A DUCTILE FRACTURE CRITERION BASED ON THREE-DIMENSIONAL VOID MODEL

A fracture criterion derived from a microscopic point of view is proposed and has proved to be effective in the analysis of uniaxial tension. On the one hand, a method of predicting a ductile fracture is proposed using a three-dimensional void model and the assumption of velocity discontinuity. The relationship between the void volume fraction and the critical strain to fracture, calculated with the help of the new model, shows the same tendency as that obtained from the modified Thomason model. On the other hand, the mechanical and metallographic analyses of the uniaxial tension experiment are performed using four kinds of carbon steel. The relationship between the void volume fraction and the critical strain to fracture, calculated from the new model, agrees better with the result obtained from the experiment, rather than that calculated by the modified Thomason model, which confirms the validity of the ductile fracture criterion based on the three-dimensional void model.     

周期电磁载荷下Nb3Sn导体耦合损耗计算模型

在国际热核试验反应堆ITER上,已采用铌三锡(Nb3Sn)复合股线来满足导体遭受12T以上的磁场冲击,但Nb3Sn运行时的洛伦兹力会导致临界电流和耦合损耗时间常数的变化,但Nb3Sn基CICC (cable-in-conduit conductor)性能退化研究还有待加强,而且应变下接触特性对耦合损耗时间常数的影响也仍在探索中。为了快速精确计算耦合损耗,本文提出新的最小二乘法计算耦合损耗时间常数的方法,它采用绞缆序列比、电磁载荷应变作用下的接触电阻和空隙率等参数的线性关系来表示。在该模型中,不仅给出绞缆序列比的计算表达,而且也给出电磁载荷周期应变下接触电阻和空隙率的经验计算。与Gandalf及传统的数值计算相比,本文采用的绞缆序列比、接触电阻和空隙率组合计算耦合损耗误差较小,非常接近工程测试值。     

Cavitation behavior in superplastic 5083 Al alloy during multiaxial gas blow forming with lubrication

Despite its importance for industrial applications, the effect of lubrication on the cavitation behavior of superplastic materials has been given little attention. In this paper, a series of experiments were performed regarding bulging superplastic 5083 Al alloy sheet into dies with a cylindrical (cup) and rectangular (pan) die cavity for forming with and without lubrication, the formed parts were then evaluated to determine the effect of lubrication on the cavitation level evolution, thickness distribution, and void distribution. It was found that void shrinkage took place in the overlaid region for both forming with and without lubrication. The maximum void volume fraction could be effectively reduced for forming with lubrication; however, reductions in the maximum void volume fractions for cup forming were less significant than those for pan forming.     

Void Shape Effects on Void Growth and Slip Systems Activity in Single Crystals

建立了含椭球形微孔的三维体胞,该模型包含了椭球形微孔的一种特例:即球形微孔。采用晶体塑性滑移理论对不同取向下,单晶合金铸造微孔形状对微孔生长和滑移系激活的影响进行了研究。结果表明,材料的晶体坐标系、椭球微孔坐标系和载荷坐标系之间的坐标转换角度以及椭球微孔的形状对于微孔的演化具有非常重要的影响。对于三维应变状态下,椭球微孔的形状、晶体取向与载荷之间的相互关系共同决定了铸造微孔体积的增长、滑移系的激活和微孔旋转。当单胞滑移系的对称性被椭球型微孔破坏,即使载荷与滑移系统具有对称性,铸造微孔也会发生旋转。虽然单晶合金具有强烈的正交各向异性,但是当铸造微孔初始形状不为球形时,材料性能的正交各向异性对铸造微孔体积增长的影响被削弱。     

The characterization and interpretation of ductile fracture mechanisms in AL2024-T351 using X-ray and focused ion beam tomography

