynamically recrystallized during creep; the stress for recrystallization was approx. 3.5 MPa. Oxygen partial pressure had no effect on the creep response. For 99.9% Ag, no dynamic recrystallization occurred; instead, steady-state creep was observed, with the rate-controlling deformation occurring primarily by dislocation motion. Long-term annealing of 99.9% Ag in increased the steady-state stress by a factor of approx. 5. Depending on test conditions, the 99.9% Ag was up to twice as strong as the 99.999% Ag. (author)
[en] Using acoustic resonators, we have investigated the mechanical stiffness of contacts between rough surfaces. In the first part, the underlying acoustic model is validated with experiments showing a transition from elastic to inertial loading. The second part is concerned with the increase in contact stiffness induced by transient exposure to a humid environment. A novel mode of surface deformation is proposed, which builds on a capillary instability. Under certain conditions, a slight decrease in the mean distance between the two surfaces may induce a rather strong increase in capillary attraction, while leaving the elastic forces of repulsion almost constant. The thus-created negative differential spring constant induces a collapse of the gap in-between neighboring load-bearing asperities. The initial decrease in distance may either be induced by local asperity creep or by distortions of the contacting surfaces on a larger scale, which improve the interlock of the asperities at a small cost of strain energy
[en] Electrochemomechanical deformation (ECMD) of the conducting polymer polyaniline film is studied to investigate the behaviour of actuation under tensile loads. The ECMD was induced by the strains due to the insertion of ionic species (cyclic strain) and a creep due to applied loads during the redox cycle. The cyclic strain was enhanced by the experience of high tensile loads, indicating a training effect. The training effect was explained by the enhanced electrochemical activity of the film. The creep was recovered by removal of the tensile load and several electrochemical cycles. This fact indicates that the creep results from the one-dimensional anisotropic deformation, and is retained (shape retention) by the ionic crosslink. The recovery of creep results from the elastic relaxation of the polymer conformation
[en] The aim of this Euromech Colloquim was to bring together researchers interested in the creep behaviour of metals, engineering ceramics and ice to examine the processes of crack growth and continuum failure. These proceedings are divided into four sections each of which contains a selection of papers which discuss the material phenomena, the development of material models and the application of these models to practical situations. The first section examines the processes of crack propagation. This is followed by two sections devoted to the behaviour of engineering ceramics and ice, with a final section on continuum damage mechanics. One paper is abstracted and indexed separately. (author)
[en] Highlights: • Long-term deformation of recycled aggregate concrete beams was examined. • Three beams were monitored for over 380 days. • Influence of recycled aggregate on the long-term performance. • Comparison of that between normal and recycled aggregate concrete beams. - Abstract: This paper presents experimental results on the long-term deformations of recycled aggregate concrete (RAC) beams for over 1 year (380 days) and flexural behavior of RAC beams after exposure to sustained loading. Three reinforced concrete (RC) beam specimens were fabricated with replacement percentage of aggregate (100% natural aggregate, 100% recycled coarse aggregate, and 50% recycled fine aggregate) and subjected to sustained loading that is 50% of the nominal flexural capacity. During the sustained loading period (380 days), the long-term deflection due to creep and shrinkage was recorded and compared with predicted behavior that was determined based on current specifications (ACI 318 Code). After measuring the long-term deflection for 380 days, four-point bending tests were conducted to investigate the flexural behavior of RC beams after exposure to sustained loading and determine any reduction in flexural capacity. A modified equation to predict the long-term deflection values for RC beams with recycled aggregate is proposed, and the experimental results are compared with the predictions calculated using the ACI 318 Code provisions