Q235覆管对AA5052铝合金基管颗粒介质胀形行为的影响

采用AA5052铝合金挤压管作内层基管、Q235碳素结构钢卷焊管作外层覆管的钢铝复合管对复合管颗粒介质胀形行为进行研究。通过塑性理论分析胀形过程中管间切向摩擦力及法向压力对基管应力大小的影响;利用数值模拟分析管间摩擦因数和覆管各向异性对基管的应变成形极限的综合影响,并给出单管、复合管胀形时的壁厚减薄情况和基管的应力、应变分布;通过管材颗粒介质内高压胀形试验,对比单管和复合管胀形条件下铝合金管的极限胀形比,分析复合管的变形协调性。结果表明：通过施加Q235碳素结构钢覆管,减小了AA5052基管胀形区中间截面处的双向拉应力,基管胀形区壁厚减薄变小,胀形比提高了22%,复合管下基管最大减薄率为17.5 %,成形性能显著提高。     

A prediction of bursting failure in tube hydroforming processes based on ductile fracture criterion

Bursting is an irrecoverable failure mode in tube hydroforming, in contrast with buckling and wrinkling. To predict bursting failure in the hydroforming processes, Oyane's ductile fracture criterion is introduced and evaluated from the results of stress and strain productions obtained from finite element analysis. The region of fracture initiation and the bursting pressures are predicted and compared with a series of experimental results. It is shown that the material parameters used in the criterion can be obtained from the forming limit diagram. From the simulation results of tube bulging, the prediction of the bursting failure based on the ductile fracture criterion was demonstrated to be reasonable. This approach can be extended to a wide range of practical tube hydroforming processes.     

Plastic buckling of tubes under axial compression and internal pressure

The plastic buckling and collapse of long cylinders under combined internal pressure and axial compression was investigated through a combination of experiments and analysis. Stainless-steel cylinders with diameter-to-thickness values of 28.3 and 39.8 were compressed to failure at fixed values of internal pressure up to values 75% of the yield pressure. The first effect of internal pressure is a lowering of the axial stress–strain response. In addition, at some plastic strain level, the cylinder develops uniform axisymmetric wrinkling. Under continued compression, the wrinkles grow stably, gradually reducing the axial rigidity of the structure and eventually lead to a limit load instability. All pressurized cylinders remained axisymmetric until the end of the test past the limit load.The critical stress and wavelength were established using classical plastic bifurcation theory based on the deformation theory of plasticity. The evolution of wrinkling, and the resultant limit state, were established by modeling a periodic domain that is one half of the critical wavelength long. The domain was assigned an initial imperfection corresponding to the axisymmetric buckling mode calculated through the bifurcation check. The inelastic material behavior was modeled through the flow theory of plasticity with isotropic hardening. The variations of the axial response and of the limit strain with pressure observed in the experiments were reproduced well by the model. Inclusion of Hill-type anisotropic yielding in all constitutive models was required for good agreement between predictions and experiments.     

多场交互作用下镁合金塑性变形研究

采用压缩圆柱形镁合金铸锭的试验方法，研究了不同变形温度和不同应变速率对AM60B铸态镁合金塑性流动应力的影响规律。采用电子显微镜观察了压缩试样微观组织，结果显示，镁合金在高温下塑性变形以基滑移为主，并出现位错攀移，在临近晶界处有明显的交滑移产生。采用数理统计的方法建立了变形温度为573-673K、应变速率为0．01～0．1s^-1条件下的AM60B铸态镁合金塑性变形本构模型，试验表明所建立的本构模型能充分反映AM60B铸态镁合金的塑性变形规律。     

Plastic buckling of conical shells under axial compression

Analysis and experimental results are presented on the plastic axisymmetric buckling of steep, truncated conical shells under axial compression. Specimens of 6061-T6 aluminum and type 416 stainless steel were tested; in spite of the considerable difference in the stress-strain curves for the two materials, the buckling modes observed in the experiments for geometrically identical cones were the same. Perturbation analysis, which takes account of the continuous change in direction of the plastic strain rate vector during buckling, is found to describe the essential features of the observed buckling deformation.     

