由拉伸试样确定材料真实应力应变关系

提出一种借助有限元分析由光滑和切口圆棒试验数据确定应力应变关系的方法，首先分析光谱和切口圆棒，得到其各自的应力应变分布，研究在颈部应力应变随变形位移和材料的分布变化，得到其间关系的曲线图，同时对圆棒颈缩变形进行模拟。对工程上常用的硬化指数差别较大的两种钢材制成的光滑试样及切口试样进行初步的计算与分析，对影响拉伸试样颈缩形成的因素进行研究，指出试样夹持端状态与外廓直径的细微减少及加载约束方式三者对颈缩有重要影响，对工程上常用的bridgeman估算公式进行分析，指出其在工程上的适用性。     

Finite-element simulation of hole-flanging process of circular sheets of anisotropic materials

A hole-flanging operation on a flat circular sheet with a hole in the center is simulated by an incremental elasto-plastic finite-element method, which incorporates strain-hardening and anisotropy in the direction normal to the sheet, with care taken to describe the boundary conditions of penetration, separation and the alternation of the sliding—sticking state of friction. The simulation clearly demonstrates the processes of generation of deformation shape until unloading. The calculated sheet geometries and the relationship of punch load to punch stroke are in good agreement with the experimental data.The stress at the hole periphery in the flange is assumed to a state of circumferential uniaxial tension, in order to simplify the fracture mode as a simple tension test. By making use of the instability of uniaxial tension, an approximate relationship to determine the onset of necking of the hole periphery in the hole-flanging process is derived and it is found to be influenced by the process geometry and the plastic properties of the material, such as the stress-concentration factor K, strain-hardening n and normal anisotropy R, and the estimated value, being obtained by the derived equation, agrees well with the experimental data.It is noted that the derived relationship for estimating the instability of the hole-flanging process can be combined into the developed finite-element model to simulate the critical condition of the limiting deformation of the hole-flanging process. This combined method could possibly be applied towards improving both the manufacturing process and the design of tools for the hole-flanging operation.     

Effect of baffles on orbital accelerations - Induced bubble oscillations in microgravity

The behavior of sloshing dynamics modulated fluid systems driven by the orbital accelerations including gravity gradient and jitter accelerations with partially-filled rotating fluids has been studied. Present study is applicable to a full-scale Gravity Probe-B Spacecraft dewar tank with and without baffle. Results of slosh wave excitation along the liquid—vapor interface induced by jitter acceleration-dominated orbital accelerations provide a torsional moment with an up-and-down movement of bubble oscillations in the rotating dewar. The results show rightward and leftward movement of bubble oscillations transverse to the rotating axis, and up-and-down movement of bubble oscillations longitudinal to the rotating axis of dewar container. The orbital accelerations also induce an eccentric contour of bubble oscillations in a horizontal r—θ plane. As viscous force between liquid—solid interface, and surface tension force between liquid—vapor—solid interface can greatly contribute to the damping effect of slosh wave excitation, the rotating dewar with baffle provides more areas of liquid—solid and liquid—vapor—solid interfaces than that of a rotating dewar without baffle. Results show that the damping effect provide by a baffle reduce the amplitude of slosh wave excitation and reduce the degree of asymmetry in liquid—vapor distribution. Computation of bubble (helium vapor) mass center fluctuations also verifies that a rotating dewar with baffle produces less fluctuations than that of a rotating dewar without baffle.     

Necking of Q&P steel during uniaxial tensile test with the aid of DIC technique

A lot of research has been focused on the necking process during the plastic deformation of sheet metals, but the localized necking is rarely distinguished form diffused necking by experiments, due to the limit of measurement equipment and method. Quenching and Partitioning (Q&P) steel is a 3rd generation advanced high strength steel (AHSS). Its good combination of high strength and ductility ensures potential application in automobile industry. Uniaxial tensile tests of QP980 steel sheet at five strain rates are performed to investigate the necking process and the effect of strain rate on necking behavior of Q&P steel. Digital image correlation (DIC) method is applied during tensile tests, and evolutions of major strain, minor strain and normal strain distributions along gauge section of the tensile specimens are obtained. The diffused and localized necking strains are determined according to SWIFT necking theory and HILL necking theory respectively. The test results indicate that with the increasing of strain rate in the investigated range, the diffused necking strain decreases from 0.152 to 0.120 and localized necking strain decreases from 0.245 to 0.137. Meanwhile, the difference of the two strains decreases form 0.096 to 0.017. Thus it can be concluded that strain rate has an influence on both necking strains during the deformation of QP980 steel sheet. Diffused and localized necking strains are determined by uniaxial tensile tests with the aid of DIC technique and the effect of strain rate on necking strains is evaluated.     