This paper describes an experimental investigation into the mechanism of ductile fracture in the aluminum alloy AL2024-T351 using a combination of synchrotron X-ray and focused ion beam tomography. Microstructural features that influence fracture at the micro- and nanoscale were characterized in virgin material in three-dimensions. The nature and volume fraction of ductile damage was then quantified as a function of distance below the fracture surfaces of tested notched and fatigue pre-cracked laboratory specimens. In both specimens the ductile fracture process initiates with the brittle fracture of large irregular intermetallic particles at low levels of plastic strain. With increasing plasticity, the resulting voids grow and combine with pre-existing porosity to increase the overall void volume fraction. Once a critical void volume fraction is attained, final failure occurs by the fracture or decohesion of dispersoids from the matrix, initiating a second population of nanoscale voids, which interlinks the larger voids. The critical void volume fraction for coalescence and the distribution of ductile damage below the fracture surface is markedly different between blunt-notched and pre-cracked specimens, with notched specimens exhibiting a significantly lower critical void volume fraction and a more extensive distribution of ductile damage below the fracture surface than is observed in pre-cracked specimens. This observation, related to the gradients in stress triaxiality and plastic strain in each specimen type, has important implications for the calibration of ductile damage mechanics models against notched-specimen data and their use to predict crack behavior in engineering structures.     

Investigations of ductile damage during the process chains of toothed functional components manufactured by sheet-bulk metal forming

Sheet-bulk metal forming processes combine conventional sheet forming processes with bulk forming of sheet semi-finished parts. In these processes the sheets undergo complex forming histories. Due to in- and out-of-plane material flow and large accumulated plastic strains, the conventional failure prediction methods for sheet metal forming such as forming limit curve fall short. As a remedy, damage models can be applied to model damage evolution during those processes. In this study, damage evolution during the production of two different toothed components from DC04 steel is investigated. In both setups, a deep drawn cup is upset to form a circumferential gearing. However, the two final products have different dimensions and forming histories. Due to combined deep drawing and upsetting processes, the material flow on the cup walls is three-dimensional and non-proportional. In this study, the numerical and experimental investigations for those parts are presented and compared. Damage evolution in the process chains is simulated with a Lemaitre damage criterion. Microstructural analysis by scanning electron microscopy is performed in the regions with high mechanical loading. It is observed that the evolution of voids in terms of void volume fraction is strongly dependent on the deformation path. The comparison of simulation results with microstructural data shows that the void volume fraction decreases in the upsetting stage after an initial increase in the drawing stage. Moreover, the concurrent numerical and microstructural analysis provides evidence that the void volume fraction decreases during compression in sheet-bulk metal forming.     

A damage model for void configuration and failure under different constrained deformation regions in ductile matrix

The constraint effects of SiC particle volume fractions and their specimen geometry are analyzed by conducting tensile tests while changing the SiC particle volume fraction 0%, 2% and 10% and the tensile specimen notch radius 0.5, 1.0, 2.0, 4.0, 8.0 and 16.0 mm. The effect of constraint on the void configuration and coalescence are investigated experimentally using 3-dimensional SEM fracture surface observations and 3-dimensional image analysis. It was found that the final void volume fraction and void shape were greatly affected by stress triaxiality and deformation constraint.     

Refractory Protection by High Speed Blowing in a PS Converter

In researching development of a high productivity converter, a six-ton Peirce-Smith converter with six tuyeres was constructed. The apparent speeds of a blast and an oxygen-enriched blast through the tuyeres were tested from MACH (MN) 0.3 to 2.5. It was found that at a speed of more than MN 1.0, tuyere blockage no longer occurs. Consequently, accretions around the tip of the tuyere did not disturb blowing, eliminating the need for punching. The life of the refractory around the tuyere zone was triple that experienced with a normal blast speed. But even with high speed blowing, refractory erosion remained severe when using oxygen enrichment in excess of 40%. This high speed blowing technique also caused an increase of splash from the converter mouth. Oxygen efficiency was, however, kept at a high level.     

Simulation and experimental research on extra-squeeze forming method during gradient sand molding

The flexible extrusion forming process (FEFP) is a sand mold patternless manufacturing technology that enables digital near-net shaping of complex sand molds. But, it is difficult to achieve the gradient sand molds with high surface strength and strong interior permeability by FEFP. To solve this problem, an extra-squeeze forming method based on FEFP for gradient sand mold was developed. To further reveal the extra-squeeze forming mechanism, based on the Johnson-Kendall-Roberts (JKR) theory and “gluing” notions, the single and double-sided squeeze models of gradient sand molds were established using the EDEM software. The squeezing processes of sand molds with different cavity depths of 60, 100, 140, 180, and 220 mm were systemically studied under single and double-sided squeeze conditions. The variation in the void fraction of sand mold as also investigated at a variety of extra-squeeze distances of 2, 3, 4, 5, and 6 mm, respectively. Simulation and test results show that a deeper cavity depth weakens the extrusion force transmission, which leads to a decrease in strength. The sand mold permeability and void fraction are identified to be positively correlated, while the tensile strength and void fraction appear to be negatively correlated. The void fraction of sand molds decreases with a longer extra-squeeze distance. A 6 mm extra-squeeze distance for the sand mold with 220 mm cavity depth results in a 26.8% increase in tensile strength with only a 5.7% reduction in the permeability. Hence, the extra-squeeze forming method can improve the quality of the sand mold by producing a gradient sand mold with high surface strength and strong interior permeability.