An experimental and numerical study of necking initiation in aluminium alloy tubes during hydroforming

In this paper, a combined experimental and numerical investigation of free hydroforming of aluminium alloy tubes is conducted. The tubes are subjected to different loading histories involving axial compression and internal pressure. The circumferential and axial strains experienced by the tubes are continuously recorded along with the pressure and axial load. The numerical simulations are carried out using both 2D axisymmetric and 3D finite-element formulations by applying the experimentally recorded axial load and internal pressure. In the latter, a geometric imperfection is introduced in the form of wall thickness reduction at the tube mid-length in order to trigger necking which happens after significant bulging and beyond the stage of peak pressure. The strain histories and peak pressures obtained from the simulations agree well with those determined from the experiments. Further, the forming limit curve predicted by the simulations as well as from a M–K analysis incorporating the computed strain paths corroborate well with the experimental data. The role of nonproportional straining on the mechanics of failure of the tubes due to bulging and necking is studied in detail.     

Axial crushing of square tubes

The mechanics of the large deformation of a square tube under axial load is discussed. Theoretical results are substantiated by experimental results. The elastic buckling load is predicted by assuming the tube to be comprised of four plates simply supported at their edges. The highest load the tube can sustain is predicted by allowing for the development of plasticity near the corners. The mean crushing load is predicted from an incremental plasticity analysis which allows for travelling plastic hinges. Comparison with circular tube behaviour is considered and an attempt to explain some of the peculiarities is made.     

Finite-element simulation of the lateral compression of aluminium tube between rigid plates

The lateral compression of aluminium and clad tubes owing to a large deformation is examined by an incremental elasto-plastic finite-element method based on an updated Lagrangian formulation in which a sliding-sticking friction mode is specially considered. It is mainly expected to predict the buckling process and load–deflection curves for energy dissipation capacity during the design stage, before trials. The high non-linearity of the process due to geometric changes, the inelastic constitutive behavior, and the deformation-dependent boundary conditions are taken into account in an incremental manner. A static explicit approach to the solution is applied, tangent stiffness matrix equation is solved without iteration and the r_min technique is employed to limit the step size to linear relation. The simulated load–deflection curve agrees with a published experimental result. The predicted geometries of the compressed tube clearly demonstrate the processes of the formation of buckling until unloading. The effects of various parameters of the process, such as elastic modulus, strain hardening exponent, tube thickness, friction coefficient and configurations of the clad tube, on the occurrence of buckling of tube are discussed and interpreted in simulation. The present work may be expected to improve the understanding of the buckling mechanism of lateral compression.     

Hydroforming of aluminum extrusion tubes for automotive applications. Part I: buckling, wrinkling and bursting analyses of aluminum tubes

Three possible failure modes have been identified in tube hydroforming: buckling, wrinkling and bursting. A general theoretical framework is proposed for analyzing these failure modes as an elastoplastic bifurcation problem. This framework enables advanced yield criteria and various strain-hardening laws to be readily incorporated into the analysis. The effect of plastic deformation on the geometric instability in tube hydroforming, such as global buckling, axisymmetric wrinkling and asymmetric wrinkling, is precisely treated by using the exact plane stress moduli tensor. A mathematical formulation for predicting the localized condition for bursting failure is established herein. Furthermore, the critical conditions governing the onset of buckling, axisymmetric wrinkling and asymmetric wrinkling are derived in closed-form expressions for the critical axial compressive stresses. Closed-form solutions for the critical stress are developed based on Neale–Hutchinson's constitutive equation and an assumed deformation theory of plasticity. It is demonstrated that the onset of asymmetric wrinkling always requires a higher critical axial compressive stress than the axisymmetric one under the context of tube hydroforming with applied internal pressure and hence the asymmetric wrinkling mode can be excluded in the analysis of tube hydroforming. Parametric studies show that buckling and axisymmetric wrinkling are strongly dependent on geometric parameters such as t₀/r₀ and r₀/ℓ₀, and that axisymmetric wrinkling is the predominant mode for short tubes while global buckling occurs for long slender tubes.     

Plastic instability in dual-pressure tube-hydroforming process

The tube-hydroforming process has become an indispensable manufacturing technique in recent years. Successful tube hydroforming requires bulging to take place without causing any type of instability such as bursting, wrinkling or buckling. The dual-pressure tube-hydroforming process was introduced to achieve a favorable tri-axial stress state in the deformation process. In this paper, the effect of applying counter pressure on plastic instability of thin-walled tubes is analyzed. It is concluded that in dual-pressure tube hydroforming, the onset of plastic instability is delayed and the ductility of the metal is increased.     

