Effects Of Stress Rate Research Articles

Push‐Pull Fatigue of Sintered Silicon Nitride Ceramics at Elevated Temperatures

The fatigue tests under push‐pull completely reversed loading and pulsating loading were performed for silicon nitride ceramics at elevated temperatures. Then the effects of stress wave form, stress rate, and cyclic understressing on fatigue strength, and cyclic straining behavior, were examined. The cycle‐number‐based fatigue life is found to be shorter under trapezoidal stress wave loading than under triangular stress wave loading, and to become shorter with increasing hold time under the trapezoidal stress wave loading. Meanwhile, the equivalent time‐based life curve, which is estimated from the concept of slow crack growth, almost agrees with the static fatigue life curve in the short and intermediate life regions, showing the small cyclic stress effect and the dominant stress‐imposing period effect on cyclic fatigue life. The fatigue strength increased in stepwise stress amplitude increasing test, where stress amplitude is increased stepwise every given number of stress cycles, at 1100° and 1200°C. Occurrence of cyclic strengthening was proved through a gradual decrease in strain amplitude during a pulsating loading test at 1200°C in this material, corresponding to the above cyclic understressing effect on fatigue strength.

Bifurcation analysis of diffuse modes by the subloading surface model with tangential stress rate effect

The tangential-subloading surface model is capable of describing the plastic strain rate due to the rate of stress inside the yield surface and the inelastic strain rate due to the stress rate component tangential to the subloading surface. The diffuse bifurcation modes of the rectangular specimen subjected to the plane strain loading is analyzed by adopting the tangential-subloading surface model, exhibiting the characteristic regimes of the governing equations (elliptic, hyperbolic and parabolic) in this article. In each of these regimes both symmetric and anti-symmetric diffuse bifurcation modes are available. Besides, the tangential strain rate makes the inception of bifurcation modes easier in not only normal-yield but also subyield states.

Shear band formation analysis in soils by the subloading surface model with tangential stress rate effect

The generalized elastoplastic constitutive equation for soils is proposed based on the subloading surface model extended so as to describe the dependence of both the magnitude and the direction of inelastic stretching on the stress rate tangential to the subloading surface [Int J Plasticity 17 (2001) 117]. It would be applicable to the analysis of deformation of soils in both normal-yield and subyield states for not only lower but also higher stress ratio than that in the critical state. Then, the shear band formation in the rectangular specimen subjected to the biaxial compression under the undrained plane strain condition is analyzed by the generalized equation, and thus the condition for shear band formation and the shear band inclination are discussed in relation to material properties and the state of stress, i.e. the stress-ratio and the normal-yield ratio. These results reveal that the tangential stretching term makes easy to fulfill the necessary condition of shear band formation for not only normal-yield but also subyield states, and further the formation is affected by the material parameter prescribing the approaching degree of the stress to the normal-yield state.

Stress rate effect on the stiffness of a soft clay from cyclic, compression and extension triaxial tests

Stress rate effect on the stiffness of a soft clay from cyclic, compression and extension triaxial tests

Stress rate effect on the stiffness of a soft clay from cyclic, compression and extension triaxial tests

Stress rate effect on the stiffness of a soft clay from cyclic, compression and extension triaxial tests

Prediction of limit strain in sheet metal-forming process by subloading surface model with tangential stress rate effect.

Prediction of limit strain in sheet metal-forming process by subloading surface model with tangential stress rate effect.

Resistance against crack nucleation and propagation in Y 2O 3 doped tetragonal zirconia ceramics

Both the nucleation and the propagation behavior of cracks have been investigated in Y-TZP ceramics, focusing on the effect of anelasticity. Indentation testing was carried out under the conditions of various loading speeds (40 and 300 μm s −1) and temperatures (198–367 K). A high resistance against cracking was observed at the lowest loading speed at higher temperature, which indicates that an activated anelasticity could release the stress concentration, resisting crack nucleation. As for the crack propagation behavior, the variation of the stress intensity factor, K, during the monotonic loading process was investigated using indentation pre-cracked sample. The effect of stress rate (8×10 −4, 8×10 −1 and 8×10 2 MPa s −1) and temperature (RT and 373 K) are reported. Extreme crack growth suppression was observed at lower stress rate at higher temperature. Under this condition, a strong resistance against the commencement of crack propagation was especially enhanced by activated anelasticity, resulting in an extended increase in K with increasing applied stress. These results suggest that the anelasticity could work as favorable effect on crack nucleation and propagation. However, it was found that exhaustion of anelasticity could cause a decrease in the fracture toughness.

