Cyclic stress induced phase transformation in super-bainitic microstructure

The effects of cyclic stress loading on the microstructual evolution and tensile properties of a medium-carbon super- bainitic steel were investigated. Experimental results show that the cyclic stress can induce the carbon gathering in austenite and phase transformation from film-like retained austenite to twin martensite, which will obviously enhance the tensile strength and the product of tensile strength and ductility. The higher the bainitic transformation temperature, the lower the transformation rate of the retained austenite. The amount and thickness of the film-like retained austenite play an important role during the cyclic stress induced phase transformation.

Modeling of Fatigue Crack Growth Closure Considering the Integrative Effect of Cyclic Stress Ratio,Specimen Thickness and Poisson's Ratio

Key components of large structures in aeronautics industry are required to be made light and have long enough fatigue lives.It is of vital importance to estimate the fatigue life of these structures accurately.Since the FCG process is affected by various factors,no universal model exists due to the complexity of the mechanisms.Most of the existing models are obtained by fitting the experimental data and could hardly describe the integrative effect of most existing factors simultaneously.In order to account for the integrative effect of specimen parameters,material property and loading conditions on FCG process,a new model named integrative influence factor model（IIF） is proposed based on the plasticity-induced crack closure theory.Accordingly to the predictions of crack opening ratio（γ） and effective stress intensity factor range ratio（U） with different material under various loading conditions,predictions of γ and U by the IIF model are completely identical to the theoretical results from the plane stress state to the plane strain state when Poisson＇s ratio equals 1/3.When Poisson＇s ratio equals 0.3,predictions of γ and U by the IIF model are larger than the predictions by the existing model,and more close to the theoretical results.In addition,it describes the influence of R ratios on γ and U effectively in the whole region from-1.0 to 1.0.Moreover,several sets of test data of FCG rates in 5 kinds of aluminum alloys with various specimen thicknesses under different loading conditions are used to validate the IIF model,most of the test data are situated on the predicted curves or between the two curves that represent the specimen with different thicknesses under the same stress ratio.Some of the test data slightly departure from the predictions by the IIF model due to the surface roughness and errors in measurement.Besides,based on the analysis of the physical rule of crack opening ratios,a relative thickness of specimen is defined to describe the influence of material property,specimen thickness and so forth on FCG characteristics conveniently.In conclusion,the relative thickness of specimen simplifies the expression of FCG characteristic and provides a general parameter to analyze the fatigue characteristics of different materials with various thicknesses under different loading conditions.The IIF model describes the integrative effect of existing influence factors explicitly and quantitatively,and provides a helpful tool for fatigue property estimation of practical component and experiment design.

Correlation between the Cyclic Stress Behavior and Microstructure in 316LN based on the Analysis of Hysteresis Loops

Total strain controlled cyclic test was performed on 316 LN under uniaxial loadings. Through the partitioning of hysteresis loops, the evolution of two components of cyclic flow stress, the internal and effective stresses, was reported. The former one determines the cyclic stress response. Based on the transmission electron microscopic（TEM） observation on specimens loaded with scheduled cycles, it is found that planar dislocation structures prevail during the entire cyclic process at low strain amplitude, while a remarkable dislocation rearrangement from planar structures to heterogeneous spatial distributions is companied by a cyclic softening behavior at high strain amplitude. The competition between the evolution of the intergranular and the intragranular components of the internal stress caused by the transition of slip mode induces the cyclic hardening and softening at high strain levels. The intergranular internal stress represents the most part of the internal stress at low strain level.

