Comparison between the Methods of Determining the Critical Stress for Initiation of Dynamic Recrystallization in 316 Stainless Steel

Several methods have been proposed to calculate the critical stress for initiation of dynamic recrystallization （σc） on the basis of mathematical methods. One＇ of them is proposed by Stewart et al. in which this critical point appears as a distinct minimum in the （-dθ/dσ vs σ） through differentiating from θ vs σ. Another one is presented by Najafizadeh and Jonas by modifying the Poliak and Jonas method. According to this method, the strain hardening rate was plotted against flow stress, and the value of σc was attained numerically from the coefficients of the third-order equation that was the best fit from the experimental θ-σ data. Hot compression tests were used in the range of 1000 to 1100℃ with strain rates of 0.01^-1 s^-1 and strain of I on 316 stainless steel. The result shows that Najafizadeh and Jonas method is simpler than the previous one, and has a good agreement with microstructures. Furthermore, the value of normalized critical stress for this steel was obtained uc=σc/σp=0.92.

STRESS－STBAIN FIELD NEAR CRACK TIP AND CALCULATION OF CRITICAL STRESS OF CRACK PROPAGATION IN A PURE BENDING BEAM OF RECTANGULAR SECTION WITH ONE－SIDED MODEICRACK

This paper studies the stress-sirain field near crack tip in a pure bending beam ofrectangular section with one-sided mode I crack by the analytic method of Rej [1],then it gives the stress and strain components at the crack tip when the crackpropagates and further it obtains the formulas of calculating the elastic deformed areawidth, the deformed intensity, area width and the equation groups of calculating thecritical stress of crack propagation, last the equation group of calculating criticalstress of crack propagation is verified by calculating instance. The maximum error is0. 18%.

Effects of temperature on critical resolved shear stresses of slip and twining in Mg single crystal via experimental and crystal plasticity modeling

Magnesium(Mg)single crystal specimens with three different orientations were prepared and tested from room temperature to 733 K in order to systematically evaluate effects of temperature on the critical resolved shear stress(CRSS)of slips and twinning in Mg single crystals.The duplex non-basal slip took place in the temperature range from 613 to 733 K when the single crystal samples were stretched along the<0110>direction.In contrast,the single basal slip and prismatic slip were mainly activated in the temperature range from RT to 733 K when the tensile directions were inclined at an angle of 45°with the basal and the prismatic plane,respectively.Viscoplastic self-consistent(VPSC)crystal modeling simulations with genetic algorithm code(GA-code)were carried out to obtain the best fitted CRSSs of major deformation modes,such as basal slip,prismatic slip,pyramidalⅡ,{1012}tensile twinning and{1011}compressive twinning when duplex slips accommodated deformation.Additionally,CRSSs of the basal and the prismatic slip were derived using the Schmid factor(SF)criterion when the single slip mainly accommodated deformation.From the CRSSs of major deformation modes obtained by the VPSC simulations and the SF calculations,the CRSSs for basal slip and{1012}tensile twinning were found to show a weak temperature dependence,whereas those for prismatic,slip and{1011}compressive twinning exhibited a strong temperature dependence.From the comparison of previous results,VPSC-GA modeling was proved to be an effective method to obtain the CRSSs of various deformation modes of Mg and its alloys.

Effect of interaction of dislocations with core-shell nanowires on critical shear stress of nanocomposites

The contribution to the critical shear stress of nanocomposites caused by the interaction between screw dislocations and core-shell nanowires (coated nanowires) with interface stresses was derived by means of the MOTT and NABARRO's model. The influence of interface stresses on the critical shear stress was examined. The result indicates that, if the volume fraction of the core-shell nanowires keeps a constant, an optimal critical shear stress may be obtained when the radius of the nanowire with interface stresses reaches a critical value, which differs from the classical solution without considering the interface stresses under the same external conditions. In addition, the material may be strengthened by the soft nanowires when the interface stresses are considered. There also exist critical values of the elastic modulus and the thickness of surface coating to alter the strengthening effect produced by it.