     

Prediction of void closure in steel slabs by finite element analysis

Closure of a spherical voids in a steel slab under plane-strain deformation was investigated using the rigidplastic finite-element method. Variations in the major and minor axes of a void from finite element analysis of a void model were related to the minimum principal strain and hydrostatic stress from finite element analysis of a non-void model. The correlation curves were obtained and a method using these curves to predict the void closure progress was proposed. The method was successfully applied to deformation processes such as simple compression, forging and rolling. Since hydrostatic stress also influenced void closure, the effective strain by itself was not sufficiently capable of predicting void closure. However, the effective strain was used to predict void closure for a specific process because it reached about 0.7 in compression or forging and about 0.78 during rolling as the void completely closed.     

Void closure prediction in cold rolling using finite element analysis and neural network

Cold rolling is used to eliminate void defects in cast materials thus improving the material performance during service. A comprehensive procedure is developed using finite element analysis and neural network to predict the degree of void closure. A three-dimensional nonlinear dynamic finite element model was used to study the mechanism of void deformation. Experiments were conducted to investigate void closure during the cold flat rolling process. Experimental results are compared to the three-dimensional finite element predictions to validate the model. The void reduction predictions from finite element analysis are in good agreement with experimental findings. Plastic strain, principal stress distribution around the void and void reduction ratio are presented for various case studies. As finite element simulation is time-consuming, a back-propagation neural network model is also developed to predict void closure behavior. Based on the correlation analysis, the reduction in sheet thickness, the dimension of the void and the size of the rollers were selected as the inputs for the neural network. The neural network model was trained based on results obtained from finite element analysis for various simulation cases. The trained neural network model provides an accurate and efficient procedure to predict void closure behavior in cold rolling.     

蠕墨铸铁生产时冲天炉风口结渣原因探析及措施

针对生产蠕墨铸铁时冲天炉风口出现结渣,风口发黑,严重时一排风口堵塞,一排风口以下的炉缸区焦炭全部烧空的现象,分别从熔剂的加入量及种类、炉壁效应、控制风焦平衡等方面研究了风口结渣的成因.结果表明:通过调整下排风口的角度、送风量以及及时清理风口等措施有效地改善了冲天炉风口结渣的现象.     

多相混输泵首级动叶轮轮毂到轮缘气相分布研究

为进一步探究流量和转速对多相混输泵内气相分布规律的影响,基于时均N-S方程和Simple算法,利用FLUENT软件对不同流量和转速下入口含气率为30%时混输泵内三维流态进行仿真。结果表明:流量对首级动叶轮不同截面轮毂处气相分布影响均较大;在进口和出口截面,随着流量的增加含气率增大;在较高转速(2 500、2 950 r/min)时,首级动叶轮不同截面从轮毂到轮缘的含气率变化较大;在不同流量和转速下,首级动叶轮轮缘附近的含气率变化均较大。研究结果可为混输泵性能优化和提高气液输送能力提供参考。     

Ductile damage and simulation of fine-blanking process by FEM

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FCC晶体中晶体取向对孔洞长大和聚合的影响

通过编制率相关有限元用户子程序，采用包含一个和两个球形孔洞的单胞探求了FCC晶体中晶体取向对孔洞长大和聚合的影响。计算结果表明：晶体取向对孔洞长大的影响较大，孔洞的形状和长大方向与晶体取向密切相关：由于变形不均匀，孔洞在晶界处产生尖角，易形成裂纹。由于约束较少，孔洞周围和两孔洞间的区域塑性变形较大，晶体的转动和滑移主要集中在孔洞周围以及两孔洞间的区域。     

Continuum simulations of the formation of Kirkendall-effect-induced hollow cylinders in a binary substitutional alloy

