Effects Of Stress Rate Research Articles

Ratcheting and fatigue properties of the high-nitrogen steel X13CrMnMoN18-14-3 under cyclic loading

Abstract High-nitrogen steel X13CrMnMoN18-14-3 has been used in manufacturing expanding metallic stents with 0.1 mm diameter, and such medical implant is subjected to complicated and asymmetrical cyclic loading during service. But there is no test data published for the thin wire of the material under cyclic loading. In this study, a series of tests were conducted on X13CrMnMoN18-14-3 stainless steel under uniaxial cyclic loading with mean tensile stress. The yield stress and ultimate strength were higher than that of large size specimen with diameters of 5 mm and 7 mm. The effects of stress amplitude, mean stress, loading history and stress rate on the ratcheting behavior of high-nitrogen steel were analyzed, respectively. It can be concluded that the ratcheting strain amplitude and ratcheting strain rate of X13CrMnMoN18-14-3 steel increases with increasing stress amplitude or mean stress correspondingly. At the meantime, experimental results reveal that the material exhibits a strong memory of the previous loading history, the stress cycling with higher stress amplitude or mean stress greatly restrains the ratcheting of subsequent stress cycling with lower ones. The ratcheting strain rate was very sensitive to the applied cyclic stress rate, and the accumulation of ratcheting strain under stress rate of 21.2 MPa/s is much faster than that under stress rate of 106 MPa/s. In addition, comparison of the fatigue life between bulk specimen and thin wire indicates that the size effect has significant influence on fatigue properties of the material. In the case of the test conducted under stress amplitude of 400 MPa, the fatigue life of small specimen is approximately ten times longer than that of bulk specimen under the same loading conditions.

Effect of LDL concentration polarization on the uptake of LDL by human endothelial cells and smooth muscle cells co-cultured

To substantiate our hypothesis that concentration polarization of low-density lipoprotein (LDL) plays an important role in the localization of atherogenesis, we investigated the effects of wall shear stress and water filtration rate (or perfusion pressure) on the luminal surface LDL concentration (c(w)) and the LDL uptake by human vascular endothelial cells and smooth muscle cells co-cultured on a permeable membrane using a parallel-plate flow chamber technique and a flow cytometry method. The results indicated that the uptake of fluorescent labeled LDL (DiI-LDL) by the co-cultured cells was positively correlated with c(w) in a non-linear fashion. When c(w) was low, the uptake increased very sharply with increasing c(w). Then the increase became gradual and the uptake was seemingly leveled out when c(w) reached beyond 160 microg/ml. The present study therefore has provided further experimental evidence that concentration polarization may occur in the arterial system and have a positive correlation with the uptake of LDLs by the arterial wall, which gives support to our hypothesis regarding the localization of atherogenesis.

Dynamic Properties of Plain Concrete in Triaxial Stress State

There was experimental investigation of plain concrete dynamic behavior at high strain rates using a custom-designed, triaxial, servo-hydraulic controlled testing machine with over 60 cubic specimens in this study. A gradually increasing force was used for axial loading of each specimen at a constant strain rate at 10 supra -5 second supra -1, 10 supra -4 second supra -1, or 10 supra -3 second supra -1 and laterally confined at a 0, 4, 8, 12, or 16 MPa (0, 0.58, 1.16, 1.74, or 2.32 ksi) constant pressure. At each strain rate, there was analysis and discussion of axial stress-strain curves under confining pressures. Test results indicated that there were significant increases according to confining pressure of both ultimate strength and its corresponding strain. There was development of empirical equations accounting for triaxial stress and strain rate effects. There is substantial ultimate strength increases under low confinement and moderate increases under high confinement with increasing strain rate.

Rate Dependence and Short-Term Creep Behavior of a Thermoset Polymer at Elevated Temperature

The inelastic deformation behavior of PMR-15 neat resin, a high-temperature thermoset polymer, was investigated at 288°C. The effect of loading rate on monotonic stress-strain behavior as well as the effect of prior stress rate on creep behavior were explored. Positive nonlinear rate sensitivity was observed in monotonic loading. Creep response was found to be significantly influenced by prior stress rate. The effect of loading history on creep was studied in stepwise creep tests, where specimens were subjected to a constant stress rate loading followed by unloading to zero stress with intermittent creep periods on both loading and unloading paths. The strain-time response was strongly influenced by prior deformation history. Negative creep was observed on the unloading path. In addition, the behavior of the material was characterized in terms of a nonlinear viscoelastic model by means of creep and recovery tests at 288°C. The model was employed to predict the response of the material under monotonic loading/unloading and multistep load histories.