The estimation of a critical shear stress based on a bottom tripod observation in the southwest off Jeju Island,the East China Sea

The resuspension and deposition of sediment within a bottom boundary layer（BBL） is the main dynamic processes that control the fate of the suspended sediment in shelf seas.The numerical study of sediment transport patterns relies on the knowledge of some critical parameters that describe sediment erosion and deposition.A critical shear stress is estimated based on field observations at the edge of a mud area southwest off Jeju Island,the East China Sea.On the basis of the simultaneous observation of velocity and suspended sediment concentrations within the BBL by means of acoustic instruments including an acoustic Doppler velocimeter and an acoustic Doppler current profiler,the settling velocity is estimated by turbulent oscillations of the SSC under the assumption of inertial-dissipation balance.This method gives a mean value of 0.91 mm/s and standard deviation of 0.20 mm/s,which is an order of magnitude larger than the value obtained by an empirical method of Soulsby and by an in situ submersible particle size analyzer.The difference is possibly due to the distinct natures of two methodologies,the inertial-dissipation method is more indicative of the BBL dynamics and is thus believed to provide reasonable in situ estimates of the settling velocity,whereas Soulsby＇s method is usually suitable for still water.A novel method for estimating the critical stresses of erosion and deposition based on statistical analyses of the temporal variability of the SSC（which is defined as a derivative of the depth-averaged SSC with respect to time） and the corresponding bottom shear stress is proposed.Both critical stresses of erosion and deposition vary between 0.11 Pa and 0.25 Pa with corresponding median values of 0.20 Pa and 0.16 Pa,respectively,which confirms that the critical stresses of erosion is somewhat higher than the critical stresses of deposition.Another method of estimating the critical shear stress by means of the settling velocity is also employed,which yields reasonable critical shear stress values of 0.06-0.17 Pa.

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.

Reflection:A Critical Look at Escaping Secondary Traumatic Stress Experienced by Law Enforcement Officers

Despite the stigma that is associated with law enforcement officers receiving mental health resources and treatment to combat secondary traumatic stress,it is widely the most effective form for recovery and having a fulfilling career.Law enforcement officers are superheroes and,they are known for saving the day.That is why it is critical for mental health offerings to be normalized in law enforcement agencies and constant evaluation of the psychological and cognitive well-being of officers.Unfortunately,the stress of the profession will not go away,but having resources and incentives to address several factors that officers face will assist with their overall feeling about the important work they do.As such,it is recommended that law enforcement agencies adopt programs to treat secondary traumatic stress,such as Critical Incident Stress Debriefing,Peer-Support Program,and Crisis Intervention Team.While there is no perfect program,these programs are designed to reduce the risk of serious injury or death during an emergency interaction between citizens and law enforcement officers.Ultimately,it is the responsibility of those charged with protecting law enforcement officers to understand the stress and how it affects the mind and body of those officers in managing life,work,and citizens.

Role of Interatomic Force to Critical Resolved Shear Stress of Single Crystals

Experimental results of the temperature dependence of critical resolved. shear stresses (CRSS)of Mo, Fe, Al and Mg single crystals are shown. Associating reports in recent years, we point out that the approximate exponential relationship between CRSS and the absolute temperatureat least in the region of the steep temperature dependence range of many materials is more common, even for bcc, fcc, and hcp single crystals. polycrystals and other covalent crystals,provided that the slip plane and slip direction are kept the same. Successful explanation with atomic force law shows that the interatomic forces (electronic structure) play a decisive role in determining the temperature dependence of yield stresses for a large number of materials.

