拉深成形对于Q&P980高强钢氢致延迟断裂影响的实验研究

目的从整车厂应用的角度出发,基于拉深工艺,研究QP980高强钢的氢致延迟断裂敏感性。方法选取1.6 mm板厚的QP980高强钢,进行拉深系数为0.56和0.63的两种冲杯实验,以电化学充氢结合摄像头定时拍摄的方法,并结合ABAQUS软件计算杯口应力集中处的应力和应变。结果对于1.6 mm板厚的QP980高强钢,在拉深试样杯口边缘应力集中处,应力大致在900～1000 MPa范围,而应变大于等于0.32,则必然发生延迟开裂;若应变小于等于0.23,则延迟断裂敏感性较低。结论应力和应变同时影响高强钢的氢致延迟断裂敏感性,即对QP 980(1. 6 mm厚度)零件,当拉深边缘应变小于0.23,则该位置氢致延迟断裂可能性低;若应力集中处残余应力达到900 MPa以上,应变达到0.3以上,则该位置氢致延迟断裂敏感性高。此结论对工程应用判断拉深零件氢致延迟断裂有一定指导意义,     

Fracture Criteria for Automobile Crashworthiness Simulation of Wrought Aluminium Alloy Components

In automobile crashworthiness simulation, the prediction of plastic deformation and fracture of each significant, single component is critical to correctly represent the transient energy absorption through the car structure. There is currently a need, in the commercial FEM community, for validated material fracture models which adequately represent this phenomenon. The aim of this paper is to compare and to validate existing numerical approaches to predict failure with test data. All studies presented in this paper were carried out on aluminium wrought alloys: AlMgSi1.F31 and AlMgSiCu‐T6. A viscoplastic material law, whose parameters are derived from uniaxial tensile and compression tests at various strain rates, is developed and presented herein. Fundamental ductile fracture mechanisms such as void nucleation, void growth, and void coalescence as well as shear band fracture are present in the tested samples and taken into consideration in the development of the fracture model. Two approaches to the prediction of fracture initiation are compared. The first is based on failure curves expressed by instantaneous macroscopic stresses and strains (i. e. maximum equivalent plastic strain vs. stress triaxiality). The second approach is based on the modified Gurson model and uses state variables at the mesoscopic scale (i. e. critical void volume fraction). Notched tensile specimens with varying notch radii and axisymmetric shear specimens were used to produce ductile fractures and shear band fractures at different stress states. The critical macroscopic and mesoscopic damage values at the fracture initiation locations were evaluated using FEM simulations of the different specimens. The derived fracture criteria (macroscopic and mesoscopic) were applied to crashworthiness experiments with real components. The quality of the prediction on component level is discussed for both types of criteria.     

Accuracy verification of a simple local three-dimensional displacement measurement method of DIC with two images coordinates

There are two methods applied for three-dimensional digital image correlation method to measure three-dimensional displacement. One is to measure the spatial coordinates of measuring points by analyzing the images. Then, the displacement vectors of these points can be calculated using the spatial coordinates of these points obtained at different stages. The other is to calibrate the parameters for individual measuring points locally. Then, the local displacements of these points can be measured directly. This study proposes a simple local three-dimensional displacement measurement method. Without any complicated distortion correction processes, this method can be used to measure small displacement in the three-dimensional space through a simple calibration process. A laboratory experiment and field experiment are carried out to prove the accuracy of this proposed method. Laboratory test errors of one-dimensional experiment are similar to the accuracy of the XYZ table; the error in Z-direction is only 0.0025% of the object distance. The measurement error of laboratory test is about 0.0033% of the object distance for local three-dimensional displacement measurement test. Test and analysis results of field test display that in-plane displacement error is only 0.12 mm, and the out-of-plane error is 1.1 mm for 20 m × 30 m measuring range. The out-of-plane error is only about 10 PPM of the object distance. These test and analysis results show that this proposed method can achieve very high accuracy under small displacement for both of laboratory and field tests.     

2219铝合金TIG和FSW接头力学及疲劳性能

目的 研究钨极氩弧焊(TIG)和搅拌摩擦焊(FSW)对2219铝合金(母材)力学及疲劳性能的影响。方法 通过拉伸试验,得到了母材、TIG和FSW接头的抗拉强度和伸长率;通过疲劳性能试验测试了母材、TIG和FSW接头在不同应力下相应的疲劳寿命,根据疲劳试验结果绘制了其试样的S-N曲线;使用扫描电子显微镜观察并分析了疲劳断口的形貌特征。结果 未焊接的铝合金母材抗拉强度和伸长率最高,分别为506 MPa和15.92%;TIG接头抗拉强度和伸长率分别为330 MPa和7.65%,FSW接头抗拉强度和伸长率分别为310 MPa和8.74%。母材、TIG和FSW接头等3种疲劳试样在2×10⁶次循环下的疲劳强度分别为129、108、115 MPa,其疲劳断口均可分为裂纹源区、裂纹扩展区和瞬间断裂区,疲劳裂纹分别起始于试样表面的局部变形区、第二相夹杂物和“吻接”缺陷。疲劳裂纹扩展区的主要形貌为疲劳辉纹和二次裂纹,瞬间断裂区以脆性断裂为主。结论 TIG和FSW等2种焊接工艺均导致了2219铝合金的强度、塑韧性和疲劳性能降低,其接头表面的第二相夹杂物和“吻接”缺陷促进了疲劳裂纹的萌生。     

