Damage percolation during stretch flange forming of aluminum alloy sheet

A multi-scale finite element (FE)-damage percolation model was employed to simulate stretch flange forming of aluminum alloys AA5182 and AA5754. Material softening and strain gradients were captured using a Gurson-based FE model. FE results were then fed into the so-called damage percolation code, from which the damage development was modelled within measured microstructures. The formability of the stretch flange samples was predicted based upon the onset of catastrophic failure triggered by profuse void coalescence within the measured second-phase particle field. Damage development is quantified in terms of crack and void areal fractions, and compared to metallographic results obtained from interrupted stretch flange specimens. Parametric study is conducted on the effect of void nucleation strain in the prediction of formability of stretch flanges to “calibrate” proper nucleation strains for both alloys.     

Suppression of necking in incremental sheet forming

Incremental sheet forming enables sheet metal to deform above a conventional strain-based forming limit. The mechanics reason has not been clearly explained yet. In this work, the stress-based forming limit was utilized for through-thickness necking analysis to explain this uncovered question. Stress-based forming limit which has path-independency shows that the stress states in top, middle and bottom surfaces did not exceed the forming limit curve at the same time and each layer has different stress state in terms of their deformation history to suppress necking. It has been found that it is important to consider the gradient stress profile following the deformation history for the proper forming limit analysis of incremental sheet forming.     

A damage accumulation model for complex strain paths: Prediction of ductile failure in metals

The characterisation of strain path with respect to the directionality of defect formation is discussed. The criterion of non-monotonic strain path is used in the scalar and tensor models for damage accumulation and recovery. Comparable analysis of models and their verification has been obtained by simulation of crack initiation in a two-stage metal forming operation consisting of wire drawing followed by constrained upsetting.     

动力松弛法在板为成形过程数值模拟中的应用

论文阐述了动力松驰法的基本原理,推导出它在中心差分动力显式算法中的应用列式,并将其用于锥形凸缘的圆形杯拉深成形过程的数值模拟研究,获得了满意的结果。     

各向异性本构关系在板料成形数值模拟中的应用

对几种能表达面内各向异性的屈服准则Hill、Barlat-Lian、Barlat进行了比较。以弹性变形服从各向同性广义虎克定律的情况下，给出了基于张量算法推导的弹塑性本构关系的一般表达式，并由此导出了相应屈服准则的弹塑性本构关系的显式表达。借助ABAQUS软件本构模块用户子程序接口，分别实现了这些屈服准则在ABAQUS的嵌入。以模拟方形盒的拉延过程为例，分析了不同的屈服准则在板料成形过程数值模拟中的应用。模拟结果表明，基于弹塑性本构关系一般表达所列出的相应屈服准则的显式表达式是正确的；在采用壳元来模拟板料成形时，采用Barlat准则的模拟结果和采用Barlat-Lian准则的结果差别不大。     

基于UMAT的形状记忆合金驱动波纹板结构的数值分析

形状记忆合金SMA主动驱动波纹板效率高,且性能稳定,在设计自适应智能结构上具有可观的前景。为有效利用有限元法对SMA波纹板结构进行计算分析,基于已有SMA本构模型推导了增量型SMA本构模型,据此编写了可由ABAQUS调用的用户材料（UMAT）子程序;利用该UMAT子程序对SMA主动驱动波纹板结构进行了数值模拟计算,与实验结果的对比验证了计算结果的有效性;在SMA波纹板原始结构基础上,提出了SMA短带错落布置型新结构,并进行了数值模拟分析与验证;提出了新结构的温度控制方案和提高驱动效果的措施,可为SMA驱动波纹板驱动器的设计与应用提供参考与借鉴。     

Investigations to the effect of heating-rate on the mechanical properties of aluminum alloy LY12

Two classes of experiments were conducted with a Gleeble 1500 thermal–mechanical testing system to investigate the effect of heating-rate and its history on the mechanical behavior of aluminum alloy LY12. In the first class of experiment, specimens were heated at different heating-rates to prescribed temperatures and then stretched until fracture. It was found that the specimen heated with higher heating-rate possesses lower rupture strength. In the second class of experiment, the specimens were preloaded and then heated at different rates until fracture. It was found that the higher the heating-rate was, the lower the failure temperature would be. Metallographical analysis showed that there are more defects in the specimens undergoing higher heating-rate. It was conjectured that higher heating-rate may cause stronger local thermal inconsistency due to the heterogeneous nature of the material. It may then cause local residual microstress fields, which, together with external thermal–mechanical load, may result in the changes in the microstructure of the material, such as recovery, recrystallization, nucleation and growth of microdefects, accounting for the changes in the macroscopic mechanical properties including hardening/softening, damage and failure, etc. A numerical simulation was performed, in which the mechanisms of local thermal inconsistency and the effect of the influencing factors were investigated.     

