Modeling of Stress Distribution During Strip Coiling Process

 Many strip materials are coiled after rolling process. The stresses are imposed on the material wound on the automatically controlled collapse mandrel under the coiling tension. The coiling process can be described by three typical cases: winding without automatic adjustment, winding with automatic adjustment and after mandrel removal. A new model of equations for predicting the stresses during the strip coiling process is built by consideration of the three cases respectively. By solving the equations of different typical cases, the radial stresses and tangential stress of the layers of coil can be calculated. Also, the coiling parameters, such as strip thickness, coiling tension and necking critical pressure, affecting the coil performance are investigated. It is believed that the present model can be used for design and control of the automatically controlled collapse mandrel.     

Creep and Stress Relaxation Anisotropy of a Single-Crystal Superalloy

In this study, the TMF stress relaxation and creep behavior at 1023 K and 1223 K (750 °C and 950 °C) have been investigated for a Ni-based single-crystal superalloy. Specimens with three different crystal orientations along their axes were tested; 〈001〉, 〈011〉, and 〈111〉, respectively. A highly anisotropic behavior during TMF stress relaxation was found where the 〈111〉 direction significantly shows the worst properties of all directions. The TMF stress relaxation tests were performed in both tension and compression and the results indicate a clear tension/compression asymmetry for all directions where the greatest asymmetry was observed for the 〈001〉 direction at 1023 K (750 °C); here the creep rate was ten times higher in compression than tension. This study also shows that TMF cycling seems to influence the creep rate during stress relaxation temporarily, but after some time it decreases again and adapts to the pre-unloading creep rate. Creep rates from the TMF stress relaxation tests are also compared to conventional constant load creep rates and a good agreement is found.     

Stress asymmetry of stoichiometric NiAl single crystals

The yield stress properties of stoichiometric NiAl single crystals were investigated in terms of crystal orientation, temperature and the deformation mode. The calculated critical resolved shear stress (CRSS) was a strong function of crystal orientation, temperature and the deformation mode whether tension or compression. The CRSS was, in a wide range of experimental conditions, higher in the sequence of {110}〈100〉, {100}〈100〉 and {hk0}〈100〉 slips. The CRSS in compression was higher particularly at low temperatures than the CRSS in tension. The tension-compression asymmetry on the CRSS was understood qualitativelys being due to the effect of the normal stress on the core structure of a 〈001〉 dislocation and a 〈111〉 dislocation. It was suggested that a compressive normal stress makes the core configuration more sessile, resulting in the increased stress effectively at low temperatures.     

Analysis of Crack Tip Stress of Transversal Crack on Slab Corner During Vertical-Horizontal Rolling Process by FEM

Behavior of transversal crack notched on slab corner during vertical-horizontal rolling process was simulated by FEM. The crack tip stress in the whole rolling process was obtained. Influences of the friction coefficient, the initial crack size, the edger roll profile, and the groove fillet radii of grooved edger roll on crack tip stress were analyzed. For vertical rolling, the tension stress appears at crack tip near the slab top surface and the compression stress appears at crack tip near the slab side surface for the flat edger roll; however, the compression stress appears at crack tip near the slab top surface and the tension stress appears at crack tip near the slab side surface in the exit stage for the grooved edger roll. For horizontal rolling, the tension stress appears at crack tip just at the exit stage for the flat edger roll, and the tension stress appears in whole rolling stage; the tension stress value near the slab side surface is much larger than that near the slab top surface for the grooved edger roll.     

Elastoplastic 3D Deformation and Stress Analysis of Strip Rolling

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Effect of Stress Triaxiality on the Flow and Fracture of Mg Alloy AZ31

