This paper provides several simple but significant formulas that shed light on the connection between the derived transverse stress and some special phenomena, including the premature fracture and largely slant fracture line, occurring during the uniaxial tensile test of sheet metals. Studying two kinds of nearly isotropic materials, theory and simulation analysis found that a non-null transverse stress (σ2) was derived on the tensile specimen after the onset of necking, which grew from 0 and was always less than half of the corresponding longitudinal principal stress. The derived transverse stress was believed to be a primary factor for hindering the development of the neck, promoting the reduction of the thickness, and determining the orientation of the fracture angle. Furthermore, a special necking phenomenon, i.e., crossed necking, was observed during the transition period of the instability in the test and was studied in this paper.

Pipelines and piping components may be exposed to extreme loading conditions, for instance earthquakes and hurricanes. In such conditions, they undergo severe plastic strains, which may locally reach the fracture limits due to either monotonic loading or ultra-low cycle fatigue (ULCF). Aiming to investigate the failure process and strain evolution of pipes enduring ULCF, a lab-scale ULCF test on an X65 steel pipeline component is simulated with finite element models, and experimental data are used to validate various material modeling assumptions. The paper focuses on plastic material modeling and compares different models for plastic anisotropy in combination with various hardening models, including isotropic, linear kinematic and combined hardening models. Both isotropic and anisotropic assumptions for plastic yielding are considered. As pipes pose difficulty for the measurement of plastic properties in mechanical testing, we calibrate an anisotropic yield locus using advanced multi-scale simulation based on texture measurements. Moreover, the importance of the anisotropy gradient across thickness is studied in detail for this thick-walled pipeline steel. It is found that the usage of a combined hardening model is essential to accurately predict the number of the cycles until failure, as well as the strain evolution during the fatigue test. The advanced hardening modeling featuring kinematic hardening has a substantially higher impact on result accuracy compared to the yield locus assumption for the studied ULCF test. Cyclic tension-compression testing is conducted to calibrate the kinematic hardening models. Additionally, plastic anisotropy and its gradient across the thickness play a notable, yet secondary role. Based on this research, it is advised to focus on improvements in strain hardening characteristics in future developments of pipeline steel with enhanced earthquake resistance.

The time-dependent springback and anelastic behavior of TC4 titanium alloy were investigated as functions of time, prescribed strain and loading strain rate by universal testing machine. In order to infer its basis, the microstructures of samples under different loading conditions were examined by optical microscope (OM) and transmission electron microscope (TEM). The results reveal that the absolute values of time-dependent springback strain (TDSS) accumulating in 10 h both present exponential growth, as the prescribed strain and loading strain rate rise. The OM and TEM results show that the evolutions of "streamline" microstructure, dislocation pile-up, and deformation twin have positive correlation with the change law of TDSS.

Abstract: In order to characterize the deformation and true stress–strain relation of metal tubes, the geometry of tube walls after necking in uniaxial tension need to be determined. The paper investigated the necking process of metal tube. A large number of tensile tests and finite element analysis of 1Cr18Ni9Ti tubes with different sizes were conducted. It was found that the geometry of outer tube wall in the necking region can be described using a logistic regression model. The final geometry of the tube is determined by original tube diameter and wall thickness. The offset of tube walls are affected by two competing factors: volume constancy and necking. The offset distances of outer and inner walls are mainly affected by original wall thickness. The length of the necking zone is more influenced by original tube diameter. Tube elongation at fracture increases slightly as tube diameter gets larger, while the wall thickness has almost no impact on the elongation.

目的 探究DP600双相钢单向拉伸时交叉颈缩现象的产生原因.方法 在单轴拉伸实验的基础上,利用ABAQUS有限元软件对拉伸过程进行模拟,对交叉颈缩区域进行应力、应变分析.结果 即将发生断裂时刻,试样标距内中心位置应力达到最大值,临近中心位置两侧由于"局部卸载"而出现应力大幅减小的情况.结论 试样标距内两侧距离中心越远的位置,越接近于单轴应力状态.DP600板材裂纹萌生时,与断裂发生部位越接近,其应力状态与平面应变状态越相似,沿着颈缩线方向基本没有应变产生,材料主要由颈缩线区域的板材厚向减薄颈缩线加宽补偿垂直于颈缩线方向上的拉伸.

In order to reveal the effect of pre-strain on the time-dependent springback, the TA2 titanium sheets were drawn under different pre-strains by the universal testing machine. The microstructures of the tested samples were both characterized by optical microscope and transmission electron microscope to infer the basis of the anelastic behavior. The results show that the time-dependent response of TA2 is obvious. The time-dependent-springback strain (TDSS) presents different exponential change laws in different plastic deformation zones. Both the TDSS and the amount of deformation twin increase as the pre-strain goes up. The loading history and the interaction between dislocation and twin affect the TDSS and the relaxation-saturation time t(IP).

