Instantaneous Creep Research Articles

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.

Deformation Behavior Immediately After Indentation Load Change in Ultrafine-Grained Al-Mg Solid Solution Alloys

Instrumented indentation tests were performed to study how grain boundaries and solute atoms affect creep and instantaneous plastic deformation in ultrafine-grained (UFG) Al-Mg solid solution alloys with average grain size d = 0.3 − 1.0 μm at T = 373 K. In the results for Al-1.0 mol% Mg, the degree of instantaneous plastic displacement generated with a rapid increase in the load was smaller when the grain diameter was smaller. On the other hand, creep occurs more readily in materials with a smaller grain diameter. When the load was rapidly decreased during creep, the indenter displacement gradually decreased over time. The degree of reverse creep that occurs is greater when the grain diameter is smaller. In light of these test results and reports in the related literature, reverse creep is thought to occur because of inverted movement of piled-up dislocations near the grain boundaries. For the case of Al-xMg (x = 0.5, 1.0, 2.0 mol%), the results show that as the solute concentration increases, the occurrence of instantaneous plastic deformation, creep, and reverse creep becomes less likely. Overall, the results indicate that the plastic deformation behavior obtained by the testing conditions of present study for UFG Al-Mg alloys could be explained based on understanding of the behavior of course-grained materials.

Investigation of squeal noise under positive friction characteristics condition provided by friction modifiers

Field application of friction modifiers on the top of rail has been shown to effectively curb squeal and reduce lateral forces, but performance can be variable, according to other relevant research. Up to now, most investigations of friction modifiers were conducted in the field, where it is difficult to control or measure important parameters such as angle of attack, rolling speed, adhesion ratio etc. In the present investigation, the effect of different friction modifiers on the occurrence of squeal was investigated on a rolling contact two disk test rig. In particular, friction-creep curves and squeal sound pressure levels were measured under different rolling speeds and friction modifiers. The results show friction modifiers can eliminate or reduce the negative slope of friction-creep curves, but squeal noise still exists. Theoretical modelling of instantaneous creep behaviours reveals a possible reason why wheel squeal still exists after the application of friction modifiers.

Leaves on the Line: Low Adhesion Detection in Railways

Abstract: Regions of extreme low-adhesion between the wheel and rail can cause critical problems in traction and braking. This can manifest in operational issues such as signals being passed at danger, or pessimistic network wide responses to mitigate for localised issues. Poor traction conditions can be caused by oil contaminants, rain, ice, condensation of water droplets (micro-wetting) or leaves on the line, where compressed leaf contamination can cause a rapid decrease in adhesion. The complexity of the problem arises as a result of the inability to directly measure and monitor all the factors involved. There remains a lack of real-time information regarding the state and location of low-adhesion areas across rail networks. On-board low adhesion detection technology installed to in-service vehicles is a suggested method to capture up-to-date adhesion information network wide and minimise significant disruptions and cancellations in railway schedules. This paper extends a principle of a model-based estimation technique previously developed in straight track running for operating in a curving scenario. The vehicle model of focus here will be a simplified single-wheelset model attached to a suspended mass via representative stiffness and damping components. It is shown that in order for instantaneous creep forces to be estimated, the radius of the curve is required.

English

Local variety of lime (Lemun tsami) was subjected to quasi-static compressive loading using Instron Universal Testing Machine at three levels of time after harvest; freshly harvested, one and two weeks after harvest. Creep parameters were determined by fitting the experimental data to the Burger's Model and the obtained model equation for freshly harvested was: , R2 =0.98. The instantaneous creep compliance (Jo) was; 1.843, 3.137 and 4.116 1/MPa for freshly harvested, one and two weeks after harvest respectively while the Corresponding instantaneous elasticity (Eo) was 0.543, 0.319 and 0.243 MPa respectively. Compliance increased with increase in time after harvest while elasticity decreased with time after harvest. The obtained free viscosity (η0) was 1160.093, 808.407 and 808.859 MPas for freshly harvested, one and two weeks after harvest respectively. This was found to decrease with increase in time after harvest.