The microscopic damage mechanisms operating in a hot-rolled magnesium alloy AZ31B are investigated under both uniaxial and controlled triaxial loadings. Their connection to macroscopic fracture strains and fracture mode (normal vs shear) is elucidated using postmortem fractography, interrupted tests, and microscopic analysis. The fracture locus (strain-to-failure vs stress triaxiality) exhibits a maximum at moderate triaxiality, and the strain-to-failure is found to be greater in notched specimens than in initially smooth ones. A transition from twinning-induced fracture under uniaxial loading to microvoid coalescence fracture under triaxial loading is evidenced. It is argued that this transition accounts in part for the observed greater ductility in notched bars. The evolution of plastic anisotropy with stress triaxiality is also investigated. It is inferred that anisotropic plasticity at a macroscopic scale suffices to account for the observed transition in the fracture mode from flat (triaxial loading) to shear-like (uniaxial loading). Damage is found to initiate at second-phase particles and deformation twins. Fracture surfaces of broken specimens exhibit granular morphology, coarse splits, twin-sized crack traces, as well as shallow and deep dimples, in proportions that depend on the overall stress triaxiality and fracture mode. An important finding is that AZ31B has a greater tolerance to ductile damage accumulation than has been believed thus far, based on the fracture behavior in uniaxial specimens. Another finding, common to both tension and compression, is the increase in volumetric strain, the microscopic origins of which remain to be elucidated.     

X型正断层交汇处应力分布及其油气地质意义

为了查明X型正断层夹角变化对应力分布特征及油气运聚的影响,应用FLAC3D软件建立了X型正断层夹角自15(°)至90(°)变化的6种模型,并对其交汇处应力场分布特征进行了三维数值模拟分析。结果表明:断裂交汇部位垂直应力表现为挤压性质的力,不明显受相交角度变化的影响,而水平应力主要为拉张性质的力,当θ大于45(°)后出现挤压应力;断裂交汇部位水平、垂直应力集中程度具有先增大后减小的变化规律,垂直应力大小较水平应力明显大一个数量级,当相交角度为45(°)时,水平、垂直应力集中程度达到最大。应力高集中区能够释放大量应变能,有利于油气的运移和聚集,因此,交角为45(°)的X型正断层是油气富集的有利区。     

Pretension-Dependent Residual Stress of Alumina Fiber-Reinforced Composite Wire

The relationship between pretension and residual stress of an aluminum wire reinforced with 45 vol pct continuous Nextel? 610 alumina fibers is investigated. It is shown that as pretension stress increases, the matrix residual stress decreases. A transition in matrix residual stress from tension to compression occurs at a pretension stress of about 80 MPa. The initial rapidly decreased residual stress caused by pretension at relatively low pretension stresses is a result of matrix elastic compressive deformation; while the later gradually decreased residual stress at higher pretension stresses comes from matrix plastic compressive deformation. As the matrix yield stress and hardening exponent increase, the decrease in matrix residual stress with pretension stress is more rapid and the absolute value of matrix residual stress increases. An analytical model suitable for fiber-reinforced metal matrix composites (MMCs) with strong interfacial bonding is developed to describe the relationship between pretension and matrix residual stress and is shown to be in good agreement with the experimental and finite-element calculated results. The pretension-dependent matrix residual stress phenomenon suggests that the mechanical properties of fiber-reinforced MMCs associated with matrix residual stress may be effectively improved by applying tensile loads.     

Suction Stress Characteristic Curve for Unsaturated Soil

The concept of the suction stress characteristic curve (SSCC) for unsaturated soil is presented. Particle-scale equilibrium analyses are employed to distinguish three types of interparticle forces: (1) active forces transmitted through the soil grains; (2) active forces at or near interparticle contacts; and (3) passive, or counterbalancing, forces at or near interparticle contacts. It is proposed that the second type of force, which includes physicochemical forces, cementation forces, surface tension forces, and the force arising from negative pore-water pressure, may be conceptually combined into a macroscopic stress called suction stress. Suction stress characteristically depends on degree of saturation, water content, or matric suction through the SSCC, thus paralleling well-established concepts of the soil–water characteristic curve and hydraulic conductivity function for unsaturated soils. The existence and behavior of the SSCC are experimentally validated by considering unsaturated shear strength data for a variety of soil types in the literature. Its characteristic nature and a methodology for its determination are demonstrated. The experimental evidence shows that both Mohr–Coulomb failure and critical state failure can be well represented by the SSCC concept. The SSCC provides a potentially simple and practical way to describe the state of stress in unsaturated soil.     

Stress levels for elastic buckling of rolled strip and plate

Abstract

Elastic buckling of a thin plate is considered in the context of flat rolling of metal plate and strip. The residual stress in the plate some distance away from the roll gap can have regions of high compression and tension whose average value is quite moderate. These are induced by mismatches between the inlet strip profile and the outlet profile defined by the roll gap. In this paper the stress levels at which elastic buckling will occur are predicted together with the form of the buckling. A fast solution technique is described in which the actual stress is replaced by piecewise constant stresses within elements across the strip. Exact solutions for the deflection due to constant stresses are employed, along with the continuity conditions at the element junctions. Various known buckling solutions are reproduced to validate the technique. Stress distributions typical of those expected to occur in flat rolling are considered and both edge and centre buckling solutions are presented.     