Statistical Analysis for Fatigue Life of Concrete

Statistical analysis is presented for 108 groups of fatigue test data of concrete, including tests of abnormal value, variance and distribution-fitted with fuzzy optimization; analysis of variance, sample size and error. It is chiefly concluded that effect of stress level of repeated loading on the fatigue life of concrete is much more significant than that of stress rate and relative stress amplitude, and effect of stress rate and relative stress amplitude are of the same significance; Relationship of sample size, relative error and significant level is also presented; fatigue life of concrete is more fitted for lognormal distribution than Weibull distribution. Suggestion for relevant code, standard and regulation are presented.

F4-6 差分-有限要素解析によるき裂先端近傍の水素凝集挙動に及ぼす負荷応力速度効果(F-4 水素脆化研究の新展開(シミュレーションと実験との対話),フォーラム)

F4-6 差分-有限要素解析によるき裂先端近傍の水素凝集挙動に及ぼす負荷応力速度効果(F-4 水素脆化研究の新展開(シミュレーションと実験との対話),フォーラム)

A simple constitutive model for cyclic compressive ratchetting deformation of polyteterafluoroethylene (PTFE) with stress rate effects

AbstractPure polyteterafluoroethylene (PTFE) were prepared by using the process of cold pressure and sintering. A series of unaxial cyclic compression tests were carried out on solid cylindrical specimens of pure PTFE. The focus of study was to investigate time‐dependent ratchetting behavior of PTFE under cyclic loading. It is shown that the stress–strain hysteresis loops exhibit pronounced nonlinearity with high stress range. The cyclic hardening is rate and stress range dependent. However, mean stresses have less effect on cyclic hardening than stress ranges. It is shown that viscoelasticity is obvious in the beginning cycles. The same stress range produces almost the same viscoelastic delay at the beginning of the compression cycles. A modified universal ratchetting model (URM), which is rate dependent is employed to predict the ratchetting strain under compressive cyclic loading condition at room temperature. POLYM. ENG. SCI., 48:29–36, 2008. © 2007 Society of Plastics Engineers

Some aspects of the mechanical response of BMI 5250‐4 neat resin at 191°C: Experiment and modeling

AbstractThe inelastic deformation behavior of BMI‐5250‐4 neat resin, a high‐temperature polymer, was investigated at 191°C. The effects of loading rate on monotonic stress–strain behavior as well as the effect of prior stress rate on creep behavior were explored. Positive nonlinear rate sensitivity was observed in monotonic loading. Creep response was found to be significantly influenced by prior stress rate. Effect of loading history on creep was studied in stepwise creep tests, where specimens were subjected to a constant stress rate loading followed by unloading to zero stress with intermittent creep periods during both loading and unloading. The strain‐time behavior was strongly influenced by prior deformation history. Negative creep was observed on the unloading path. In addition, the behavior of the material was characterized in terms of a nonlinear viscoelastic model by means of creep and recovery tests at 191°C. The model was employed to predict the response of the material under monotonic loading/unloading and multi‐step load histories. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

187 き裂先端近傍の水素凝集挙動に及ぼす負荷応力速度効果(学生賞I)

187 き裂先端近傍の水素凝集挙動に及ぼす負荷応力速度効果(学生賞I)

Size Effects and the Dynamic Response of Plain Concrete

This paper investigates the specimen size effect on the dynamic response of plain concrete. The report is based upon experimental data by the writers and others and considers results from creep tests on beams, beams under flexural impact, and cylinders under axial impact loading. Size effect is examined using Bažant’s size effect law and the multifractal scaling law, and both scaling models are able to capture the size effect on strength. For fracture energy, on the other hand, the size effect manifests itself only at impact rates. Under quasi-static loading, plain concrete in compression is less sensitive to the specimen size. But under impact, the compressive response appears to be more size dependent than flexure. However, upon accounting for the stress rate effects, the flexural response depicts a more significant size effect, similar to that seen at quasi-static rates.