Critical Cyclic Stress Ratio of Undisturbed Saturated Soft Clay in the Yangtze Estuary under Complex Stress Conditions

There exists a critical cyclic stress ratio when sand or clay is subjected to cyclic loading. It is an index dis-tinguishing stable state or failure state. The soil static and dynamic universal triaxial and torsional shear apparatus de-veloped by Dalian University of Technology in China was employed to perform different types of tests on saturated soft marine clay in the Yangtze estuary. Undisturbed samples were subjected to undrained cyclic vertical and torsional coupling shear and cyclic torsional shear after three-directional anisotropic consolidation with different initial consoli-dation parameters. The effects of initial orientation angle of major principal stress, initial ratio of deviatoric stress,initial coefficient of intermediate principal stress and stress mode of cyclic shear on the critical cyclic stress ratio wereinvestigated. It is found that the critical cyclic stress ratio decreases significantly with increasing initial orientation angle of major principal stress and initial ratio of deviatoric stress. Compared with the effects of the initial orientationangle of major principal stress and initial ratio of deviatoric stress, the effect of initial coefficient of intermediate prin-cipal stress is less evident. Under the same consolidation condition, the critical cyclic stress ratio from the cyclic cou-pling shear test is lower than that from the cyclic torsional shear test, indicating that the stress mode of cyclic shear has an obvious effect on the critical cyclic stress ratio. The main reason is that the continuous rotation in principal stressdirections during cyclic coupling shear damages the original structure of soil more than the cyclic torsional shear does.

Experimental study on the slip evolution of planar fractures subjected to cyclic normal stress

The frictional rupture mechanisms of rock discontinuities considering the dynamic load disturbance still remain unclear.This paper investigates the transitional behaviors of slip events happened on a planar granite fracture under cyclic normal stress with diferent oscillation amplitudes.The experimental results show that the activations of fast slips always correlate with unloading of normal stress.Besides,the intensive normal stress oscillation can weaken the shear strength which is recoverable when the normal stress return to constant.The rupture patterns are quantifed by stress drop,slip length and slip velocity.With the efect of small oscillation amplitudes,the slip events show chaotic shapes,compared to the regular and predictable style under constant normal stress.When the amplitude is large enough,the big and small slip events emerge alternately,showing a compound slip style.Large amplitude of the cyclic normal stress also widens the interval diferences of the slip events.This work provides experimental supports for a convincible link between the dynamic stress disturbance and the slip behavior of rock fractures.

Assessment of cyclic deformation and critical stress amplitude of jointed rocks via cyclic triaxial testing

Jointed rock specimens with a natural replicated joint surface oriented at a mean dip angle of 60were prepared,and a series of cyclic triaxial tests was performed at different confining pressures and cyclic deviatoric stress amplitudes.The samples were subjected to 10,000 loading-unloading cycles with a frequency of 8 Hz.At each level of confining pressure,the applied cyclic deviatoric stress amplitude was increased incrementally until excessive deformation of the jointed rock specimen was observed.Analysis of the test results indicated that there existed a critical cyclic deviatoric stress amplitude(i.e.critical dynamic deviatoric stress)beyond which the jointed rock specimens yielded.The measured critical dynamic deviatoric stress was less than the corresponding static deviatoric stress.At cyclic deviatoric stress amplitudes less than the critical dynamic deviatoric stress,minor cumulative residual axial strains were observed,resulting in hysteretic damping.However,for cyclic deviatoric stresses beyond the critical dynamic deviatoric stress,the plastic strains increased promptly,and the resilient moduli degraded rapidly during the initial loading cycles.Cyclic triaxial test results showed that at higher confining pressures,the ultimate residual axial strain attained by the jointed rock specimen decreased,the steadystate dissipated energy density and steady-state damping ratio per load cycle decreased,while steadystate resilient moduli increased.

Effect of Cyclic Stress Reduction on the Creep Characteristics of AZ91 Magnesium Alloy

The creep deformation behavior of aged AZ91 magnesium alloy under the cyclic stress reduction has been investigated in the temperature range 353–383 K. The microstructural evolution of the AZ91 magnesium alloy at different heat treatment conditions has been studied using optical microscopy and scanning electron microscopy. The minimum strain rate, e_min, was found to be higher under cyclic stress reduction condition than that under static creep condition for the same maximum stress. Cyclic creep acceleration is interpreted using the interaction of moving dislocations with the existing b-phase（Mg17Al12） precipitates. The mean value of the activation energy of secondary creep stage was found to be equal to that quoted for lattice self-diffusion of magnesium mechanism.