目的 研究直径Φ20 mm、壁厚2 mm的5A06铝合金管在单向拉伸试验下的表现以及其在外支承球形缩口成形中的变形特点.方法 利用管材准静态单向拉伸试验测试其力学性能,获得真实应力-应变曲线,再结合有限元软件ABAQUS分析管缩口成形的应力应变特征.结果 5A06管在拉伸中表现出良好的应变硬化能力,并呈明显的锯齿波动屈服特性.模拟结果显示缩口中的管坯整体处于周向、轴向压应力状态;主变形区内,轴向应变从缩口口边的拉应变逐渐变为主变形区向直壁过渡区域的压应变;除主变形区外,直壁部分的壁厚也有所增加.Φ20 mm×2 mm的5A06管在模拟中的极限缩口系数约为0.410.结论 Φ20 mm×2 mm的5A06管在外支承球形缩口中的模拟成形极限较高,有限元分析作为缩口成形模具和工艺设计的前一步工作,其结果有一定的参考价值.

在对所研究材料双相钢DP600的力学性能测试基础上,完成双相钢的非回转对称性拉深的有限元模拟,对非回转对称性盒3条特征曲线上的厚度分布、应力应变状态三方面进行分析.详细解释断裂点的产生原因及断裂本质,可为预测类似形状零件的拉深缺陷进而提高成形极限提供一定的参考.

The instability and fracture process during uniaxial tension was observed from load-stop tensile tests and finite element simulation. The results indicate that at the end of instability, the direction of the maximum principle stress near the necking groove turns to being perpendicular to the groove. This tensile stress is critical to the growth of fracture. The fracture initiates from the internal of the sheet at the center of volume where the two local necking grooves intersect. Material here is under triaxial tensile stress state and the principle stresses in all three directions are the largest. Once the initial crack occurs, it propagates along the zero-strain-rate necking groove. Moreover, the final fracture angle between the fracture plane and the tensile axis is always larger than theoretical value. An important reason is the ignorance of the triaxial stress state evolution during instability in theoretical calculation.

ABSTRACT:In order to investigate the stress and strain distribution and shape of the fracture in the instability process of the pipes under uniaxial tension load,finite element analysis for whole process of uniaxial tension for the tube was carried out based on a series of experiments on axial tensile of 1Cr18Ni9Ti tube.During the instability process of the tube,three principal stress close to the inner tube wall were greater than the corresponding principal stress of the outer wall. Initial crack firstly occurred in the area which was close to the inner tube wall.The fracture of tube under uniaxial tension has a tendency to shear fracture,which forms a degree of plus or minus 45 ° between fracture section and tensile axis,and crack forming close to the inner tube wall gradually propagates to the outer tube wall,so the final crack lines are often not in the same plane.

To initially explore the time-dependent springback change law and reason of the blank at temperature. Different bending angels of three-point bending tests were taken of high strength steel sheet DP600,then the instantaneous springback angels and time-dependent springback angels for up to a mouth were measured at room temperature,meanwhile the finite element analogy analysis was proceeded with creep model of ABAQUS software. Time-dependent springback angels changed in a exponential growth law over time and finally trended to be stable. The correction coefficient k was introduced to the original creep model,the calculation result of the update model has good agreement with the experimental values. Stress distribution in the plate gradually flattened out. The release of residual stress is one kind of causes of the time-dependent springback.

DP600 steel sheet with high strength has drawn much attention in the automotive industry, but the shape change following forming and unloading has not been known widely. The time-dependent springback of DP600 steel sheet was investigated under different pre-strains by uniaxial tension. According to the viscous behaviors under the elastic and plastic loading tests, the lower limit of integration in the constitutive equation of linear viscoelasticity was modified and the creep compliance was gained from the creep curve at a constant stress level of 309 MPa at room temperature. The predicted curve was acquired by using the superposition of the unloading impulse and the historical loading curve. The results reveal that the strain rates with high initial values gradually decreased following unloading at room temperature. As the pre-strain went up, both the absolute anelastic strain of time-dependent springback and the anelastic proportion of the total springback increased. Meanwhile, the direction of the time-dependent springback was same as that of the initial springback and opposite to the loading direction. In the same springback period, the ratio of the unloading stress to the creep springback strain tended to vary more linearly with the pre-strain than those obtained from immediate unloading. The simulated results using the revised model are in good agreement with the experimental data.

The time-dependent springback and anelastic behavior of AC170PX aluminum alloy were investigated as a function of time, prescribed strains and loading strain rates by uniaxial tensile tests. In order to quantify the effect and infer its basis, the X-ray diffraction tests for the lattice-strain recovery were conducted. The measured anelastic strain accumulated in 10h increased, as the prescribed strain went up. A non-monotonic behavior, showing a maximum anelastic recoverable strain at an intermediate strain rate followed by onset of a decreasing trend, is observed. The XRD results show the changes of different interplanar spacings in the time-dependent response do not follow the elastic law. There exists an anisotropy in recovery behavior with respect to lattice plane.