Effects of Shear Stress and Pressure Drop Rate on Microcellular Foaming Process

The effect of shear stress and pressure drop rate on cell nucleation density during the foaming process has been studied using a foaming process simulator. The major part of this simulator is a test chamber that can be temperature controlled and pressurized with gas. Shear stress can be applied to the polymer melt through a rotor. With this simulator, the effects of shear stress and pressure drop rate can be investigated separately. The three materials tested were PS, unfilled HDPE, and HDPE filled with 5% talc. It was found that both shear stress and pressure drop rate have significant effect on cell nucleation density, although shear stress is generally dominant. The effect of shear stress or pressure drop rate becomes more critical when the saturation pressure is low. The transition pressure is about 1500 psi with PS or unfilled HDPE. The transition pressure reduces to about 500 psi with the filled HDPE, since the filler acts as a nucleation agent and reduces the requirement on saturation pressure for sufficient nucleation.

Microscopic mechanisms of low-temperature yield stress anomaly in solids

The effect of temperature ( T=0.5–300 K ), stress rate ( 10 −4–10 4 MPa s −1 ) and shear stress ( τ) on the mean path of mobile dislocations ( l), slip bands ( L), macroscopic strain, and relaxation stress have been studied in pure (P) and high purity (HP) single crystals: LiF, NaCl, KBr, KJ, RbJ. The temperature-dependencies of the starting stresses for dislocation motion, multiplication and the yield stress are common for various crystals. They increase with decreasing temperature and drastically drop at T< T 0. It is the deformation stress that governs T 0; T 0 usually increases with crystal softening and decreases with the rise of crystal hardening. It is governed by the competing mechanisms of dislocation bowing around obstacles, climb, and dislocation double cross-slip (DDCS). DDCS is initiated by local stresses at the interfaces of matrix-impurity micro-precipitates due to their thermal expansion mismatch. The stress (strain) rate controls the contribution of thermal DDCS, climb, or athermic bowing to the temperature dependence of crystal hardening or softening and irrefutably contradicts the quantum and inertial mechanisms in the low-temperature anomaly of plastic flow in solids. The data confirm the extremely low estimates of the Peierls–Nabarro stress in various crystals: τ P <(10 −8–10 −6) G , where G is the shear modulus.

Post-localization Analysis by the Subloading Surface Model with Tangential Stress Rate Effect

Various constitutive models have been applied to the analyses of localized deformation of solids. However, most of them fall within the framework of the conventional plasticity premising that the interior of yield surface is an elastic domain and obeys the plastic potential theory in which the plastic stretching is independent of the stress rate component tangential to the yield surface. Therefore, they predict a stiff elastic response until the stress reaches the yield state and further a stiff elastoplastic response after yielding. On the other hand, the subloading surface model falling within the unconventional plasticity would be only the model capable of describing pertinently the plastic deformation induced by the rate of stress within the yield surface in general loading process including the unloading, reloading and inverse loading. Further, the numerical calculation by this model is quite efficient disusing the special algorism, e.g. the mean normal method and the radial return method in order to make the stress lie just on the yield surface in the plastic loading process since it contains the controlling function to make the stress approach automatically the yield surface in the plastic loading process. Further, this model is recently extended so as to describe the tangential stress rate effect, i.e. the inelastic deformation induced by the stress rate component tangential to the subloading surface. In this article the post-localized deformation of metal due to the shear band formation is analyzed by the finite element method incorporating the subloading surface model with the tangential stress rate effect. Thus, the influence of the tangential stress rate effect on the shear band formation is discussed exhibiting several examples of the deformation patterns.

Elastoplastic constitutive equation with tangential stress rate effect

Abstract In traditional elastoplastic constitutive equation with a single smooth plastic potential surface, the plastic stretching is independent of the tangential stress rate , i.e. the component of stress rate which is tangential to the yield surface. This traditional model predicts an unrealistically stiff response when a loading path deviates significantly from the proportional loading. In order to overcome this defect various constitutive models have been proposed. However, a pertinent model applicable to the description of the deformation behavior in a general loading process has not been proposed up to the present. In this article, an elastoplastic constitutive equation with the inelastic stretching induced by the deviatoric stress rate component tangential to the subloading surface is formulated by extending the subloading surface model with a smooth elastic–plastic transition. This model is applicable to the analysis of deformation in a general loading process of materials with an arbitrary yield surface. Based on this equation, a constitutive equation of metals with isotropic-kinematic hardening is formulated and its basic characteristics are examined in detail.