Effects of Cyclic Electron Flow Inhibitor(Antimycin A)on Photosystem Photoinhibition of Sweet Pepper Leaves upon Exposure to Chilling Stress Under Low Irradiance

In chloroplast, there were two pathways involved in the cyclic electron flow around photosystem 1 （PS 1）. One was the NADH dehydrogenase （NDH）-dependent flow and the other was the ferredoxin quinone reductase-dependent flow. It was proposed that the NDH-dependent cyclic electron flow around PSI was related to the xanthophyll cycle-dependent non-photochemical quenching （NPQ） at chilling temperature under low irradiance （CL）. The function of the chloroplastic cyclic electron flow around PS 1 was examined by comparing sweet pepper （Capsicum annuum L.） control with its antimycin A （AA）-fed leaves upon exposure to CL stress. During CL stress, the maximum photochemical efficiency of PS2 （Fv/Fm） decreased markedly in both controls and AA-fed leaves, and P700＋ was also lower in AA-fed leaves than in controls. These results implied that cyclic electron flow around PS 1 functioned to protect the photosynthetic apparatus from CL stress. Under such stress, NPQ and PS2-driven electron transport rate were different between AA-fed leaves and controls. The lower NPQ in AA-fed leaves might be related to an inefficient proton gradient across thylakoid membranes （ApH） because of inhibiting cyclic electron flow around PS 1 under CL stress.

EFFECT OF ORIENTATION ON STRESS RESPONSE,FRICTION STRESS AND BACK STRESS DURING CYCLIC DEFORMATION IN Al SINGLE CRYSTALS

The symmetrical push-pull fatigue tests at strain amplitude of 2×10~3 with different slip orientation of Al single crystals,were carried out at room temperature in air.The peak stresses of various cycles were measured and the stress-strain hysteresis loops for selected cy- cles were recorded.The energy losses.friction stresses,back stresses and shape

EFFECT OF SLIP ORIENTATION ON STRESS RESPONSE,INTERNAL FRICTION AND ULTRASONIC ATTENUATION DURING CYCLIC DEFORMATION IN Al SINGLE CRYSTALS

The variation of cyclic stress,internal friction and ultrasonic attenuation during cyclic deformation and relations among them have been investigated with different slip orientation Al single crystals.The results indicate that the value of cyclic stress σ,internal friction Q^(-1)and ultrasonic attenuation △α depend obviously on the slip orientation.There are large differences in above three parameters for different slip orientation Al crystals.In early stage of fatigue life,σ and △α increase and Q^(-1)decreases with cycles N,and △α reached maxi- mum before σ,while Q^(-1)and σ get the valley and the peak,respectively,at same cycles.

Effects of cyclic uniaxial stress on mitosis orientation

Background:Cell division is one of the key roles in the cell development,cell differentiation,embryogenesis and recovery of tissues.Independent studies have shown that spindle alignment during not only asymmetric but also symmetric cell divisions is essential