We examine binary substitutional diffusion in cylindrical diffusion couples in which free surfaces are considered explicit vacancy sources and sinks. The central region of the cylinder is initially occupied by an atomic species with a larger hop frequency, while the outer region is occupied by another atomic species with a smaller hop frequency. Equilibrium vacancy concentration is maintained at free surfaces that serve as vacancy sources and sinks. In the crystal, diffusion is governed by the standard diffusion equations with analytically evaluated diffusion coefficients. The void growth dynamics and hollow cylinder formation stemming from the Kirkendall effect are simulated. Our results show that the Kirkendall void growth involves two competing factors. One is the net inward vacancy flux that favors void growth. The other is the Gibbs–Thomson effect that favors void shrinkage. We compute the critical initial radius for void growth above which the Kirkendall effect dominates over the Gibbs–Thomson effect. The fully grown void radius and the elapsed time to the fully grown size are also predicted for different fast-diffuser volume fractions and fast-to-slow diffuser atomic hop frequency ratios.     

Void formation in nanocrystalline Cu film during uniaxial relaxation test

Void formation in nanocrystalline Cu thin films with a grain size of 100 nm during uniaxial tensile relaxation experiments is quantitatively studied. Cu thin films with a two-dimensional fiber structure were deposited on heat-resistant polyimide substrates and subject to various subcritical uniform uniaxial tensile strains at an elevated temperature (∼0.3T_m), to observe void formations in nanocrystalline metals with a reduced amount of dislocation-based deformation. Microstructural observations were carried out at several stages of deformation, and the evolutions of void formation in subcritical strain levels are quantitatively discussed. A void formation model is proposed for approximating the nucleation and growth rate of voids. The resulting model shows a reasonable agreement with the observed number density and area fraction of voids for various strain levels and grain sizes. On the basis of the results, the stress and grain size dependences of the void formation process are further discussed.     

An analysis of the size effect on void growth in single crystals using discrete dislocation dynamics

The effect of size on the mechanical behavior and the void growth rate in a voided single crystal was studied using two-dimensional discrete dislocation dynamics. The simulations were based on the methodology developed by Van der Giessen and Needleman [Van der Giessen E, Needleman A. Modell Simul Mater Sci Eng 1995;3:689], which was extended to non-convex domains through the use of finite elements with embedded discontinuities [Romero I, Segurado J, LLorca J. Modell Simul Mater Sci Eng 2008;16:035008]. Square crystals (in the range 0.5–2.5 μm) with an initial void volume fraction of 10% were deformed under plane strain conditions in uniaxial tension, uniaxial deformation and biaxial deformation. The results of the simulations show two size effects, one on the initial flow stress and strain-hardening rate of the voided crystal (“smaller is stronger”) and another on the void growth rate (“smaller is slower”). The magnitude of both size effects increased with triaxiality. The physical micromechanisms responsible for these size effects were elucidated from the simulation results.     

Effects of residual S on Kirkendall void formation at Cu/Sn–3.5Ag solder joints

Solder joints of Cu/Sn–3.5Ag were prepared using Cu foil or electroplated Cu films with or without SPS additive. With a high level of SPS in the Cu electroplating bath, voids tended to localize at the Cu/Cu₃Sn interface during subsequent aging at 150 °C, which was highly detrimental to the drop impact resistance of the solder joints. In situ Auger electron spectroscopy of fractured joints revealed S segregation on the Cu/Cu₃Sn interface and void surfaces, suggesting that segregation of S to the Cu/Cu₃Sn interface lowered interface energy and thereby the free energy barrier for Kirkendall void nucleation. Once nucleated, voids can grow by local tensile stress, originating from residual stress in the film and/or the Kirkendall effect. Vacancy annihilation at the Cu/Cu₃Sn interface can induce tensile stress which drives the Kirkendall void growth.     

Proposal and use of a void model for the simulation of ductile fracture behavior

A ductile fracture model derived from a microscopic point of view is proposed and is proved to be effective in the analysis of multipass drawing. First, a method of predicting ductile fracture is proposed using a model from a microscopic point of view. The relationship between the void volume fraction and the strain to fracture, calculated from the model, agrees well with the relationship obtained from the Thomason model. Next, the analysis and experiment of multipass drawing are performed using various reductions in area and die angles. The inner diameter of the die at which the material fractures and the material density distribution in the radial direction, calculated from the analysis, agree with those obtained from the experiment. Finally, a simulation of drawing at the die at which the material fractures is performed and inner fracture defects are observed to appear periodically in the drawing direction.