FAST ACCURATE PREDICTION OF BLANK SHAPE IN SHEET METAL FORMING

By using the Finite Element Inverse Approach based on the Hill quadratic anisotrop-ically yield criterion and the quadrilateral element, a fast analyzing software-FASTAMP for the sheet metal forming is developed. The blank shapes of three typical stampings are simulated and compared with numerical results given by the AUTOFORM software and experimental results, respectively. The comparison shows that the FASTAMP can predict blank shape and strain distribution of the stamping more precisely and quickly than those given by the traditional methods and the AUTOFORM.     

Multi-Axial Deformation Setup for Microscopic Testing of Sheet Metal to Fracture

While the industrial interest in sheet metal with improved specific-properties led to the design of new alloys with complex microstructures, predicting their safe forming limits and understanding their microstructural deformation mechanisms remain as significant challenges largely due to the inadequacy of the existing experimental tools. The investigation of the strain-path dependent failure mechanisms requires miniaturized testing equipment, which can be placed in a scanning electron microscope for in situ experiments. So far, such tests could only be carried out for a single strain path (uniaxial tension). In this work, in order to fill this gap, a miniaturized Marciniak test setup is designed, built and tested. With this setup real-time, multi-axial tests of industrial sheet metal can be carried out to the point of fracture within a scanning electron microscope. Proof-of-principle experiments demonstrate that a realm of information can be obtained, crucial for the understanding of the mechanical behavior of new alloys.     

无铆连接模具参数对异种金属板料接头成形的影响

为了研究无铆连接模具参数对304不锈钢和AL6061板料接头成形的影响,基于DEFORM-2D建立了有限元模型,分析了凹模深度、凹槽深度和凸模直径三个主要模具参数对接头成形的影响.研究结果表明,凹模深度越大,越有利于下板料变形,反之,则有利于上板料变形.当凹模直径一定时,凹槽深度和凸模直径过小不利于形成互锁,而凹槽深度不宜过大,避免出现充不满现象,凸模直径过大导致接头颈厚值过小.通过正交试验发现,对接头成形的影响程度从大到小依次为凸模直径、凹槽深度和凹模深度,确定了最优模具参数,并通过无铆连接实验验证了正交试验的可靠性.此外,通过剪切实验获得了接头最大剪切失效载荷为1.8 kN.     

韧性材料颈缩主应力面上应变分布假定的研究

为了深入研究韧性材料主应力面的应变分布情况,讨论Bridgman关于主应力面上应变均布假定的精度．本文首先结合断口学基本理论与在室温条件下拉伸断裂的Q235钢、40Cr钢和45#钢断口电子显微镜扫描结果,从微观角度分析其韧窝分布后发现应变均布假定存在一定的误差;然后基于体积守恒和Aramis三维应变观测系统测得的试验结果,将计算值和试验值进行对比分析发现,三种材料中除含碳量较高的45#钢外,应变均布假定并不适用于分析颈缩主应力面上应变分布．本文的研究成果可为进一步研究韧性材料颈缩大塑性阶段的应力、应变本构关系提供一定的参考．     

高强钢及软钢断裂失效行为表征与仿真预测

针对汽车碰撞、结构强度等分析中无法准确模拟高强钢及软钢的断裂失效,研究了高强钢及软钢的断裂失效特性以及其仿真预测方法．以宝钢高强钢HC340/590DP及软钢DC05为研究对象,首先通过不同应变率的拉伸试验和不同应力状态的断裂失效试验,来表征两种钢材的弹塑性及断裂失效行为．并从试验数据中得到LS-DYNA中的Damage Initiation and Evolution Criteria (DIEC)失效模型所需参数,建立失效模型,通过子系统试验及仿真计算对DIEC失效模型进行验证．结果表明基于准确的材料试验数据,正向建立的DIEC失效模型可以准确预测HC340/590D及DC05的断裂失效行为．本研究方法可推广到其他高强钢和软钢的断裂失效仿真预测中．     

Fracture Prediction for an Advanced High-Strength Steel Sheet Using the Fully Coupled Elastoplastic Damage Model with Stress-State Dependence

In this study,the numerical simulations of sheet metal forming processes are performed based on a fully coupled elastoplastic damage model.The effects of stress...     