Cyclic behavior of reinforced sand under principal stress rotation

Although the cyclic rotation of the principal stress direction is important,its effect on the deformation behavior and dynamic properties of the reinforced soil has not been reported to date.Tests carried out on large-scale hollow cylinder samples reveal that the cyclic rotation of the principal stress direction results in significant variations of strain components(ε,ε,εand γ) with periodic characteristics despite the deviatoric stress being constant during tests.This oscillation can be related to the corresponding variations in the stress components and the anisotropic fabric that rotate continuously along the principal stress direction.Sand under rotation appears to develop a plastic strain.Similar trends are observed for reinforced sand,but the shear interaction,the interlocking between particles and reinforcement layer,and the confinement result in significant reductions in the induced strains and associated irrecoverable plastic strains.Most of the strains occur in the first cycle,and as the number of cycles increases,the presence of strains becomes very small,which is almost insignificant.This indicates that the soil has reached anisotropic critical state(ACS),where a stable structure is formed after continuous orientation,realignment and rearrangement of the particles accompanied with increasing cyclic rotation.Rotation in the range of 60°-135° produces more induced strains even in the presence of the reinforcement,when compared with other ranges.This relates to the extension mode of the test in this range in which σ>σand to the relative approach between the mobilized plane and the weakest horizontal plane.Reinforcement results in an increase in shear modulus while it appears to have no effect on the damping ratio.Continuous cycles of rotation result in an increase in shear modulus and lower damping ratio due to the densification that causes a decrease in shear strain and less dissipation of energy.

Involvement of cyclic electron flow in irradiance stress responding and its potential regulation of the mechanisms in Pyropia yezoensis

Pyropia yezoensis, belongs to the genus of P orphyra before 2011, inhabit on intertidal zone rocks where irradiation changes dramatically, implying that the seaweed has gained certain mechanisms to survive a harsh environment. Based on the photosynthetic parameters with or without the inhibitors determined by a Dual-PAM-100 apparatus, we investigated the photosynthetic performance and the changes in electron fl ow that occurred during the algae were stressed with dif ferent light intensities previously. When the irradiation saturation was approaching, the CEF around PS I became crucial since the addition of inhibitors usually led to an increase in non-photochemical quenching. The inhibitor experiments showed that there were at least three dif ferent CEF pathways in Py. yezoensis and these pathways compensated each other. In addition to maintaining a proper ratio of ATP/NAD(P)H to support effi cient photosynthesis, the potential roles of CEF might also include the regulation of dif ferent photoprotective mechanisms in Py. yezoensis. Under the regulation of CEF, chlororespiration is thought to transport electrons from the reduced plastoquinone(PQ) pool to oxygen in order to mitigate the reduction in the electron transfer chain. When irradiation was up to the high-grade stress conditions, the relative value of CEF began to decrease, which implied that the NADP+ pool or PQ + pool was very small and that the electrons were transferred from reduced PS I to oxygen. The scavenging enzymes might be activated and the water-water cycle probably became an ef fective means of removing the active oxygen produced by the irradiation stressed Py. yezoensis. We believe that the dif ferent mechanisms could make up the photoprotective network to allow Py. yezoensis for survival in a highly variable light stress habitat, which may enlighten scientists in future studies on irradiance stress in other algae species.

INFLUENCE OF TEMPERATURE-DEPENDENT PROPERTIES ON THERMAL STRESSES RESPONSE AND OPTIMUM DESIGN OF C/M FGMS UNDER THERMAL CYCLIC LOADING

The influence of temperature-dependent properties on thermal stresses response and optimum design of newly developed ceramic-metal functionally graded materials under cyclic thermal loaning and high temperature gradient environment is studied. The thermal conductivity of material is considered to be dependent on the temperature. In this paper, the thermal stresses response of the material is calculated rising a nonlinear finite element method. Emphasis is placed on the influence of temperature-dependent properties on the thermal stresses response characteristics, the thermal stresses relaxation property and the thermal stresses history under the different graded compositional distributions and different heat flux magnitudes. Through tile analysis. it is suggested that the influence of temperature-dependent properties can not be neglected In the thermal stresses response analysis and the optimum design process of the material must be based on the temperature-dependent thermo-elastic-plastic theory.