STRESS RELAXATION BEHAVIOUR OF TITANIUM

Studies of stress relaxation were carried out on commercially pure titanium.It was found that the stress relaxation behaviour relates closely to the level of deformation.The amount of re- laxed stress (?)σ_R is independent of the amplitude of stress dip in a certain range of deformation.However,it depend on the stress level at which stress relaxation begins in a cer- tain relaxation time t_R.Both back(critical)stress σ_c and flow stress σ_o vary with the strain in a similar manner.The effective stress on dislocations σ~* increases with increasing strain under low strains,but keeps constant under higher strains.

H_2S Stress Corrosion Tests of Welded Joint for X65 Pipeline Steel and Finite Element Numerical Analysis of Crack Tip

The microstructure of welded joint is surveyed and the mechanical properties of X65 pipeline steel are studied in this paper, which provides experimental basis of performance effect on stress corrosion. H 2S stress corrosion cracking (SCC) tests on the steel are carried out in the environment based on NACE TM- 01-77 solution. The threshold stress intensity factor and crack propagation velocity for base metal and HAZ are obtained. The susceptibility of welded joint for X65 pipeline steel to H 2S stress corrosion cracking is investigated. The programming package ANSYS of finite element model (FEM) is used to perform the three-dimensional elastic-plastic finite element analysis of WOL specimens. Stress field and concentration of hydrogen distribution property of the crack tip are obtained.

Study on H_2S stress corrosion test of welded joint for X65 pipeline steel and numerical analysis

The susceptibility of welded joint for the X65 pipeline steel to H_2S stress corrosion cracking (SCC) is investigated. SCC tests on the steel are carried out in the environment based on NACE TM-01-77 solution with saturated gaseous H_2S. The threshold stress intensity factor and crack propagation velocity are calculated according to wedge-opening loading (WOL) specimens. The three-dimensional elastic-plastic finite element analysis of WOL specimens is performed by using the FEM programming package ANSYS. Stress field and concentration of hydrogen distribution property ahead of the crack tip are obtained. This paper surveyed the microstructure of welded joint and studied on the mechanical properties of X65 pipeline steel. It provides experimental basis for studying stress corrosion. The results of numerical analysis are consistent with conclusions of stress corrosion test.

Mathematical Modeling of Shear Stress and Direct Shear Test for Compressible Soil: Case of Soil Bordering the Wouri River

This paper focuses on the development of the mathematical model of shear stress by direct shear test for compressible soil of the littoral region, which will be a great tool in the hand of geotechnical engineers. The most common use of a shear test is to determine the shear strength which is the maximum shear stress that a material can withstand before the failure occurs. This parameter is useful in many engineering designs such as foundations, roads and retaining walls. We carried out an experimental laboratory test of ten samples of undisturbed soil taken at different points of the border of Wouri river of Cameroon. The samples were collected at different depths and a direct shear test was conducted. The investigations have been performed under constant vertical stresses and constant sample volume with the aim to determine the frictional angle and the cohesion of the compressible soil which are so important to establish the conditions of buildings stability. Special care was taken to derive loading conditions actually existing in the ground and to duplicate them in the laboratory. Given that the buildings constructed in this area are subjected to settlement, landslide, and punch break or shear failure, the cohesion and the frictional angle are determined through the rupture line after assessed the mean values of the shear stress for the considered ten samples. The bearing capacity of the soil, which is the fundamental soil parameter, was calculated. From the laboratory experimental results, the least squared method was used to derive an approximated mathematical model of the shearing stress. Many optimizations methods were then considered to reach the best adjustment.

Evaluation of visual stress symptoms in age-matched dyslexic, Meares-Irlen syndrome and normal adults