A non-associated constitutive model with mixed iso-kinematic hardening for finite element simulation of sheet metal forming

In this paper an anisotropic material model based on non-associated flow rule and mixed isotropic–kinematic hardening was developed and implemented into a user-defined material (UMAT) subroutine for the commercial finite element code ABAQUS. Both yield function and plastic potential were defined in the form of Hill’s [Hill, R., 1948. A theory of the yielding and plastic flow of anisotropic metals. Proc. R. Soc. Lond. A 193, 281–297] quadratic anisotropic function, where the coefficients for the yield function were determined from the yield stresses in different material orientations, and those of the plastic potential were determined from the r-values in different directions. Isotropic hardening follows a nonlinear behavior, generally in the power law form for most grades of steel and the exponential law form for aluminum alloys. Also, a kinematic hardening law was implemented to account for cyclic loading effects. The evolution of the backstress tensor was modeled based on the nonlinear kinematic hardening theory (Armstrong–Frederick formulation). Computational plasticity equations were then formulated by using a return-mapping algorithm to integrate the stress over each time increment. Either explicit or implicit time integration schemes can be used for this model. Finally, the implemented material model was utilized to simulate two sheet metal forming processes: the cup drawing of AA2090-T3, and the springback of the channel drawing of two sheet materials (DP600 and AA6022-T43). Experimental cyclic shear tests were carried out in order to determine the cyclic stress–strain behavior and the Bauschinger ratio. The in-plane anisotropy (r-value and yield stress directionalities) of these sheet materials was also compared with the results of numerical simulations using the non-associated model. These results showed that this non-associated, mixed hardening model significantly improves the prediction of earing in the cup drawing process and the prediction of springback in the sidewall of drawn channel sections, even when a simple quadratic constitutive model is used.     

7A04铝合金的本构关系和失效模型

使用万能材料试验机、扭转试验机和Taylor撞击实验研究了高强铝合金7A04在常温至250 ℃的 准静态、动态本构关系和失效模型。基于实验结果,修改了Johnson-Cook强度模型中的应变强化项以及 Johnson-Cook失效模型中的温度软化项,并结合数值模拟标定了模型参数。实验结果表明,7A04铝合金的 应变和应变率强化效应不显著,失效应变随温度的增加、应力三轴度的减小和应变率的减小而增加。     

Surface hardness increase of 2024 aluminum alloy subjected to cyclic loading

Constant amplitude fatigue tests at R = 0.1, conducted on the aircraft aluminum alloy 2024 T3, have revealed an appreciable surface hardness increase of the alloy at the nano- and meso-scale during fatigue. The observed surface hardness changes could be monitored with confidence by means of nanoindentations. The degree of hardening increases with increasing number of fatigue cycles following exponential relations. With increasing fatigue stress level degree of hardening increases as well. The observed results provide a basis for developing concepts to early detect and also monitor fatigue damage accumulation in aluminum aircraft structures based on measurements of the material’s hardness changes by means of nanoindentations.     

A constitutive model of aluminum foam for crash simulations

A new constitutive model for metallic foams is developed to overcome the deficiency of existing models in commercial finite element codes. The proposed constitutive model accounts for volume changes under hydrostatic compression and combines the hydrostatic pressure and von Mises stress into one yield function. The change of the compressibility of the metallic foam is handled in the constitutive model by allowing for shape changes of the yield surface in the hydrostatic pressure-von Mises stress space. The backward Euler method is adopted to integrate the constitutive equations to achieve numerical accuracy and stability. The model is implemented into LS-DYNA as a user-defined subroutine, verified with existing solutions, and validated with foam testing data. The verified and validated model is then utilized in the crushing simulations of foam-filled columns with square and hexagonal cross-sections. Two constitutive models are studied: the first using an exponential function to describe the relationship of plastic Poisson's ratio with respect to true strain and the second using linear interpolation function as an alternative approximation. The new foam model provides satisfactory prediction of crushing forces and deformed shapes compared to experimental results. Additionally, the new foam model was shown to have better numerical stability and accuracy than existing LS-DYNA built-in material models.     

A physico-mechanical approach to modeling of metal forming processes—Part II: damage analysis in processes with plastic flow of metals

A damage analysis is presented for the extrusion of a case-shaped cylindrical part by using a physico-mechanical approach for modeling metal forming processes. Two integral measures related to the hydrostatic and deviatoric parts of the damage tensor are used for the calculation of strain damage. The combined use of two damage measures in contrast to only one allows us to assess not only a risk of macro-fracture of the deformed material but also the stage of formation of large cavernous defects due to coalescence of ellipsoidal voids. Such a refined prediction of the actual quality of the material’s micro-structure is important when producing metalware that is supposed to operate under intense loading and thermal conditions. In case study of this paper the kinetic equations of damage are solved by using mutually consistent fields of stresses, flow velocities, and strains. It is shown that the predicted damage is less than its permissible value since a high hydrostatic pressure in the plastic zone heals the micro-defects, prevents their growth, and, thereby, increases the processing ductility of deformed metals during extrusion.      

