Instantaneous Creep Strain Research Articles

Field-Induced Viscoelastic and Creep and Recovery Behavior of Water-Based MR Fluids Using Bentonite and Oleic Acid as an Additive

The viscoelastic and creep and recovery behaviors of a carbonyl iron (CI)-water-based magnetorheological fluid (MRF) were studied under dynamic and constant loading conditions. The feature of MR fluid is to change from liquid to semi-solid state just in a few milliseconds after applying a magnetic field. ThereforeMR fluid is a kind of smart material whose rheological properties change with step-change in a magnetic field. We prepared MR fluids comprising CI 65 wt%, water 35 wt %, bentonite 3 wt %, and oleic acid 1 wt %. Because bentonite with nanosized fills the voids between the CI particles, it was used with oleic acid to enhance the MR response of the CI/water suspension. The strain amplitude tests reveal that MR fluid behaves as a viscoelastic material in the LVE range and a transition of fluid occurred from linear viscoelastic to non-linear viscoelastic behavior at the critical strain of 0.1%. Its storage moduli confirmed a steady plateau region for the entire angular frequency range, suggesting the well-known solid-like behavior of the MR suspension. The creep and recovery result signified that as the magnetic field increased, the instantaneous creep strain contributions decreased dramatically.

Improved creep performance of melt‐extruded polycaprolactone/organo‐bentonite nanocomposites

AbstractIn this work, biodegradable nanocomposites based on polycaprolactone reinforced with pristine and organo‐modified bentonites are prepared by melt extrusion. Bentonite is exchanged with benzalkonium chloride (CBK) in a pilot plant scale reactor. The influence of clay type and loading on morphology, rheology, mechanical properties, and creep performance of the resulting materials is analyzed. Besides, several theoretical models then applied to experimental creep data and master curves are used to relate time and temperature with the compliance of the materials. The morphology characterization of the nanocomposites show that the organo‐modification of the clay greatly improves its dispersion in the polymer matrix. As a consequence, it is demonstrated that reinforcement of PCL with 3 wt% loading of organoclay produces the strongest improvement in creep resistance. The instantaneous creep strain and the experimental creep rate decrease more than 9% and 27%, respectively, in the range of temperatures analyzed. Moreover, the experimental values are used to adequately fit theoretical creep models for different clay loadings. On the other hand, the material with optimal creep behavior also shows the greatest improvements in tensile mechanical properties.

Dynamic mechanical analysis and creep-recovery behavior of agglomerated cork

The mechanical behavior of agglomerated cork, made of cork granules bound with polyurethane moisture-cured adhesive is investigated and compared to natural cork in the small strain regime (strain <5%). Dynamic mechanical analysis (DMA) of the agglomerated material revealed two distinct thermal transitions, one at −45 °C, related to the glass transition of polyurethane, and one at 3 °C, associated with melting of suberin, a natural polyester that is the main component of cork’s cell walls. Natural cork showed the latter transition to occur at a higher temperature range, between 10 and 25 °C, probably due to a different crystalline arrangement being formed upon cooling the cork granules under pressure in the mold. The storage modulus of agglomerated cork was found to be similar to that of natural cork. Creep and recovery experiments were well described by Burgers model and Weibull distribution function, respectively. Agglomerated cork showed higher instantaneous creep strain and viscous flow than natural cork, probably due to relative displacement and slippage of the granules being allowed by the binder. In all cork materials, not all the instantaneous creep strain was instantaneously recovered. A fraction underwent delayed recovery and another turned into permanent strain. This behavior was associated with the deformation of corrugations in the cork cell walls. Cyclic creep-recovery tests showed for all cork materials recoveries above 90%, except for the first cycle.

Flexural creep behaviour of long glass fibre reinforced polyamide 6.6 under thermal-oxidative environment

This article aims to investigate the flexural creep behaviour as a function of temperature of long glass fibre polyamide 6.6 taking into account the thermal-oxidative degradation occurring during the test. The mould geometry has been chosen so as to reproduce some geometrical accidents (e.g. sharp frontal and tangential steps) occurring on industrial moulds. The nominal fibre content (10, 40 and 55 wt%), initial fibre length (short glass fibre, long glass fibre), load rate (up to 70%) and creep temperature (23℃, 100℃ and 130℃) have been considered to estimate the Findley’s model coefficients. A first investigation on the polyamide 6.6 degradation under thermo-oxidative environment has been led to understand the mechanisms of thermal-degradation of the polyamide 6.6 composites. The pure polyamide 6.6 matrix has shown a 20% increase of flexural modulus during the first period of ageing attributed to a combined chain scissions and cross-linking reactions. Then, a decrease of properties attributed to predominant chain scission mechanism was noticed after 1000 h of thermal exposure reaching up to 30% after 5000 h. In case of reinforced polyamide 6.6, the flexural properties tend to increase (+6.5%) up to 2000 h of exposure. The least square method has then permitted to evaluate the material coefficients from the experimental data; the instantaneous creep strain has been estimated from a power law representation. In any cases, the calculations are in a good accordance with experimental measurements.

Creep properties and a creep equation of delay outburst coal and its adjacent mudstone

The study of the creep properties of coal and its adjacent mudstone is very important for understanding the mechanism of delay outburst coal. The samples of delay outburst coal and its adjacent mudstone collected from Yongshanqiao mine were used to carry out triaxial creep tests. The influence of confining pressure and axial compression on the creep test was analyzed. An accelerated creep model was constructed in parallel with a nonlinear viscous component and plastic component. It is connected with the traditional Burges creep model in series. A creep model which can describe the nonlinear visco-elastic-plastic creep model of rock was established and the corresponding creep equation was derived. According to the results of the creep test, the related parameters of the equation were fitted. The results show that, under the same confining pressure, instantaneous creep strain, creep strain of deceleration phase and constant rate creep of the coal and its adjacent mudstone are increased with an increase in the deviatoric stress. But at the same axial pressure, all of the above decrease with an increase of confining pressure. The duration time of the deceleration creep phase increases with the increase in the deviatoric stress. The theoretical values of the creep equation are in good agreement with the experimental results. It indicates that the creep properties of the delayed outburst coal and its adjacent mudstone can be well described by the creep model established in this paper.

Primary creep behavior of Ti-48Al-2W as a function of stress and lamellar morphology

The fully lamellar microstructure of powder metallurgy Ti-48Al-2W after cooling from the α region to 1280 °C, followed by air cooling and aging at 950 °C for up to 96 hours, is presented. Aging times as short as 5 hours result in acicular-shaped precipitates of W-rich β0 along lamellar interfaces, with the β0 size increasing with aging time. The β0 precipitates nucleate and grow in the α2 lamellae. Concurrently, with the formation of β0, the α2 decomposes into discontinuous lamellae. Aging to precipitate β0 along lamellar interfaces increases the 760 °C tensile strength (with a slight reduction of ductility) and reduces the instantaneous creep strain, since β0 precipitates at lamellar interfaces hinder interface dislocation mobility. The deformed microstructures from interrupted creep tests at 140 to 276 MPa at 760 °C indicate that the precipitation of β0 along interfaces substantially reduces the primary creep strain, primarily due to the influence of β0 on interface dislocation emission and motion. These results are discussed in terms of the influence of lamellar morphology on the instantaneous creep strain and primary creep transient, and the possible creep mechanisms are highlighted.

The observation of anomalous instantaneous creep strain in Cu-30% Zn after a stress change

The observation of anomalous instantaneous creep strain in Cu-30% Zn after a stress change