AIM： To examine the prevalence of dyslexia and Meares-Irlen syndrome（MIS） among female students and determine their level of visual stress in comparison with normal subjects.·METHODS： A random sample of 450 female medical students of King Saud University Riyadh（age range, 18-30y） responded to a wide range of questions designed to accomplish the aims of this study. The detailed questionnaire consisted of 54 questions with 12 questions enquiring on ocular history and demography of participants while 42 questions were on visual symptoms. Items were categorized into critical and non-critical questions（CQ and NCQ） and were rated on four point Likert scale. Based on the responses obtained, the subjects were grouped into normal（control）, dyslexic with or without MIS（Group 1） and subjects with MIS only（Group 2）. Responses were analysed as averages and mean scores were calculated and compared between groups using one way analysis of variance to evaluate total visual stress score（TVSS =NCQ ＋CQ）, critical and non-critical visual stress scores. The relationship between categorical variables such as age, handedness and condition were assessed with Chi-square test.·RESULTS： The completion rate was 97.6% and majority of the respondents（92%） were normal readers, 2%dyslexic and 6% had MIS. They were age-matched. More than half of the participants had visited an eye care practitioner in the last 2y. About 13% were recommendedeye exercises and one participant experienced pattern glare. Hand preference was not associated with any condition but Group 1 subjects（3/9, 33%） were significantly more likely to be diagnosed of lazy eye than Group 2（2/27, 7%） and control（27/414, 7%） subjects. The mean±SD of TVSS responses were 63±14 and it was 44±9for CQ and 19 ±5 for NCQ. Responses from all three variables were normally distributed but the CQ responses were on the average more positive（82%） in Group 2 and less positive（46%） in Group 1 than control. With NCQ,the responses were equally less positive in Group 1 and2 than control. Group 2 subjects showed significantly higher TVSS（P =0.002）, NCQ（P =0.006） and CQ（P =0.008）visual stress scores than control but no difference between Group 1 and control subjects, was observed for all scores（P 〉0.05, for all comparisons）.·CONCLUSION： The prevalence of dyslexia and MIS among Saudi female students was 2% and 6%,respectively. Critical questions performed best for assessing visual stress symptoms in dyslexic and MIS subjects. Generally, students with MIS were more sensitive to visual stress than normal students but dyslexics were more likely to present with a lazy eye than MIS and normal readers.

Determining the Critical Fracture Stress of Al Dendrites near the Melting Point via Synchrotron X-ray Imaging

Dendrites are the most common microstructural features in the cast metals,significantly affecting the structure integrity and mechanical properties of the castings.In this study,the in situ synchrotron X-ray radiographic and tomographic imaging techniques were combined to evaluate the critical fracture stress of the growing dendrite tip during the solidification of an Al-15 wt%Cu alloy under an external electromagnetic force.Two dendritic 3D models have been proposed to simulate the dendrite 3D morphologic characteristics and thus revealed that the critical fracture stresses of the Al dendrites at temperatures close to its melting point were in the range of 0.5 kPa–0.05 MPa.The present results demonstrate the feasibility of measuring the high-temperature mechanical properties of the metallic dendrites.

Modelling erosion of a single rock block using a coupled CFD-DEM approach

Rock block removal is the prevalent physical mechanism for rock erosion and could affect the stability of dam foundations and spillways.Despite this,understanding of block removal is still inadequate because of the complex interactions among block characteristics,hydraulic forces,and erosive processes acting on the block.Herein,based on a previously conducted physical experiment of erosion of a single rock block,the removal processes of two different protruding blocks are represented by a coupled computational fluid dynamics-discrete element model(CFD-DEM)approach under varied flow conditions.Additionally,the blocks could be rotated with respect to the flow direction to consider the effect of the discontinuity orientation on the block removal process.Simulation results visualize the entire block removal process.The simulations reproduce the effects of the discontinuity orientation on the critical flow velocity inducing block incipient motion and the trajectory of the block motion observed in the physical experiments.The numerical results present a similar tendency of the critical velocities at different discontinuity orientations but have slightly lower values.The trajectory of the block in the simulations fits well with the experimental measurements.The relationship between the dimensionless critical shear stress and discontinuity orientation observed from the simulations shows that the effect of block protrusion becomes more dominant on the block incipient motion with the increase of relative protrusion height.To our knowledge,this present study is the first attempt to use the coupled finite volume method(FVM)-DEM approach for modelling the interaction behavior between the block and the flowing water so that the block removal process can be reproduced and analyzed.