Quasi-static compression of electric resistance welded mild steel tubes with axial gradient-distributed microstructures

This paper presents the deformation behavior and crashworthiness of electric resistance welded mild steel tubes with axial gradient microstructures in quasi-static compression. Three sets of tubes were prepared, and regions of each tube were Induction heated and directly quenched (IH-DQ). The effect of the length to diameter (L/D) ratio, and length of the IH-DQ region on crushing characteristics was investigated, and compared with untreated tubes. The compression tests revealed that improved energy absorption can be obtained in IHDQ tubes if the collapse is controlled by the formation of a concertina buckling mode. However, there was a tendency to produce mixed or Euler buckling modes as the ratio of L/D increased. Meanwhile, the results of the crush experiments and the FEM models showed that the heat-treatment process should be precisely controlled to produce the correct type of microstructure, and circumferential uniformity of microstructure distribution.     

Axial crushing of wood-filled square metal tubes

The behaviour of thin tubes made of sheet metal and not so thin extruded tubes filled with wood and subjected to axial crushing is studied. Experiments show that the mode of elastic buckling is changed by the presence of the wood filler. The plastic crushing of thin tubes resulted in Euler-type buckling, while a considerable enhancement in the load carrying capacity and energy absorption was seen in the case of thicker walled tubes which were examined both under quasi-static and dynamic loading conditions. A new idealized deformation mechanism for the progressive crushing of the wood-filled tube is suggested and analysed. The results obtained for the mean loads agree reasonably with experimental observations. An alternative method employed to predict the mean crushing load of empty tubes and using a thickness that provides an equivalent stiffness to that of a filled tube is also seen to produce reasonable agreement with the experiment.     

Post-buckling and delamination propagation behavior of composite laminates with embedded delamination

Delamination occurred due to poor manufacturing process or in-service actions significantly affects the mechanical and failure behavior of laminated composite structures. In this study, the buckling and post-buckling delamination behavior of laminated composite with an embedded initial delamination under in-plane compression was studied experimentally and numerically. First, compression tests for laminated composite specimens with embeded initial delamination were performed and the buckling and delamination responses were obtained. Then the experimental test was numerically simulated using finite element methods with the progressive failure accounted for by using cohesive zone modeling. The load-displacement curve, strain behavior and delamination shapes of experimental specimens obtained from load cells, strain gages installed at different locations, and C scan images, respectively, were compared with the FEM results, and good agreements were attained. The effect of the buckling modes, laminate stacking sequence and shape of initial delamination on the buckling load and propagation behavior was studied by considering different ply stacking and shapes of initial delaminations. It was found that the buckling mode determined the growth direction of the delamination propagation, and the stacking sequence influenced the extent of the propagation area, while the orientation of the delamination affected the buckling loads.     

Reproducing hoop stress–strain behavior for tubular material using lateral compression test

Identification of material properties in the hoop direction, such as stress–strain behavior, is essential in tube hydroforming processes. Conventional tests such as uniaxial tension and compression tests have some drawbacks and limitations. In the current investigations a simple technique to identify the stress–strain behavior in the hoop direction for tubular material is introduced, based on the experimental data obtained from tube lateral compression test. In the proposed technique, an assumed stress–strain curve is used in finite element simulation to predict the load deflection curve of the tube lateral compression. An iterative algorithm is used to compare the calculated and experimental load deflection curves until a good agreement with a percentage deviation less than 4% is obtained. The suggested technique was used to obtain the material properties of Cu–40%Zn brass tube. The predicted stress–strain curve was compared with that obtained from uniaxial compression test. Comparison between the experimental and predicted stress–strain curve showed that the proposed technique is effective in the prediction of the material properties from the tube lateral compression test with percentage deviation less than 1%.     

加载路径对AA5083管件热态非金属颗粒介质成形性能的影响

将颗粒介质作为传力介质,应用于铝合金管件内高压热成形工艺。通过热单向拉伸试验建立AA5083板材的本构模型。通过管材热态颗粒介质胀形数值模拟,结合AA5083理论成形极限图的分析,研究了不同加载路径对管件壁厚分布、管端缩料量和主应变曲线的影响规律,并进行了相应的工艺试验验证。研究结果表明,合理匹配初始压头力和管端进给量参数,使预成形管坯在胀形区形成有益皱纹,可为胀形区管坯变形提供聚料作用,从而提高管件成形质量和胀形极限。     

Numerical simulation of rotational symmetric tube bulging with inside pressure and axial compression