Combined gravity gradient and jitter accelerations acting on liquid-vapor interface oscillations in reduced gravity

The dynamical behavior of fluids affected by the asymmetric combined gravity gradient and jitter accelerations, in particular the effect of surface tension on partially-filled rotating fluids applicable to a full-scale Gravity Probe-B Spacecraft dewar tank, have been investigated. Three different cases of accelerations, one gravity gradient-dominated, one equally weighted between gravity gradient and jitter, and the others gravity jitter-dominated are studied. Results of slosh wave excitation along the liquid—vapor interface induced by gravity gradient-dominated acceleration indicate that the gravity gradient-dominated acceleration is equivalent to the combined effect of a twisting force and torsional moment acting on the spacecraft. Results of the slosh wave excitation along the liquid—vapor interface induced by gravity jitter-dominated acceleration indicate that the gravity jitter-dominated acceleration is equivalent to time-dependent oscillatory forces which push the bubble in the combined directions of down-and-up and sideward -and-middleward as the bubble is rotating with respect to rotating dewar axis. This study discloses the slosh wave excitation along the liquid—vapor interface driven by the combined effects of gravity gradient and jitter accelerations which are two major driving forces affecting the stability of the fluid system in microgravity.     

Characterization of the 3Dimension optical,geometrical, and mechanical profiles of iPP fiber with necking deformation

We present a method for evaluating the 3D refractive indices and 3D true stress and/or 3D true strain profiles of “isotactic polypropylene iPP” fibers during necking deformation. Observing the changes in geometrical shape during the deformation process is necessary to understand the mechanical performance of iPP fibers. 3D geometric shape profile and actual stress and strain profiles were measured for iPP fibers during the propagation of neck deformation. These measurements were performed with the aid of an in‐situ opto‐ mechanical device to dynamically characterize different properties of fibers at different strain rates. A software image analysis program was used to calculate the 3D opto‐mechanical properties of iPP fibers. The obtained results show that the used dynamic stretching device can be easily used to monitor the deformation process with high accuracy. The effective stress and strain can be determined from the filaments profile. For illustration microinterferograms are given.     

Modeling of anisotropic plastic behavior of ferritic stainless steel sheet

Modeling of anisotropic plastic behavior of ferritic stainless steel sheet (Type 409) was investigated using the three yield functions of Hill [A theory of the yielding and plastic flow of anisotropic metals. Proceedings of Royal Society of London, Series A 1948;193:281–97.], Barlat and Lian [Plastic behavior and stretchability of sheet metals. Part I: A yield function for orthotropic sheets under plane stress conditions. International Journal of Plasticity 1989;5:51–66] and Barlat et al. [Plane stress yield function for aluminum alloy sheet. Part I: Theory. International Journal of Plasticity 2003;19:1297–319.] (referred to as Yld2000-2d) criteria. Mechanical behaviors were characterized based on uniaxial tension, balanced biaxial bulge, and disk compression tests. Directionalities of yield stresses and r values were predicted from the three criteria and compared with experimental results. In order to verify the modeling accuracy of the three functions under complex loading conditions, cylindrical cup drawing and limiting dome height tests were carried out numerically and experimentally. It has been demonstrated that the result from Yld2000-2d criterion exhibits good agreement with experimental data. The effects of anisotropic hardening on earing and necking were also investigated based on the different levels of plastic work.     

Deformation by moving interfaces from single crystal experiments to the modeling of industrial SMA

Microstructure-based micromechanics models have to be developed in order to successively predict complex thermomechanical behaviors of shape-memory alloys (SMA). However, a proper implementation of the deformation mechanism—motion of internal interfaces in solid state—into the micromechanics models of SMA polycrystals is very important. In this work, experimental characteristics of the deformation mechanism surveyed from literature data obtained from uniaxial loadings of SMA single crystals are summarized. Original results of thermomechanical tension/compression loading experiments on CuAlNi and CuAlZnMn single crystals and CuAlZnMn polycrystals are also reported. Oriented single crystals may transform into different martensitic phases under various orientations of the uniaxial loading axes and sense of the applied stress. This phenomenon is called martensite formation anisotropy and possible consequences for thermomechanical behaviors of CuAlZnMn polycrystals are suggested.     