Experimental and Numerical Analysis of an In-Plane Shear Specimen Designed for Ductile Fracture Studies

An in-plane shear specimen made of dual phase steel designed for ductile fracture studies is presented and then analyzed experimentally and numerically. In the experiment, digital image correlation (DIC) technique is utilized to measure the deformation of the specimen. Based on the implicit nonlinear FE solver Abaqus/Standard, numerical analysis of the specimen is performed by using plane stress and solid elements respectively. The elongation of the specimen’s gauge length and the shear strain distribution within the shear zone are compared between the experimental and numerical results and a general good agreement is obtained. Thereafter, based on calculated results, the stress state of the shear zone is investigated in detail. It is shown that the shear stress is dominant within the shear zone despite of the emergence of normal stresses. The deformation is concentrated in the shear zone, where the incipient fracture is most likely to occur. The stress triaxiality and the Lode parameter at the fracture initiation are found to be maintained at a relatively low level, which implies that the stress state achieved by the specimen is close to pure shear. The present study demonstrates that the proposed in-plane shear specimen is suitable for investigation of the fracture behavior of high strength materials under shear stress states.     

Mechanical Testing of Polymers in Pressurized Hydrogen: Tension,Creep and Ductile Fracture

The paper aims at characterizing some key features of the mechanical behavior of two semi-crystalline polymers in weakly pressurized hydrogen. The opportunity to use hydrogen as an alternative energy strengthens the need for reliable data on possible coupling effects between gas diffusion and mechanical properties, especially for safe design purpose. However, such effects have not been really quantified in polymers. In the present study, a hydraulic testing machine has been fitted with a pressure hydrogen chamber, and three major aspects of the mechanical behavior have been investigated in polyethylene and polyamide 11: monotonic tension, long-term creep (based on a time-temperature superposition principle) and ductile fracture (evaluated from an essential work of fracture method). Suitable protocols have been defined to take into account specificity like temperature and pressure history dependence and gas saturation kinetics of the sample.     

Out-of-plane Testing Procedure for Inverse Identification Purpose: Application in Sheet Metal Plasticity

Many recent works in inverse identification of constitutive parameters have pointed to the need of tests which exhibit heterogeneous strain paths. The present study details a new testing procedure based on out-of-plane motion capture by Stereo-Image Correlation (SIC). With the original test proposed hereby, a unique sample is deformed on a tensile machine along two perpendicular tensile directions, two perpendicular shear directions and one expansion area. The choice of the sample shape is discussed along with the stereo imaging device, 3D reconstruction and measurement uncertainties. The test sample is made from a sheet of commercially pure titanium. A Finite-Element updating inverse method is applied in order to identify six parameters of an anisotropic plastic constitutive model. Results show that this new testing procedure allows every constitutive parameter of the model to be identified from one and only one test.     

On the Cover: Analysis of Damage and Failure in Anisotropic Ductile Metals Based on Biaxial Experiments with the H-Specimen

Experimental Mechanics -     

Analysis of Damage and Failure in Anisotropic Ductile Metals Based on Biaxial Experiments with the H-Specimen

Background

Dependence of strength and failure behavior of anisotropic ductile metals on loading direction and on stress state has been indicated by many experiments. To realistically predict safety and lifetime of structures these effects must be taken into account in material models and numerical analysis.

Objective

The influence of stress state and loading direction on damage and failure behavior of the anisotropic aluminum alloy EN AW-2017A is investigated.

Methods

New biaxial experiments and numerical simulations have been performed with the H-specimen under different load ratios. Digital image correlation shows evolution of strain fields and scanning electron microscopy is used to visualize failure modes on fracture surfaces. Corresponding numerical studies predict stress states to explain damage and fracture processes on the micro-scale.

Results

The stress state, the load ratio and the loading direction with respect to the principal axes of anisotropy affect the width and orientation of localized strain fields and the formation of damage mechanisms and fracture modes at the micro-level.

Conclusions

The enhanced experimental program with biaxial tests considering different loading directions and load ratios is suggested for characterization of anisotropic metals.

     

电子封装技术中金属基板结构的热失效行为研究

本文将电子散斑干涉与有限元数值计算相结合，对金属基板结构的热失效行为进行了研究。获得了低维氧化物绝缘薄膜的热失效温度，以及失效过程中基板表面的变形分布；测定了25μm厚度绝缘氧化薄膜的热膨胀系数，室温下弹性模量和失效应力等三个重要的材料常数；结合实验所测定的材料常数和研究的基板结构及性能，用有限元数值方法，模拟计算了绝缘氧化薄膜在不同缺陷形状时的失效过程。并给出了不同温度下绝缘氧化薄膜的最大应力集中值。     

Comparison of Two Unsteady Intermittency Models for Bypass Transition Prediction on a Turbine Blade Profile