Predicting Critical Conditions and Stress-Strain Curves for Dynamic Recrystallization in SPHC Steel

The hot compression tests on an SPHC steel were carried out in the temperature range of 900-1150 ℃ and strain rate range of 0.1-10 s-1,in which the maximum true strain is 0.8.The activation energy of test steel was calculated,to be 299.4 kJ/mol.The critical stresses and strains for initiation of dynamic recrystallization were determined based on changes of the work hardening rate（θ）as a function of the flow stress（σ）or strain（ε）,respectively.The dependence of the peak strain（εp）,the peak stress（σp）,and the steady state stress（σs）were determined based on the Zener-Hollomen parameter.The mathematical models of the flow stress evolution were established in the hardening and dynamic recovery region and dynamic recrystallization region,respectively.The average error between experimental curves and predicted ones was around 3.26%.

Prediction of mode I fracture toughness of rock using linear multiple regression and gene expression programming

Prediction of mode I fracture toughness(KIC) of rock is of significant importance in rock engineering analyses. In this study, linear multiple regression(LMR) and gene expression programming(GEP)methods were used to provide a reliable relationship to determine mode I fracture toughness of rock. The presented model was developed based on 60 datasets taken from the previous literature. To predict fracture parameters, three mechanical parameters of rock mass including uniaxial compressive strength(UCS), Brazilian tensile strength(BTS), and elastic modulus(E) have been selected as the input parameters. A cluster of data was collected and divided into two random groups of training and testing datasets.Then, different statistical linear and artificial intelligence based nonlinear analyses were conducted on the training data to provide a reliable prediction model of KIC. These two predictive methods were then evaluated based on the testing data. To evaluate the efficiency of the proposed models for predicting the mode I fracture toughness of rock, various statistical indices including coefficient of determination(R2),root mean square error(RMSE), and mean absolute error(MAE) were utilized herein. In the case of testing datasets, the values of R2, RMSE, and MAE for the GEP model were 0.87, 0.188, and 0.156,respectively, while they were 0.74, 0.473, and 0.223, respectively, for the LMR model. The results indicated that the selected GEP model delivered superior performance with a higher R2value and lower errors.

Mechanical analysis of folds in compressive belt and its significance to earthquakes

The paper proposes the conception of Beads type fold system. The mechanical analyses of the typical tectonic system are made by means of elastic stability theory, mathematical and mechanical method and rheology. The relation among the deflections of folds and time, external forces, and distribution of stresses, strain energy density are analyzed to explain the causing mechanism of folding earthquake.

Effect of Grain Size and Compression Direction on the Hot Deformation Characteristics of High-Cr Ultra-Super-Critical Rotor Steel with Columnar Grains

The columnar grains with a duplex grain size distribution in the ingot increase the difficulty of the hot forging of the as-cast high Cr ultra-super-critical rotor steel.The hot deformation behaviors of the high Cr steel with different initial grain sizes under various compression directions were investigated.The results show that the hot deformation characteristic is strongly grain size and compression direction dependent.The finer grain size and compression direction perpendicular to the columnar grains increase the flow stress and activation energies of hot deformation,comparing with the large grain and deformation direction parallel to the columnar grains.The relationships between flow stress and deformation parameters for the different initial structures conform to the established constitutive equations.The former enhances the critical stress of the dynamic recrystallization(DRX),inhibiting the occurrence of DRX and reducing the dimensions of DRX grains.

Incipient Motion of Sediment Under Currents

Summarized in the paper are the author's studies on incipient motion of sediment in recent 40 years. In addition to the forces of gravity, drag and lift, the cohesive force and the additional static pressure are important for fine particles. The relations between three stages of incipient motion are defined by use of instantaneous velocity. Formulas for initial velocity and critical shear stress are given and overall verified by the author's and others' experimental data.