Post-anthesis Water Stress and Nitrogen Rate Effects on Dry Matter and Nitrogen Remobilisation in Wheat Cultivars

Post-anthesis Water Stress and Nitrogen Rate Effects on Dry Matter and Nitrogen Remobilisation in Wheat Cultivars

載荷速度が凝灰岩の応力‐ひずみ特性に及ぼす影響

Much research has been carried out on the time-dependent behavior of rocks. It is well known that uni axial compressive strength increases as the strain rate becomes larger. Rock in general mainly contains microscopic cracks. In process to failure, cracks seem to develop parallel to the loading direction from the tip of microscopic cracks. Therefore, it is important to pay attention to the behavior of the circumferential strain when the time-dependency of stress-strain characteristics of rocks is examined. In this study the uni axial compression test are carried out using tuff , and the effect of stress rate on the stress-strain behavior is described.

Influence of Stress Rate on Rheology – An Experimental Study on Rocksalt of Simla Himalaya, India

The stress-strain behaviour and mechanical properties were investigated on rocksalt samples collected from Guma rocksalt Mine, District Mandi, Himachal Pradesh, India. Cylindrical samples cored perpendicular to the bedding planes, were compressed using an automated closed loop servo-controlled testing machine at stress rates ranging from 9.12×10−3 MPa/minute to 9.12 MPa/minute. The effects of stress rate on stress-strain behaviour as well as on mechanical properties were investigated by controlling the time of deformation. The stress-strain behaviour of rocksalts were verified by theoretical analysis with the help of the derivation of equations for Maxwell and Kelvin-Voigt Models connected in series.

Shear stress and shear rate differentially affect the multi-step process of leukocyte-facilitated melanoma adhesion.

Previous studies have shown that neutrophils (PMNs) facilitate melanoma cell extravasation [M.J. Slattery, C. Dong, Neutrophils influence melanoma adhesion and migration under flow conditions, Intl. J. Cancer 106 (2003) 713-722] Little is known, however, about the specific interactions between PMNs, melanoma and the endothelium (EC) or the molecular mechanism involved under flow conditions. The aim of this study is to investigate a "two-step adhesion" hypothesis that involves initial PMN tethering on the EC and subsequent melanoma cells being captured by tethered PMNs. Different effects of hydrodynamic shear stress and shear rate were analyzed using a parallel-plate flow chamber. Results indicate a novel finding that PMN-facilitated melanoma cell arrest on the EC is modulated by shear rate, which is inversely-proportional to cell-cell contact time, rather than by the shear stress, which is proportional to the force exerted on formed bonds. Beta2 integrins/ICAM-1 adhesion mechanisms were examined and the results indicate LFA-1 and Mac-1 cooperate to mediate the PMN-EC-melanoma interactions under shear conditions. In addition, endogenously produced IL-8 contributes to PMN-facilitated melanoma arrest on the EC through the CXC chemokine receptors 1 and 2 (CXCR1 and CXCR2) on PMN. These results provide new evidence for the complex role of hemodynamic forces, secreted chemokines and PMN-melanoma adhesion in the recruitment of metastatic cancer cells to the EC.

Modelling the rheological behaviour of fresh concrete: An elasto-viscoplastic finite element approach

Rheological behaviour of fresh concrete is an important factor in controlling concrete quality. It is recognized that the measurement of the slump is not a sufficient test method to adequately characterize the rheology of fresh concrete. To further understand the slump measurement and its relationship to the rheological properties, an elasto-viscoplastic, 2-D axisymmetric finite element (FE) model is developed. The FE model employs the Bingham material model to simulate the flow of a slump test. An experimental program is carried out using the Slump Rate Machine (SLRM_II) to evaluate the finite element simulation results. The simulated slump-versus-time curves are found to be in good agreement with the measured data. A sensitivity study is performed to evaluate the effects of yield stress, plastic viscosity and cone withdrawal rate on the measured flow curve using the FE model. The results demonstrate that the computed yield stress compares well with reported experimental data. The flow behaviour is shown to be influenced by the yield stress, plastic viscosity and the cone withdrawal rate. Further, it is found that the value of the apparent plastic viscosity is different from the true viscosity, with the difference depending on the cone withdrawal rate. It is also confirmed that the value of the final slump is most influenced by the yield stress.