Physicochemical characterization of white, yellow and purple maize flours and rheological characterization of their doughs

White, yellow and purple maize flours were obtained after dried kernels milling with two different sieves (200 and 500μm). Hygroscopic characteristics, particle size distribution, colour and total starch and damaged starch (DS) of flours were determined. Maize flour doughs were obtained by mixing of flour and water in a laboratory kneader (Mixolab®) at constant dough consistency (1.10±0.07Nm). Dough properties like water absorption (WA), development and stability times were determined. Rheological characterization was carried out at 30°C by means of oscillatory frequency sweep (1-100rads(-1)) at 0.1% strain and creep (50Pa, 60s) - recovery (0Pa, 180s) tests using a controlled stress rheometer. No significant differences were observed among water desorption isotherms of maize varieties and Halsey model was satisfactorily employed. Under the same milling conditions, white maize flours showed higher average particles size than purple and yellow maize flours. A model to predict flours colour involving colour parameters of the particle size fractions is proposed. Flours obtained with smaller particle size showed higher DS content and WA. For tested doughs, the mechanical spectra showed that elastic component was dominant over the viscous one. Damping factor varied slightly with angular frequency. Moduli values depended on average particle size and WA of dough. Creep-recovery data were satisfactorily fit with Burgers model. Instantaneous creep compliance varied with the same trend than elastic modulus. Viscoelastic creep compliance increased linearly with WA of the tested doughs and, at constant average flour particle size, increased with increasing DS.

Hybrid materials design to control creep in metallic pipes

Abstract A hybrid material has been developed to improve creep performance in pressurized metallic pipes subjected to high-temperatures. Model materials were selected for an investigation of reinforcement design parameters in architectured materials. Brass pipes (65 wt.% Cu/35 wt.% Zn) with austenitic stainless steel reinforcement were pressurized and creep rupture tested at 673 K. Compared to unreinforced pipes of equal dimensions, a 47-times reduction in the effective strain rate was observed with a 50° reinforcement angle. A ‘neutral angle’ of 54.7 ± 1.5° was determined experimentally, where tangential (hoop) and longitudinal stresses on the pipe can be balanced and strains minimized. For initial angles below the neutral angle, creep strain was shown to facilitate a shift in orientation towards the neutral angle. For an initial angle of 42°, this shift towards the neutral angle resulted in instantaneous creep rate dropping from 170% of the mean creep rate to 60% of the mean creep rate over 820 h, when the final angle was measured to be 50°. A high-temperature prototype (tungsten braid oriented at 53° over a 253MA stainless steel pipe) was shown to give a creep life extension in excess of 300-times at 1313 K.

Irradiation creep of nano-powder sintered silicon carbide at low neutron fluences

The irradiation creep behavior of nano-powder sintered silicon carbide was investigated using the bend stress relaxation method under neutron irradiation up to 1.9dpa. The creep deformation was observed at all temperatures ranging from 380 to 1180°C mainly from the irradiation creep but with the increasing contributions from the thermal creep at higher temperatures. The apparent stress exponent of the irradiation creep slightly exceeded unity, and instantaneous creep coefficient at 380–790°C was estimated to be ∼1×10−5[MPa−1dpa−1] at ∼0.1dpa and 1×10−7 to 1×10−6[MPa−1dpa−1] at ∼1dpa. The irradiation creep strain appeared greater than that for the high purity SiC. Microstructural observation and data analysis indicated that the grain-boundary sliding associated with the secondary phases contributes to the irradiation creep at 380–790°C to 0.01–0.11dpa.

Instantaneous creep in face-centered cubic metals at ultralow strain rates by a high-resolution strain measurement

Instantaneous creep in face-centered cubic metals, 5N Al (99.999%), 2N Al (99%) and 4N Cu (99.99%) with different grain sizes, was firstly investigated by sudden stress-change experiments at ultralow strain rates $\dot \varepsilon $ ⩽ 10−10 s−1 and temperature T < 0.32 T m. The experimental results indicate that the observed instantaneous creep is strongly dependent on grain size, the concentration of impurity, and stacking fault energy. Creep in high-purity aluminum, 5N Al, with a very large grain size, d g > 1600 μm, shows non-viscous behavior, and is controlled by the recovery of dislocations in the boundary of dislocation cells. On the other hand, for 5N Al with a small grain size, d g=30 μm, and low-purity aluminum, 2N Al, with d g= 25 μm, creep shows viscous behavior and may be related to ‘low temperature grain boundary sliding’. For high-purity copper, 4N Cu, with d g= 40 μm and lower stacking fault energy, creep shows a non-viscous behavior, and is controlled by the recovery process of dislocations. For all of the samples, creep shows anelastic behavior.