Axisymmetric deformation of circular elastic-plastic tubes under axial tension and internal pressure

The axisymmetric bifurcation and post-bifurcation behaviour of circular tubes subjected to the combined action of axial tension and internal pressure are investigated numerically using the finite element method. It is assumed that the tubes are made of elastic-plastic strain hardening material with a smooth yield surface and that they deform without shear stress at both ends under the proportional stress path based on the stress values averaged over the cross-section.The bifurcation point and associated mode shape are obtained for each stress path. The initial to medium nonuniform deformation was studied for several specific stress paths and the growth of necking of the tube wall and swelling of the middle plane due to bifurcation are clarified.     

Elastoplastic buckling of rectangular plates in biaxial compression/tension

This paper examines the elastoplastic buckling of a rectangular plate, with various boundary conditions, under uniform compression combined with uniform tension (or compression) in the perpendicular direction. The analysis is based on the standard linear buckling equations and material behaviour is modelled by the small strain J₂ flow and deformation theories of plasticity. A detailed parametric study has been made for Al 7075 T6 over a range of plate geometries (a/b=0.25–4,a/h≈20–100) and with three sets of boundary conditions (four simply supported boundaries and the symmetric combinations of clamped/simply supported sides). For sufficiently thin plates we recover with both theories the classical elastic results. However, for thicker plates there is a remarkable difference in the buckling loads predicted by these two theories. Apart from the expected observation that deformation theory gives lower critical stresses than those obtained from the flow theory, we report on the existence of an optimal loading path for the deformation theory model. Buckling loads attained along the optimal path—specified by particular compression/tension ratios—are the highest possible over the entire space of loading histories. By contrast, no similar optimum has been found with the flow theory. This discrepancy in the buckling behaviour, obtained from the two competing plastic theories, provides a possibly new illustration of the plastic buckling paradox.     

Modelling grain growth evolution and necking in superplastic blow-forming

A stability analysis is carried out to investigate necking in superplastic materials characterised by the sinh-law constitutive equation . The effects of load and the strain rate sensitivity parameter β on necking are quantitatively studied and a necking map is obtained for conditions of uniaxial loading. Finite element simulations of a superplastic blow-forming process are carried out in order to investigate both non-uniform thinning and grain size distribution which result from the use of the sinh-law constitutive equation. The pressure cycle required to ensure a target maximum strain rate is not exceeded in the material is obtained. The effects of strain rate and the magnitude of the parameter β on the grain size and through-thickness strain distributions for the formed part are investigated.     

Characterization and modeling of the mechanical behavior and formability of a 2008-T4 sheet sample

A 2008-T4 sheet sample has been characterized and its mechanical behavior and formability have been modeled. Uniaxial tensile and equal biaxial tensile stress-strain data, compressive yield strengths, crystallographic texture, earing and the forming limit curve were experimentally determined. Bulge test specimen shape and thickness profiles were also measured after various amounts of biaxial strain. A recently developed phenomenological constitutive model of anisotropic mechanical behavior was used to predict the directionality of strength, plastic strain ratio (R) and shear strain ratio (Г) values. In addition, this model was used to predict the forming limit curve for this sample. Predictions made with the recent model generally compare favorably with experimental results and with predictions made using the Taylor/Bishop and Hill theory. According to the data obtained in hydraulic bulge testing, the 2008-T4 exhibited apparent isotropic behavior. However, in cup drawing—another axisymmetric deformation mode—this material exhibited anisotropic behavior, as indicated by the formation of ears and troughs.     

The stability of plastic deformation in tension

Different concepts of the term “stability” are discussed and the resulting stability conditions in tension are derived. Particular importance is given to the relation between stability and the type of heterogeneities that may occur in tensile specimens. It is shown that a general instantaneous criterion of stability, in terms of the deformation properties of the material, cannot be formulated. However, if finite deformations are considered it is possible to derive general necessary criteria, which depend, of course, on the definition of stability adopted.The results obtained can explain the experimental observations concerning the phenomenon of necking in tensile tests, particularly its “weak” relation to the point of maximum load.     

Numerical studies on crack growth in a steel tubular T-joint

The results of a numerical investigation on fatigue crack growth of an offshore tubular T-joint under the action of axial, in-plane and out-of-plane bending loads are presented in this paper. Extensive stress analysis has been carried out to determine the location of the hot spot stress along the brace—chord intersection for each load case. Semi-elliptical cracks with varying crack lengths and crack depths were introduced at the hot spot location by means of line spring elements for stress intensity factor evaluation. The line springs were properly constrained to prevent the problem of crack surface penetration. The stress intensity factors obtained are then used in a crack growth law for life estimation.     