内爆炸载荷作用下7A55铝合金的动态性能及断裂行为

采用圆筒爆炸实验研究了内爆炸载荷作用下7A55铝合金的动态性能,用扫描电镜和光学显微镜 对破裂样品的断口形貌、金相组织等进行了微观分析。结果表明:在本文实验条件下,7A55铝合金能够承受 360MPa的内部爆炸加载;合金的断裂方式为剪切断裂,裂缝与筒壁的径向近似成45角;靠近圆筒内侧组织 中存在剪切变形带、绝热剪切带和裂纹,沿最大剪切应力面向外扩展。     

A simple isotropic-distortional hardening model and its application in elastic–plastic analysis of localized necking in orthotropic sheet metals

In the present work an elastic–plastic constitutive model including mixed isotropic-distortional hardening is presented. The approach is very simple and requires only experimental data that are part of the standard characterization of sheet metals. It is shown that the distortional hardening contribution can be of considerable importance for localized necking prediction in orthotropic sheet metals.     

宽应变率范围下2A16-T4铝合金动态力学性能

为了研究2A16-T4铝合金的动态力学性能,利用电子万能试验机、高速液压伺服试验机及霍普金森压杆(SHPB)装置进行常温下准静态、中应变率和高应变率的动态力学性能实验,得到不同应变率下的应力应变曲线,基于修正的Johnson-Cook本构模型对它进行拟合,并分析材料中应变率力学特性对模型应变率敏感参量的影响。结果表明:2A16-T4铝合金在应变率10-4~102 s-1范围内应变率敏感性较弱,而在102~103 s-1范围内应变率敏感性较强,且应变率强化效应随塑性应变的增大而减小;同时,在10-4~103 s-1范围内具有较强的应变硬化效应,且应变硬化效应随应变率的增大而减小;此外,修正Johnson-Cook本构模型的拟合结果与实验结果吻合很好,能够很好表征材料的动态力学行为,且考虑材料中应变率力学特性可提高本构模型参量的准确性。     

A new crystal plasticity scheme for explicit time integration codes to simulate deformation in 3D microstructures: Effects of strain path,strain rate and thermal softening on localized deformation in the aluminum alloy 5754 during simple shear

The predominant deformation mode during material failure is shear. In this paper, a crystal plasticity scheme for explicit time integration codes is developed based on a forward Euler algorithm. The numerical model is incorporated in the UMAT subroutine for implementing rate-dependent crystal plasticity model in LS-DYNA/Explicit. The sheet is modeled as a face centered cubic (FCC) polycrystalline aggregate, and a finite element analysis based on rate-dependent crystal plasticity is implemented to analyze the effects of three different strain paths consisting predominantly of shear. Finite element meshes containing texture data are created with solid elements. The material model can incorporate information obtained from electron backscatter diffraction (EBSD) and apply crystal orientation to each element as well as account for texture evolution. Single elements or multiple elements are used to represent each grain within a microstructure. The three dimensional (3D) polycrystalline microstructure of the aluminum alloy AA5754 is modeled and subjected to three different strain rates for each strain path. The effects of strain paths, strain rates and thermal softening on the formation of localized deformation are investigated. Simulations show that strain path is the most dominant factor in localized deformation and texture evolution.     

A 2D finite element based on a nonlocal constitutive model describing localization and propagation of phase transformation in shape memory alloy thin structures

Here, the effects of localization and propagation of martensitic phase transformation on the response of SMA thin structures subjected to thermo-mechanical loadings are investigated using nonlocal constitutive model in conjunction with finite element method. The governing equations are derived based on variational principle considering thermo-mechanical equilibrium and the spatial distribution of the nonlocal volume fraction of martensite during transformation. The nonlocal volume fraction of martensite is defined as a weighted average of the local volume fraction of martensite over a domain characterized by an internal length parameter. The local version of the thermo-mechanical behavior model derived from micromechanics considers the local volume fraction of martensite and the mean transformation strain. A 4-noded quadrilateral plane stress element with three degrees of freedom per node accounting for in-plane displacements and the nonlocal volume fraction of martensite is developed. Numerical simulations are conducted to bring out the influence of material and geometrical heterogeneities (perturbations/defects) on the localization and propagation of phase transformation in SMA thin structures. Also, a sensitivity analysis of the material response due to the localization and the other related model parameters is carried out. The detailed investigation done here clearly shows that the localization of phase transformation has significant effect on the response of shape memory alloys.