A 3-Dimensional Model Related to Stress-Strain-Time of Rock Salts

To build a stress-strain-time relationship of salt rock with unified instantaneous law and creep law, we combine the stress-strain curve with whole process of creep curve, looking for inner relation between softening curve and creep deformation. Finally, the stress-strain-time space surface is obtained. The researching results demonstrated that time and space development law of rock salts can be well reflected by it.

Creep Characteristics of Mudstone under Devastating or Integrated States by Uniaxial Tests

Based on general instantaneous strength test and uniaxial creep tests under devastating or integrated states, strain characteristics of mudstone in different stages have been determined. Analyzing strain features of mudstone developing in every stage of different tests, evolving creep law of mudstone under uniaxial devastating state may be validated, which axial limited creep strain under the decided loading standard is equal to that value at uniaxial complete strain-stress curve rearwards ultimate strength. Relationship between designed loading stress and its corresponding creep strain can be linear in uniaxial creep test of mudstone under integrated state, while must not be a determinate secant of uniaxial complete strain-stress curve. Studying results present that terminal creep strain of rock with certain loading levels and under devastating state can be only corresponded with an exclusive point of traditional uniaxial complete strain-stress curve rearwards the ultimate strength, and the extended limited creep courses of mudstone will answer for Boltzmann function.

Strain model for traditional and self-compacting concrete during fire

In this paper, a complete strain model is derived that describes the strains developed during fire (up to 400 °C) and is based on the model of Anderberg et al., published in 1976. This model describes the total measured deformation as a superposition of four strain types: free thermal strain, instantaneous stress-related strain, creep strain and transient strain. The strains are derived from loading tests on cylinders with dimensions O106 × 320 mm, submitted to load ratios of 0%, 20% and 30% of the initial strength before heating. For damage to the oven to be avoided, the instantaneous stress-related strain is found from Young's modulus tests immediately after cooling. All tests occurred at a heating rate of 5 °C/min, and the specimens were pre-dried to avoid explosive spalling. The investigated concretes are a traditional and a self-compacting concrete with a testing age of about 30 months. Only small differences between both concrete types are found for the transient strain despite the different fracture of cement matrix. For the studied test conditions, the modern siliceous concretes such as self-compacting concrete yielded similar results as the traditional siliceous concretes tested in the 1970s. Copyright © 2012 John Wiley & Sons, Ltd.

Study on the Relationship between Parameters of Deterioration Creep Model of Rock under Different Modeling Assumptions

Based on the analysis to the characteristics of two kinds of rock steady creep curves, the nBurgers model that can describe the uniaxial creep properties of rock is presented. Considering the external influences on rock deformation as instantaneous elastic deterioration defined by the reduction of instantaneous elastic modulus and long-term creep deterioration defined by the reduction of long-term creep modulus, kBurgers model with deterioration is presented too. On this basis, three-dimensional model considering the Poisson’s ratio and Bulk modulus as the constant are established separately, and the relationship between their parameters is obtained. Finally, According to a calculation example, the deterioration evolution of model parameters considering water content changing under the assumption of constant bulk modulus is analyzed. In addition, the deterioration evolution equations of instantaneous elastic and long-term creep under the assumption of constant bulk modulus are obtained. It is verified that conversion relationships of model parameters are reasonable.