低作用应力引起的溶质在晶界非平衡偏聚或贫化

提出了关于应力时效（高温及作用应力）引起溶质原子的晶界非平衡偏聚或贫化机理的一个新模型。此模型基于如下假设：晶界相对于完整晶体而言在强度上是弱化区。当多晶体受到一个在弹性范围内的作用应力时,晶界优先变形。若压应力作用时,晶界作为激发射空位;若拉应力作用时,晶界作为阱吸收空位,应力引起的过饱和空位与溶质原子形成复合体,此复合体在基体中的扩散速率远高于溶质原子。作用应力对晶界溶质偏聚或贫化的影响,是由复合体的扩散和溶质原子的反向扩散之间的平衡所决定。根据此模型,应力时效过程中存在一个临界时间,在此时间,应力引起的晶界偏聚或贫化程度将达到一个极大值。此模型已由ShinodaT等所作的磷在钢中的应力时效实验和MisraR D K所作的硫在钢中的应力时效实验结果所证实。     

Stress Wave Method for Measuring Elastic Moduli of Powder Compacts in Tension and Compression

Abstract

A test is described in which a cylindrical powder pellet is sandwiched between two steel rods (input and output bar), aligned linearly. The free end of the input bar is struck by a small projectile, propagating a low intensity, short duration, compressive stress pulse across the specimen to the free end of the output bar. This pulse is then reflected, causing the pellet to be loaded in tension. The strain pulses in the two bars are used to determine the elastic Young's modulus in tension and compression and the tensile strength of the pellet. Details are given of the apparatus used and the theory for calculation of the specimen stress and strain. The results are found to compare well with both uniaxial tension and diametral ‘Brazilian’ tests. The method is believed to be applicable to a wide range of powder compacts and porous materials. PM/0503     

Hysteresis of Capillary Stress in Unsaturated Granular Soil

Constitutive relationships among water content, matric suction, and capillary stress in unsaturated granular soils are modeled using a theoretical approach based on the changing geometry of interparticle pore water menisci. A series of equations is developed to describe the net force among particles attributable to the combined effects of negative pore water pressure and surface tension for spherical grains arranged in simple-cubic or tetrahedral packing order. The contact angle at the liquid–solid interface is considered as a variable to evaluate hysteretic behavior in the soil–water characteristic curve, the effective stress parameter χ, and capillary stress. Varying the contact angle from 0 to 40° to simulate drying and wetting processes, respectively, is shown to have an appreciable impact on hysteresis in the constitutive behavior of the modeled soils. A boundary between regimes of positive and negative pore water pressure is identified as a function of water content and contact angle. Results from the analysis are of practical importance in understanding the behavior of unsaturated soils undergoing natural wetting and drying processes, such as infiltration, drainage, and evaporation.     

Experimental study on the thermoelastic martensitic transformation in shape memory alloy polycrystal induced by combined external forces

Combined tension and torsion experiments with thin wall specimens of Cu-Al-Zn-Mn polycrystalline shape memory alloy (SMA) were performed at temperatureT =A _f + 25 K. The general stress-strain behaviors due to the thermoelastic martensitic transformation, induced by a combination of external forces of axial load and torque, were studied. It is shown that the progress of martensitic transformation (MT) at general stress conditions can be well considered as triggered and controlled by the supplied mechanical work (a kind of equivalent stress) in the first approximation. Pseudoelastic strains in proportional as well as nonproportional combined tension-torsion loadings were found fully reversible, provided that uniaxial strains were reversible. The axial strain can be controlled by the change of torque andvice versa due to the coupling among tension and torsion under stress, not only in forward transformation, but also in reverse transformation on unloading. The pseudoelastic strains of SMA polycrystal are path dependent but well reproducible along the same stress path. The evolution of macroscopic strain response of SMA polycrystal, subjected to the nonproportional pseudoelastic loading cycles with imposed stress path, was systematically investigated. The results bring qualitatively new information about the progress of the MT in SMA polycrystal, subjected to the general variations of external stress. PETR SITTNER, Research Associate, formerly with the Faculty of Engineering, Mie University     