ABSTRACT:The aim of this work was to study the drawing forming property of rectangular case of St16 steel blank. Based on a series of quasi-static uniaxial tensile test of St16 steel plates, eta/DUNAFORM was used to analyze the influ-ence of two kinds of process programs on the drawing forming property of rectangular case, including segmented blank hold-er with variable force control technology and optimizing blank shape. Both methods could improve the limit drawing depth of rectangular case, moreover, the effect of optimizing blank shape was better. Integrated application of these two programs could greatly improve the drawing property of rectangular case. In conclusion, the drawing forming property of rectangular case could be greatly enhanced by ameliorating the flange flow condition, providing reference for determination of the form-ing process as well as mould designing and manufacturing.

Effects of stress level,loading velocity and loading process on room temperature creep properties of 1Cr18Ni9Ti steel tube and influences of room temperature creep and loading process on yield stress were studied by using microcomputer control electric tensile tester.The results show that room temperature creep strain of the steel tube increased with the increase of stress level and loading velocity.The loading process could decrease the secendary room temperature strain .The density of the fixed dislocation increased after different loading processes were carried on the steel tube,and the subsquent secondary tensile process could raise the yield strength of the steel tube.

To investigate the time-dependent springbackof 1Cr18Ni9Ti stainless steel tubes under tension and rotary bending, and to predict it by FEM. The two-layer viscoplasticity model whichis available in the finite-element analysis soft-ware ABAQUS is applied. Parameters of this model are obtained based on the results of a series of uniaxial tension and stress relaxation tests carried out at different strain rates. The time-dependent springbackquantitiesof tubes are simulated by ABAQUS based on thiselasto-viscoplasticity model. It′s revealed that simulated time-dependentspringbacklaw and quantities are close to the experimental data. Basedon thiselasto-viscoplasticity material model, it seems that the reverse residual stress appeared in the elastic-plastic network and visco-elastic network caused the time-dependent springback. Thenumericalre-sults are in good agreement with experimental results with this model by taking account of the viscous behaviors in the form-ing process.

目的 研究汽车车身用5B003A板的成形性能.方法 在进行单轴拉伸试验的基础上,利用软件eta/DYNAFORM模拟了板成形极限曲线、破裂点应变路径、圆筒拉深过程,并进行了相应变形参数的优化.结果 5B003A板单向拉伸和有限元模拟均出现“交叉颈缩”现象,并且在与轧制呈45°角方向上的冲压性能优于0°和90°方向;5B003A板成形极限破裂点的应变路径漂移倾向较明显,双拉区中均呈ε2=const的应变状态;在与拱顶高实验相近变形条件下,优化得到最佳凸模圆角半径为20 mm时,与极限拉深系数0.43对应的无凸缘拉深的最大板坯尺寸为233 mm,相应最佳压料力为68 kN.结论 5B003A板在45°方向上有较好的冲压性能,且凸模圆角半径、板坯直径、压料力等工艺参数对其拉深成形性能影响较大.

在金属滞后回弹研究中,对DP600、BUFD及AC600汽车用板材施加相同应变5％,在室温下测试卸载后瞬时回弹及32 h内滞后回弹.基于一维线粘弹性与粘塑性理论引入谐振子模型,构建DP600的滞后回弹本构模型.试验及拟合结果表明,DP600、BUFD及AC600板材的瞬时回弹与弹性模量及卸载应力有关;而滞后回弹在回复前期均具有较高回复速率,方向与瞬时回弹相同,采用粘弹性理论的拟合优度较好.分析卸载2h后的回弹规律,AC600板材没有明显反回弹趋势,而DP600、BUFD钢板变化规律不再单调,在较长时间跨度内呈现一定波动性.

According to the working way of the numerically controlled (NC) tube bender and taking the additional tensile force into account, the formulas are derived to calculate the average principal stress in different directions of the bending tube surface, the equivalent stress, the variation of wall thickness, and the ratio of the minor axis to the original radius of outer contour and validated by the experiments. The corresponding experiments and simulation analysis are performed to compare with the calculated results, and it is revealed that the equivalent stress of the bending tube surface is non-uniform and the maximum equivalent stress is localized in smaller deformation field around the bending terminating end. The consideration of the additional tensile force makes the equivalent stress in the outer convex portion of tube remarkably greater than that in the inner concave portion of tube, accelerating the thinning of the tube’s wall and the ovalization of tube’s outer contour. The approximate calculation formulae of additional tensile force is also derived and based on the deformation analysis; the proper adjustment of the additional tensile force could improve the quality of the NC bending tube.

Objective To investigate the mechanical behavior,the buckling form and influencing factors of the pipes under axially compressive load. Methods Based on a series of experiments on quasi- static tensile and compression test of 5A03 aluminum alloy tube,finite element analysis of axial compression for this tube was carried out. Results The results showed that there were differences between tensile and compressive mechanical properties of 5A03 tube,such as tendency of tensile- compressive bi- modulous,the tensile yield strength was slightly higher than the compressive yield strength, the tensile strength was slightly lower than the compressive strength,and both the tensile and compression tests exhibited certain yield characteristics of saw tooth wave. Height- diameter ratio affected the form of compression instability: when the ratio was lower than one,the test specimen developed into a figure of single bulge,while it turned into a figure of double bulge as the ratio became larger than one. The bulge preferred to occur at the end where the friction was larger. Conclusion The tube exhibited a certain regular non- axisymmetric buckling crushing after compression instability.