Generalized subloading surface model with tangential stress rate effect

The traditional elastoplastic constitutive equation, which is inde- pendent of the stress rate component tangential to the yield surface predicts an unrealistically stiff mechanical response for the nonproportional loading proc- ess in which the stress rate has a component tangential to the yield surface. In this article, the generalized constitutive equation is then formulated by incor- porating the inelastic strain rate due to the stress rate tangential to the subload- ing surface into the subloading surface model exhibiting a smooth elastic- plastic transition.

Distinct role of hydrodynamic shear in leukocyte-facilitated tumor cell extravasation

Previously, we found polymorphonuclear neutrophils (PMNs) increased melanoma cell extravasation under flow conditions (Intl J Cancer 106: 713-722, 2003). In this study, we characterized the effect of hydrodynamic shear on PMN-facilitated melanoma extravasation using a novel flow-migration assay. The effect of shear stress and shear rate on PMN-facilitated melanoma extravasation was studied by increasing the medium viscosity with dextran to increase shear stress independently of shear rate. Under fixed shear rate conditions, melanoma cell extravasation did not change significantly. In contrast, the extravasation level increased at a fixed shear stress but with a decreasing shear rate. PMN-melanoma aggregation and adhesion to the endothelium via beta(2)-integrin/intracellular adhesion molecule-1 (ICAM-1) interactions were also studied. Lymphocyte function-associated molecule-1 (LFA-1; CD11a/CD18) influenced the capture phase of PMN binding to both melanoma cells and the endothelium, whereas Mac-1 (CD11b/CD18) affected prolonged PMN-melanoma aggregation. Blockage of E-selectin or ICAM-1 on the endothelium or ICAM-1 on the melanoma surface reduced PMN-facilitated melanoma extravasation. We have found PMN-melanoma adhesion is correlated with the inverse of shear rate, whereas the PMN-endothelial adhesion correlated with shear stress. Interleukin-8 (IL-8) also influenced PMN-melanoma cell adhesion. Functional blocking of the PMN IL-8 receptors, CXCR1 and CXCR2, decreased the level of Mac-1 upregulation on PMNs while in contact with melanoma cells and reduced melanoma extravasation. We have found PMN-facilitated melanoma adhesion to be a complex multistep process that is regulated by both microfluid mechanics and biology.

Modeling rate effects in undrained loading of sands

The present technical note extends the previous work by the authors concerned with formulation of a constitutive model of elastoplastic response of sands (Superior sand model) and its application to the analyses of static liquefaction and instability states in triaxial compression and extension occurring in the undrained deformation of saturated granular materials. To account for time-dependent behavior and strain rate effects, an elastic, viscoplastic extension of the model to triaxial compression is proposed. The constitutive equations derived are used to predict the model response in different loading histories. In particular, strain rate and stress rate effects and undrained creep deformation for specified stress components are discussed in detail. Comparison of model predictions with available experimental data also is provided.Key words: saturated sand, constitutive model, elastic–viscoplastic behavior.

Notch Effects on Impact Tensile Properties in A6061-T6 Aluminum Alloy

The strain rate sensitivity of a A6061-T6 aluminum is examined as a function of triaxial stress. The triaxial stress state is changed by changing the notch acuity of notched specimens. Under the high strain rate conditions, increasing triaxiality increases flow stress. Plastic constraint factor decreases in notched specimens under high strain rate conditions. Reduction-of-area in notched specimen increases with increasing strain rate. The relaxation of plastic constraint takes place under high strain rate in the notched specimens. The effects of triaxial stress and strain rate on the fracture surfaces are discussed.

下負荷面モデルによる繰返し円形応力負荷を受ける砂の変形挙動の予測および接線応力速度の効果

The subloading surface model which fulfills the mechanical requirements for constitutive equations, i. e. the continuity, the smoothness' conditions, the work rate-stiffness and the Masing effect relaxation has been extended so as to describe the inelastic stretching due to the deviatoric tangential stress rate for the subloading surface. The model is called thetangential-subloading surface model. In this article, the subloading surface model enabling the description of deformation behavior of a cyclic loading with the tangential rate effect is applied to the prediction of the deformation behavior of sand subjected to a cyclic loading of circular stress path in the g-plane deviating significantly from the proportional loading. The validity of the model and the tangential stress rate effect are verified comparing with the test data.