Stress Zones near Displacement Piers: II. Radial Cracking and Wedging

Transient liquefaction of saturated soils near Rammed Aggregate Piers is described in Part I on the basis of radial stress measurements. This is supported by dynamic pore-water pressure measurements, as peak pore pressures approximately equal radial stresses imposed at the pier surface by ramming. Stress measurements outside of the liquefied/plastic zone indicate radial tension cracking in the elastic zone, which is consistent with the observation that pore pressures abruptly drop and momentarily can even become negative as soon as ramming stops. The drainage field created by extended radial cracking and hydraulic fracturing allows Rammed Aggregate Piers to be effective in saturated, fine-grained soils where other dynamic methods are reported to be less effective. Stress measurements indicate that liquefied soil injected into open tension cracks causes stress to be retained in the elastic zone through arching action. A stress path analysis indicates that lateral stress may play an important role in control of foundation settlement, by simulating an increase in the preconsolidation pressure without vertically surcharging the soil or waiting for it to consolidate.     

Stress dependence of creep in nanocrystalline Ni-P alloy

The stress dependence of the steady state creep rate has been studied at temperatures near 0.5 Tm for nano-grained (28 nm) and coarse-grained (257 nm) samples of Ni-P alloy. The stress exponent is found to be 1.2 for the former, and 2.5 for the latter. By considering the data of the stress exponent obtained in this study and the activation energy in our previous paper, we suggest that the creep rate of the nano-grained samples is mainly controlled by grain (and/or phase) boundary diffusion, while the creep rate of the coarsegrained samples is possibly controlled by a combined mechanism involving dislocation creep.     

Applied Stress Affecting the Environmentally Assisted Cracking

Stress corrosion cracking (SCC) is affected by the mode of applied stress, i.e., tension, compression, or torsion. The cracking is measured in terms of initiation time to nucleate a crack or time to failure. In a simple uniaxial loading under tension or compression, it is observed that the initiation time can vary in orders of magnitude depending on the alloy and the environment. Fracture can be intergranular or transgranular or mixed mode. Factors that affect SCC are solubility of the metal into surrounding chemical solution, and diffusion rate (like hydrogen into a tensile region) of an aggressive element into the metal and liquid metallic elements in the grain boundaries. Strain hardening exponent that affects the local internal stresses and their gradients can affect the diffusion kinetics. We examine two environments (Ga and 3.5 pct NaCl) for the same alloy 7075-T651, under constant uniaxial tension and compression load. These two cases provide us application to two different governing mechanisms namely liquid metal embrittlement (7075-Ga) and hydrogen-assisted cracking (7075-NaCl). We note that, in spite of the differences in their mechanisms, both systems show similar behavior in the applied K vs crack initiation time plots. One common theme among them is the transport mechanism of a solute element to a tensile-stress region to initiate fracture.     

Stress Dilatancy and Fabric Dependencies on Sand Behavior

A stress dilatancy model with embedded microstructural information, originally developed by the writers, is used to illustrate the pivotal importance of dilatancy and fabric on the behavior of sand. Fabric, as a second-order tensor, enters into the stress dilatancy equation obtained from a microscopic analysis of an ensemble of rigid particles. Model simulations of sand behavior are carried out in triaxial stress conditions along strain paths with varying degrees of controlled dilation (or compaction) including isochoric deformations as a particular case. Under particular strain paths and fabric conditions, it is shown that a relatively dense sand can succumb to instability or liquefaction under other than isochoric (undrained) conditions. This phenomenon is in accord with laboratory experiments in which dilation or compaction is controlled by modulating the amount of water flowing in or out of a sand specimen during shearing. Mixed drained–undrained loading paths are also simulated with particular reference to fabric dependence at a fixed void ratio. Model simulations capture most of the observed characteristics of sand response, such as instability and asymptotic behavior under various conditions.     

Effect of Stress State on Fracture Features

Present article comprehensively explores the influence of specimen thickness on the quantitative estimates of different ductile fractographic features in two dimensions, correlating tensile properties of a reactor pressure vessel steel tested under ambient temperature where the initial crystallographic texture, inclusion content, and their distribution are kept unaltered. It has been investigated that the changes in tensile fracture morphology of these steels are directly attributable to the resulting stress–state history under tension for given specimen dimensions.