Consideration of shear band formation by elastoplastic constitutive equation of soils with tangential stress rate effect.

Consideration of shear band formation by elastoplastic constitutive equation of soils with tangential stress rate effect.

Effect of shear rate within the spinneret on morphology, separation performance and mechanical properties of ultrafiltration polyethersulfone hollow fiber membranes

The effect of shear rate within a spinneret during hollow fiber spinning on the morphology, permeability and separation performance, and mechanical properties of ultrafiltration polyethersulfone hollow fiber membranes has been investigated. Hollow fiber UF membranes were prepared from a dope solution containing Polyethersulfone (PES)/N-methyl-2-pyrrolidone (NMP)/Diethylene Glycol (DG) with a weight ratio of 13/45/42. This dope formulation was very close to its cloud point (binodal line) in order to speed up the coagulation of nascent fibers as much as possible so that the relaxation effect on molecular orientation was reduced. The wet-spinning process was purposely chosen to fabricate the hollow fibers without drawing and water was used as the external coagulant. Therefore, it is in the belief that the effects of gravity and elongation stress on fiber formation could be significantly reduced and the orientation induced by shear stress within the spinneret could be frozen into the wet-spun fibers. In order to minimize the complicated coupling effects of elongation stress and uneven external solvent exchange rate on fiber formation and the effect of inner skin resistance on separation performance of final fibers while the shear rate within the spinneret was increased from 245 to 2568 s −1, an optimized bore fluid of 86 wt% NMP in water was employed and a constant ratio of dope fluid flow rate to bore fluid flow rate was kept. Experimental results showed that a higher shear rate in the spinneret apparently resulted in a hollow fiber UF membrane with a thicker and/or a denser skin due to a greater molecular orientation. As a consequence, when the shear rate increased, pure water flux, coefficient of thermal expansion (CTE) and elongation of the final fibers decreased, but storage modulus, tensile strength and Young's modulus increased. For the first time, it was found that there was a certain critical value of shear rate below which the separation performance of fibers increased obviously while the flux decreased dramatically with an increase in shear rate but above which the separation decreased slightly while the flux did not change. The results suggested that it was possible to dramatically enhance the production efficiency of hollow fiber UF membranes for the separation of high Mw (>40 K) solutes with the same fiber dimension and similar separation performance by the approach proposed in this paper.

The Extended Elastoplastic Constitutive Equation with Tangential Stress Rate Effect

The Extended Elastoplastic Constitutive Equation with Tangential Stress Rate Effect

Changes with growth rate in the membrane lipid composition of and amino acid utilization by continuous cultures of Campylobacter jejuni.

Methods and media (defined and complex) are described which permit studies designed to determine the influence of single environmental factors on the survival and virulence of Campylobacter jejuni. The effect of growth rate on selected physiological traits (amino acid utilization, membrane lipid composition, motility, cell morphology) was studied in continuous culture. In both media, growth was at the expense of amino acid (serine, aspartate, glutamate and proline) catabolism. Slow growth in the complex medium shifted amino acid utilization from more (serine and aspartate) to less preferred substrates (glutamate, proline and possibly amino acids from the proteolysis of peptones). Low growth rates promoted the conversion of unsaturated 11-octadecenoic acid substituted phosphatidyl ethanolamines to corresponding 11-methylene substituted species, a feature correlated with stationary phase and exposure to environmental stress in other organisms. During continuous growth, cells lost motility although they still possessed flagella. Slow growth resulted in longer cells. Future studies will investigate the independent effects of nutrient stress and growth rate on the virulence and persistence of cells.

Effects of Stress Waveform on Push-Pull Cyclic Fatigue Properties of Sintered Silicon Nitride Ceramics at Elevated Temperatures.

Push-pull cyclic fatigue tests were performed for silicon nitride ceramics with stress-rate-controlled triangular and trapezoidal stress waves at 1 200°C at stress ratio R=-1.0. Then the effects of stress rate and stress hold time on the cyclic high-temperature fatigue were examined. The cyclic fatigue life is shorter under trapezoidal stress wave loading than under triangular stress wave loading, and becomes shorter with increasing hold time under the trapezoidal stress wave loading. Cyclic fatigue lifetime is successfully estimated through the crack extension law based on a slow crack growth mechanism, showing a small cyclic stress effect and dominant stress-application-period effect on cyclic fatigue lifetime. This was also evident from scanning electron microscope observations of fracture surface. Cyclic fatigue life was influenced little by stress waves which have hold time on both the tension and compression sides or only on the compression side. Thus compressive stress hold seems not to induce cyclic fatigue damage.