The process of rotational symmetric tube bulging with inside pressure and axial compression enables the standard tubes to be formed into different rotational symmetric hollow parts in such a way that their central part is expanded into a desired shape while the ends remain unchanged. The superposition of axial compression contributes to a more favorable forming stress state, which is reflected in larger forming limits and smaller wall thinning in the widened area. The problems characterizing the process are a limited range of compression stability and difficulties met when establishing and optimizing the technological parameters of the process whose course cannot be defined in an analytical way. Based on a physical model of the forming process a numerical model was built. Using ABAQUS code the model was simulated over the entire stress/forming region. The comparison of the computer simulated forming process with the experimentally obtained results showed that the model was highly accurate. Finally, the paper studies the influences of particular parameters on the stability of the process, showing on a practical example how it is possible to achieve tube bulging without wall thinning.     

BT25钛合金动态再结晶行为的元胞自动机模拟

为研究热加工工艺参数对钛合金塑性成形过程中微观组织的影响,利用Gleeble-3500型热模拟试验机对BT25钛合金进行单道次等温恒应变压缩试验。分析真应力-应变曲线,建立JMAK动态再结晶动力学方程;通过对热变形行为的分析,推导出钛合金的位错密度模型、再结晶形核和晶粒长大模型;结合元胞自动机的算法,建立元胞自动机(Cellular automata, CA)模型并利用该模型模拟和验证了BT25钛合金热变形过程中动态再结晶行为。结果表明,BT25钛合金的流动应力对应变速率和变形温度非常敏感;提高变形温度或降低应变速率均有利于材料发生动态再结晶;CA模型模拟晶粒尺寸误差约为3%,预测DRX体积分数误差在10%以内。该模型具有良好的预测精度,为合金材料在塑性加工过程中优化工艺参数和控制锻件微观组织演变提供了可靠性依据。     

铝合金搅拌摩擦焊接加筋板剪切稳定性能研究

对2024-T3铝合金搅拌摩擦焊接加筋壁板的剪切稳定性能进行试验研究,得到该结构的剪切失稳形式及屈曲载荷、承载能力及破坏形式。应用有限元法对该焊接加筋壁板结构进行简化建模,对该结构的稳定性和承载能力进行计算,将计算结果与试验结果进行比较。试验及有限元计算结果表明,结构的剪切失稳形式表现为筋条间平板的局部屈曲;屈曲后结构进入张力场受力状态,破坏形式主要表现为平板的塑性变形和边缘撕裂、筋条的弯扭变形以及焊接区的局部脱焊;出现脱焊现象的试验件其承载能力较未出现脱焊现象的试验件下降7.7%;线性和非线性屈曲计算所得屈曲载荷分别比试验平均值高出18.4%和26.2%,而非线性计算所得承载能力比试验平均值高出5.7%,焊接引起的初始缺陷对结构承载能力的影响小于对屈曲载荷的影响;有限元分析得到的结构屈曲形式和失效形式与试验现象吻合,验证了有限元模型的合理性,但其仍需要进一步改进以考虑初始缺陷来减小计算误差。     

基于三维成形极限应力图的AA6061挤压管材成形性能分析

为解决AA6061挤压管材成形性能极差的问题,建立了“固溶水淬+颗粒介质胀形+人工时效”的铝合金管件成形工艺流程。研究了固溶水淬和人工时效工艺参数对材料性能影响规律,并建立了考虑厚向应力的三维成形极限应力图;建立了管件颗粒介质胀形有限元仿真模型,分析管件变形特征质点的运动轨迹和应力应变状态,并应用理论成形极限图对管件破裂失稳点和胀形极限进行分析和预判。四方截面管件胀形工艺试验结果表明,颗粒介质胀形工艺与合适的热处理工艺相结合能够有效地解决AA6061挤压管材的成形问题;考虑厚向应力的三维成形极限应力图可作为铝合金管件胀形工艺方案制定的破裂失稳判据。     

Fretting Wear Behavior and Mechanism of Inconel 690 Alloy Related to the Displacement Amplitude

Fretting wear tests on Inconel 690 alloy were carried out at different displacement amplitudes. The results indicated that with an increase in displacement amplitude, the ratio of tangential force to normal load and wear volume increased. The fretting mode gradually transformed from mostly stick, mixed stick–slip, to full sliding, showing the competition of fretting-induced fatigue cracking and fretting-induced wear. There was a gradient plastic strain created by fretting, which resulted in the formation of a nanocrystalline tribologically transformed structure (TTS) and plastic deformation layers. The plastic strain in the plastic deformation layer gradually increased with an increase in displacement amplitude.