Study on true stress correction from tensile tests

Average true flow stress-logarithmic true strain curves can be usually obtained from a tensile test. After the onset of necking, the average true flow stress-logarithmic true strain data from a tensile specimen with round cross section should be modified by using the correction formula proposed by Bridgman. But there have been no firmly established correction formulae applicable to a specimen with rectangular cross section. In this paper, a new easy-to-use formula is presented based on parametric finite element simulations. The new formula requires only incremental plastic strain and hardening exponents of the material, which are simply presented from a tensile test. The newly proposed formula is verified with experimental data for high strength steel DH32 used in the shipbuilding and offshore industry and is proved to be effective during the diffuse necking regime.     

基于纵向模态超声导波陷频特性的钢绞线拉力测量新方法

试验发现了钢绞线结构中L(0,1)模态导波存在陷频这一固有声学特性,由此提出一种钢绞线拉力检测新方法。为测试钢绞线中导波陷频特性与拉力的关系,研制了一款柔性低频磁致伸缩传感器,可在30~160 k Hz范围内激励/接收L(0,1)模态导波。在直径17.8 mm和15.2 mm的两种7芯钢绞线上进行导波陷频检测试验,结果表明陷频特征频率与拉力的自然对数值呈良好线性关系,陷频旁侧峰值比随拉力增大而线性增加。试验中1 k N的拉力增量(应力增量约3 MPa)引起的声学参数(陷频特征频率和陷频旁侧峰值比)变化可被检测到并较好的符合标定关系方程。在额定工作载荷内,拉力估算相对误差小于1.5%,这表明新方法具有较高的拉力检测精度。同时,依据试验所得结论,给出了钢绞线结构中L(0,1)模态导波陷频中心频率与拉力关系的修正公式。新方法实施过程简洁,具有很好的工程应用前景。     

包含颈缩失稳的中厚钢板应力应变关系

为弥补传统引伸计在颈缩区域测量方面的不足,提出一种基于三维数字图像相关(3D-DIC)技术的截面分析方法。以轧制方向为0°的QStE700中厚钢板为研究对象,采用截面分析方法获得单向拉伸过程中截面几何形貌及面积,确定颈缩失稳发生时刻,并分析包含颈缩失稳的应力-应变演化过程及截面积变化规律。利用改进的Bridgeman方程对颈缩失稳后的真实应力-应变曲线进行校正,获得包含颈缩失稳的中厚钢板等效应力-应变曲线。分别以传统的幂指硬化曲线和改进的等效应力-应变曲线为本构模型,建立中厚钢板单向拉伸有限元仿真模型,获得截面积随时间变化曲线及载荷与位移关系曲线。结果表明,以改进的等效应力-应变曲线为本构模型的仿真结果与试验结果吻合较好,验证了基于3D-DIC技术的截面分析方法的正确性。此方法可用于描述包含颈缩失稳的整个拉伸过程。     

Entasis: Mechanics and illusion

After defining entasis—the swelling and necking of the stone columns which support the roof and entablature of ancient Greek stone temples—the reasons for its use are discussed. Historical mathematical-structural calculations pertaining to these quasi-cylindrical elements to account for their shape (due to Lagrange and others), are instanced, but do not encourage us to set useful store by them. General opinion about the best proportions for column surface curvature to give the greatest aesthetic architectural pleasure seems to be empirically founded and the product of little-understood psychological sense data processing.     

A taper tension profile maker in a converting machine

Winding is an integral operation in almost every web handling process, and a center-wound roll is one of the suitable and general schemes in a winding system. However, improper internal stresses within a center-wound roll can cause damage such as buckling, spoking, cinching, etc. Wound roll quality and performance are known to be related to the distribution of in-roll stresses. It is therefore necessary to analyze the relationship between taper tension in the winding section and internal stress distribution within the center-wound roll to prevent winding failure. But it is hard to compensate for an undesirable winding roll shape such as starring, buckling, and telescoping. This is because the winding section is the final process in a roll to roll system and has no feedback control system to correct winding roll shape directly. A time varying tension profile and accurate control of it in a winding section is one way to shape the fail-safe in-roll stress distribution of a winding roll. In this study, a new taper tension profile making method is aimed for designing high quality wound rolls. A new method to determine the proper taper tension profile was designed by analyzing the winding mechanism which includes the stress model in center-wound rolls, nip induced tension model, relationship between taper tension profile and telescoping, relationship between taper tension type and internal stress distribution. An auto taper tension profile making method was proposed not only to optimize radial stress distribution but also to minimize lateral error (telescoping). Simulation results show that the proposed method is very useful for determining the desirable taper tension profile during the winding process and preventing defects of winding roll shape such as telescoping, starting, and dishing and so on.