Vascular bundle shape in cross-section and relaxation properties of Moso bamboo ( Phyllostachys pubescens)

The variation in the longitudinal and radial direction of the R/T ratio which is the ratio of the radially maximum length (R) to the tangentially maximum width (T) of a vascular bundle and area ratio of vascular bundles, which are cross-sectional image features of bamboo, and the relationships between the R/T ratio or area ratio and relaxation behavior were investigated. These image features varied characteristically in the longitudinal and radial directions. Relationships between these image features and relaxation behavior were evaluated using the instantaneous creep compliance ln[J (0)] and the creep intensity ln [J(3 × 10 4) − J (0)]. Although both instantaneous compliance and creep intensity decreased as density increased, their dependence properties were remarkably different. Instantaneous compliance was strongly correlated with R/T ratio and density, which was related to the area ratio, whereas creep intensity was weakly correlated with density in a given range. The results indicate a difference between the two relaxation properties, because creep intensity depends more on microstructures, such as the conformation of molecular chains in the substance, or interactions among cells than on R/T ratio and area ratio image features.

Finite-element analysis of the hot-pressing consolidation of continuous Al2O3fibers-reinforced NiAl composites

Abstract The finite-element method was used to model the consolidation process of continuous Al2O3 fiber-reinforced NiAl composites. Actual creep behavior of NiAl was accounted for including the instantaneous strain and primary creep behavior. Primary creep played a very important role in the consolidation process at high temperatures and high pressures. Various processing parameters were investigated to evaluate the effects of temperature and hot-pressing stress on the consolidation behavior. It was found that the effect of pressure on consolidation was more significant at high temperatures due to its strong influence on the creep rate during power-law creep. The effects of fiber volume fraction and the arrangement of the matrix-coated fibers were also analysed. The simulations recommend an optimal fiber volume fraction in the range of 30%–50 %. Fully dense composites were fabricated according to the predicted processing conditions, and good agreement between simulation and experiment was achieved.

Tensile mechanical and creep properties of Descemet's membrane and lens capsule

Descemet's membrane (DM) and the lens capsule (LC) are two ocular basement membrane structures which in comparison with other basement membranes have exceptional thicknesses which increase with age. Both membranes are supposed to contain networks of type IV collagen and laminin linked together with nidogen/entactin and containing other glycoproteins and proteoglycans. DM is a unique basement membrane which in addition contains fine filaments of type VIII collagen arranged in a hexagonal lattice. The mechanical functions of the LC are in lens suspension and accommodation, and its mechanical properties, previously investigated, are of great interest from a surgical point of view. DM serves as an endothelial basement membrane. Otherwise, its physiological function is unknown but may be one of mechanical support, filtration, or fluid barrier. Data on the mechanical properties of DM or the supramolecular assembly of type VIII collagen are very scarce or absent. The aim of this study was to determine and compare the mechanical properties of the two ocular membranes in order to elucidate the properties of DM in the light of those of LC. The human eyes were from testamentary donors and rat, cow, and sow eyes were obtained from normal animals. The tensile mechanical properties were determined by a volume–strain procedure and creep properties by subjecting the membranes from the latter three species to a constant axial stress. In rat, cow, and sow, DM was less strained to obtain a fixed moderate stress value (0·5 MPa) and showed to be 3·4- to 5·2-fold stiffer and to attain 2·7- to 4·6-fold higher stress at a strain value of 0·10 when compared with LC. The maximal strain, stiffness and stress were found to be less than those of the LC. In humans, DM and LC showed very similar mechanical properties. The instantaneous creep of DM was found to be less than that of LC indicating a higher stiffness of DM in the axial direction. In conclusion, depending on the species, DM showed to possess from one-fourth to similar mechanical strength as that of LC, and, in rat, cow, and sow, DM appeared to be stiffer in both lateral and axial directions at moderate strain values when compared with LC.

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.

Rate of diffusion creep accompanied by grain boundary sliding in elongated microstructures

The instantaneous creep rate is evaluated for various two-dimensional elongated microstructures, when deformation occurs by grain boundary diffusion accompanied by grain boundary sliding. The strain contributions of grain boundary diffusion and grain boundary sliding are evaluated separatively, and the total creep rate is obtained as a sum of the two. The analysis for the diffusion on elliptical grain shows an increasing creep rate with grain elongation, while the creep rate for hexagonal microstructures increases or decreases depending on grain orientation. It is shown that the instantaneous creep rate calculated for hexagonal microstructures is reasonably consistent with the reported values in a steady state. The instantaneous creep rate in a polycrystal decreases with grain elongation, but the decrement is small enough to allow the application of the original constitutive equation to diffusion creep without significant modification.