The Cyclic Feeding Regime Induces Decaying Age-Dependent Oxidative Stress and Regulates the Cell Chain of the Immunity

We developed an experimental model of a cyclic feeding regime (CFR) that increased a lifespan in rats. The manifestations of oxidative stress and their interrelation with parameters of cell immunity were assessed in rats at CFR. It is shown that changes of body mass, liver mass and indexes of pro/antioxidant system after periods of starving—ad libitum nutrition at CFR diet in old animals were less pronounced than in young animals. The body mass loss of 30% in 14 days was accompanied by oxidative stress. Indexes of phagocytosis did not change, but activity of oxidase system of neutrophil was increased in 2 times. The response of metabolic and physiological systems on repeat starving—ad libitum nutrition cycles differs from the response to the initial cycle of CFR. This is interpreted as a change of adaptation strategy and the effect of metabolic memory, which influences the choice of organism strategy of adaptation for subsequent starving. The dynamics of change of the studied indexes in response to CFR was age-dependent. It was supposed that different answer to CFR in young and old animals is determined by the different amount of carbon and fat depots in young and old animals.

A NEW METHOD FOR STRESS ANALYSIS OF A CYCLICALLY SYMMETRIC STRUCTURE

In this paper, a computational method for finite element stress analysis of a cyclically symmetric structure subjected to arbitrary loads is provided. At first, using discrete Fourier transformation technique, the complete structure is analyzed by considering only one sector with appropriate complex constraints on its boundary with the adjacent sectors. Next, an imaginary structure which is composed of two identically overlapping sectors is constructed, and that the complex constraints mentioned above can be equivalently replaced by a set of real constraints on this imaginary structure is proved. Therefore, the stress analysis of a cyclically symmetric structure can be solved conveniently by most of finite element programs.

循环荷载下双裂隙砂岩弹性模量及裂纹动态演化

为研究不同岩桥倾角砂岩在循环荷载作用下的弹性模量变化和裂纹动态演化规律,对裂隙倾角固定为45°,岩桥倾角分别为15°,45°,75°,90°,105°的砂岩试件开展了单轴循环加、卸载试验和PFC^(2D)模拟.结果表明,不同裂隙砂岩的弹性模量随循环荷载作用得到了明显的强化;在循环荷载作用下,应力-应变曲线与离散元模拟得到的试件瞬时微裂纹演化曲线存在对应关系;岩桥倾角对砂岩的破坏模式有影响,PFC^(2D)模拟的裂纹动态演化可以合理地反映岩石中裂纹的萌生位置、扩展方向.基于微裂纹数目所定义的损伤与破坏模式很好地对应,岩桥倾角为45°时原始损伤是最大的.

列车荷载作用下压实花岗岩残积土累积变形特性试验研究

为研究列车荷载作用下应力及湿化条件对压实花岗岩残积土累积变形特性的影响,以围压、动应力幅值、含水率为变量,对压实花岗岩残积土进行循环空心扭剪和循环定向加载试验。结果表明:循环扭剪应力路径下主应力轴旋转效应增加了试样的累积变形量,甚至改变了累积变形的发展模式,该影响程度随试样含水率的增大而增大;试样的累积应变发展速率在加载初期(N<100)时完成大幅度衰减,围压增大可加快累积应变发展速率的衰减,动应力幅值的增加会延长累积应变发展速率的衰减时间,循环扭剪加载时试样的累积应变发展速率衰减程度小于循环定向加载试验,含水率越高该效应越显著。通过预测模型能够对两种动力加载应力路径下的稳定型和发展型累积应变发展曲线能够进行拟合,对压实花岗岩残积土的累积变形作出较好预测。

循环加载下压实黄土的边界面塑性本构模型

压实黄土的动应力应变关系预测是黄土高原平山填方工程长期运营涉及的关键问题之一,已有模型预测压实黄土的动本构关系难度较大。该文在边界面塑性框架下,引入动态映射法则,提出了一个反映应变累积的循环加载塑性模量表达式,构建了循环加载条件下压实黄土边界面塑性本构模型,分析了循环应力比与围压对应力应变关系与模量的影响规律,采用压实粉质黄土动三轴试验结果进行了验证。结果表明,该循环加载塑性模量基本反映了压实黄土的应变累积行为,所构建的本构模型考虑了循环应力比对动应力应变关系与模量的影响,可为填方压实黄土长期变形分析提供参考。