Effects of shear stress and substrate loading rate on Pseudomonas aeruginosa biofilm thickness and density

Thickness, roughness, and density data were obtained from monopopulation Pseudomonas aeruginosa biofilms grown in an annular reactor under different substrate loading rates and shear stresses. Biofilm thickness showed significant increases, up to approx. 30 μm, with increasing substrate loading rate. Shear stress had no significant effect on thickness. Biofilm roughness, as measured by the standard deviation of the thickness, increased with increasing thickness. Areal mass density significantly increased with substrate loading rate and was not significantly affected by shear stress. Substrate loading and shear stress did not significantly affect volumetric mass density. A numerical expression that describes changes volumetric density as a function of biofilm depth is presented.

Fundamental Study on the Application of the As-Quenched Low Carbon Martensitic Steels to Structural Materials.

The mechanical properties of low-carbon martensitic steels were studied with the aim of their application to structural materials. The seven kinds of steels containing different carbon contents up to 0.3 mass percent were examined. The carbon content dependences of hardness, tensile properties and impact toughness in as-quenched martensite are shown. The low-carbon martensite is plastically deformable, i. e., more than 60% cold rolling is easily achieved in steels containing less than 0.2 mass percent carbon and the strength is explained reasonably from the viewpoint of dislocation density. The 0.1 mass percent carbon as-quenched martensite is revealed to show comparable impact value to those of quenched and tempered steels at strength level of more than 1 GPa. Several points relevant to practical use, including, residual stress and strain rate effect, are discussed.

Fatigue crack growth analysis in PEEK

Experimental results on fatigue crack propagation in polyether-etherketone (PEEK) under different load levels, specimen thicknesses, and environmental temperatures reported in Refs 1, 2 are analysed. Energy release rates evaluated from load displacement curves in the ‘ductile’ phase of crack growth are nearly equal to J 1 = σ y δ (where σ y is the yield stress of the material and δ is the crack tip opening displacement). Thus the material around the crack tip behaves as an elastic-perfectly plastic material under plane stress. When the kinetic data are plotted against J 1 the crack speed is practically independent of the load level, specimen thickness, and of environmental temperatures up to 75 °C. Both cycle- and time-dependent crack growth are observed when the environmental temperature is 100 °C. The effects of stress rate are investigated by keeping the stress levels of the fatigue cycle the same and changing the frequency. Values of the stress rate σ dot are obtained from the formula σ dot = 2ν ( σ max − σ min), where ν is the frequency and σ max, σ min are the maximum and minimum stresses of the fatigue cycle. The results show that when the kinetic data are plotted as Δl/ Δt against the crack length, an increase in crack speed is observed with stress rate. If the same data are treated as Δl/ ΔN against the crack length a decrease in Δl/ ΔN with test frequency is seen. Crack growth data under different thicknesses and temperatures are correlated with a power relation and the kinetic equation of a crack layer. The present analysis indicates that both a power relation and the kinetic equation of the crack layer model describe well crack growth data under a wide spectrum of loading conditions.

A statistical analysis of the effect of substrate utilization and shear stress on the kinetics of biofilm detachment

One of the least understood processes affecting biofilm accumulation is detachment. Detachment is the removal of cells and cell products from an established biofilm and subsequent entrainment in the bulk liquid. The goal of this research was to determine the effects of shear stress and substrate loading rate on the rate of biofilm detachment. Monopopulation Pseudomonas aeruginosa and undefined mixed population biofilms were grown on glucose in a RotoTorque biofilm reactor. Three levels of shear stress and substrate loading rate were used to determine their effects on the rate of detachment. Suspended cell concentrations were monitored to determine detachment rates, while other variables were measured to determine their influence on the detachment rate. Results indicate that detachment rate is directly related to biofilm growth rate and that factors which limit growth rate will also limit detachment rate. No significant influence of shear on detachment rate was observed.A new kinetic expression that incorporates substrate utilization rate, yield, and biofilm thickness was compared to published detachment expressions and gives a better correlation of data obtained both in this research and from previous research projects, for both mono- and mixed-population biofilms.