The present paper evaluates two unsteady transition modelling approaches: the prescribed unsteady intermittency method PUIM, developed at Cambridge University and the dynamic unsteady intermittency method developed at Ghent University. The methods are validated against experimental data for the N3-60 steam turbine stator profile for steady and for unsteady inlet flow conditions. The characteristic features of the test case are moderately high Reynolds number and high inlet turbulence intensity, which causes bypass transition. The tested models rely both on the intermittency parameter and are unsteady approaches. In the prescribed method, the time-dependent intermittency distribution is obtained from integral relations. In the dynamic method, the intermittency distribution follows from time-dependent differential equations. For unsteady computations, self-similar wake profiles are prescribed at the inlet of the computational domain. Joint validation of the prescribed and the dynamic unsteady intermittency models against experimental data shows that both methods are able to reproduce the global features of the periodical evolution of the boundary layer under the influence of a periodically impinging wake. The overall quality of the dynamic method is better than that of the prescribed method.     

Nonclassical Problems of Fracture/Failure Mechanics: on the Occasion of the 50th Anniversary of the Research (Review)

International Applied Mechanics - The principal results of research on some nonclassical problems of fracture/failure mechanics have been analyzed. These results have been obtained by the author...     

基于应力三轴度和罗德参数的6061和7075铝合金材料断裂失效分析

分别对6061铝合金和7075铝合金材料的缺口圆棒试件和凹槽平板试件进行准静态拉伸试验,并采用ABAQUS软件对拉伸过程进行数值模拟。模拟结果与试验测试结果吻合很好,验证了有限元模型的合理性和可靠性。通过有限元模拟,分别给出了不同试件的应力三轴度和罗德参数随等效塑性应变的变化曲线以及两种材料的失效轨迹,并对它们进行了分析讨论。结果表明:形状相同、材料性质不同的试件,应力三轴度的演化规律不同;材料的失效应变受应力三轴度和罗德参数的影响,并且不同性质的材料对罗德参数的敏感性不同。     

Fracture Characterization of Rolled Sheet Alloys in Shear Loading: Studies of Specimen Geometry,Anisotropy, and Rate Sensitivity

Two different shear sample geometries were employed to investigate the failure behaviour of two automotive alloy rolled sheets; a highly anisotropic magnesium alloy (ZEK100) and a relatively isotropic dual phase steel (DP780) at room temperature. The performance of the butterfly type specimen (Mohr and Henn Exp Mech 47:805–820, 16; Dunand and Mohr Eng Fract Mech 78:2919-2934, 17) was evaluated at quasi-static conditions along with that of the shear geometry of Peirs et al Exp Mech 52:729-741, (27) using in situ digital image correlation (DIC) strain measurement techniques. It was shown that both test geometries resulted in similar strain-paths; however, the fracture strains obtained using the butterfly specimen were lower for both alloys. It is demonstrated that ZEK100 exhibits strong anisotropy in terms of failure strain. In addition, the strain rate sensitivity of fracture for ZEK100 was studied in shear tests with strain rates from quasi-static (0.01 s^?1) to elevated strain rates of 10 and 100 s^?1, for which a reduction in fracture strain was observed with increasing strain rate.     

Analyzing the Laws of Stable Subcritical Growth of Cracks in Polymeric Materials on the Basis of Fracture Mesomechanics Models. Theory and Experiment

The laws of stable crack growth are analyzed using fracture mesomechanics models for polymeric materials under long-term subcritical tension. A review is given of experimental and theoretical studies of crack-tip process zones. The studies were conducted using physical and mechanical methods and fracture mesomechanics models, allowing for the structural and rheological features of process zones. Theoretical and experimental results on the behavior of process zones are generalized and the theory of stable crack growth in viscoelastic polymers, which assumes the autonomy of the process zone during crack development, is justified     

Nonclassical Problems of Fracture/Failure Mechanics: On the Occasion of the 50th Anniversary of Research (Review). III

International Applied Mechanics - The main results of research on some nonclassical problems of fracture/failure mechanics are analyzed. These results have been obtained by the author and his...     

三维复合型断裂的试验方法探讨

探讨了一种能够揭示三维复合型断裂现象并能记录这些现象的三维复合型断裂研究的实验方法，用此方法完成了LC4－CS航空结构铝合金不同厚度紧凑拉伸试样的I＋Ⅱ复合型断裂试验。结果表明：此方法能够简便地确定出复合型裂纹的起裂载荷、破坏载荷、裂尖张开位移、滑移位移和起裂角，基本解决三维复合型断裂研究所需实验数据问题。同时发现此种材料试样的承载能力和超裂方向的确具有明显的厚度效应。