不锈钢拉伸试验条件与断后伸长率测量值的关系

采用单向拉伸试验研究了不同拉伸速度、不同试样规格对不锈钢冷轧薄板断后伸长率测量值的影响。结果表明:不锈钢的断后伸长率测量值随拉伸速度的提高而下降、随试样横截面的增大而提高。说明拉伸试验条件对不锈钢薄板的断后伸长率测量值影响较大,只有在拉伸试验条件一致的情况下,断后伸长率测量值才能作为选材的依据之一。     

数字图像相关法测量聚碳酸酯板应力强度因子

本文以数字图像相关方法测量了聚碳酸酯板的张开型应力强度因子（SIF）。实验过程中以聚碳酸酯标准紧凑拉伸式样为样本,由数字图像相关（DIC）方法提取不同载荷下裂纹附近的全场位移。在已计算得到全场位移的情况下,由线弹性断裂理论可知,弹性区域内每一点位移可表示为SIF的函数。在计算SIF时考虑了实验中不可避免存在的刚体平移与转动的影响,应力强度因子、刚体平移与转动可由最小二乘法计算得到;还讨论了位移场的选择及位移展开项项数对SIF的影响。实验结果与理论值进行了比较,结果显示DIC方法准确地测量了聚碳酸酯板的应力强度因子,因此DIC为测量SIF提供了一种有效的试验方法。     

Stress and strain based fracture forming limit curves for advanced high strength steel sheet

In this work, strain based fracture forming limit curve (FFLC) of advanced high strength (AHS) steel grade 980 was determined by means of experimental Nakajima stretch-forming test and tensile tests of samples under shear deformation. During the tests, a digital image correlation (DIC) technique was applied to capture the developed strain histories of deformed samples up to failure. The gathered fracture strains from different stress states were used to construct the FFLC. Subsequently, the FFLC in the strain space was transformed to a principal stress space by using plasticity theories. As a result, the fracture forming limit stress curve (FFLSC) of examined steel was obtained. Furthermore, fracture locus (FL) as a relationship between stress triaxialities and critical plastic strains was determined. Hereby, two anisotropic yield functions, namely, the Hill’48 and Yld89 model were taken into account and their effects on the calculated curves were investigated. To verify the applicability of the obtained limit curves, rectangular cup drawing test and forming tests of so-called Diabolo and mini-tunnel samples were performed. Obviously, the FFLSCs and FLs more accurately described the failure occurrences of 980 steel sheets than the FFLCs. In addition, it was found that the drawing depths predicted by the FLs and the Yld89 yield criterion slightly better agreed with the experimental results than those from the FFLSCs and the Hill’48 model, respectively.     

Evaluation of a miniaturized disc test for establishing the mechanical properties of low-alloy ferritic steels

A miniaturized disc test has been used to measure load-displacement curves for a 2 1/4Cr1Mo steel, from which are derived values for the Young's modulus of elasticity, yield stress and ultimate tensile strength over the temperature range -196 to 25 °C. The miniaturized test uses disc specimens 3 mm diameter and 0.25 mm thick and a test jig that applies a load via a 1 mm diameter steel ball at a constant displacement rate. The reproducibility of the method has been determined by testing a large number of the 2 1/4Cr1Mo steel specimens at each temperature considered. Comparisons have been made between analytical and empirical methods of evaluating the tensile yield stress and ultimate tensile strength from the measured load-displacement data. In addition, consideration has been given to the total energy to fracture, K*, and the variation with temperature which confirms a transition from low-energy cleavage to high-energy ductile fracture.     

An optical method of measuring anisotropic deformation and necking in material testing

The mechanical properties of structural engineering materials are often highly anisotropic, particularly in wrought products. Characterisation of the material behaviour and the identification of the fracture and damage parameters of structural metals require load‐displacement data from smooth and notched tensile tests. Modern optical methods, such as 3D digital image correlation (DIC), enable full field displacement data to be collected. However, the measurement of diametral contraction in round tensile specimens is not straightforward for anisotropic materials. In this paper a method for measuring diametral contractions simultaneously in two orthogonal directions using a standard 3D DIC system in conjunction with an edge detection algorithm is described. The results show that deformation anisotropy can be readily quantified through these orthogonal diametral displacements.     

Nonlinear compressive stiffness in impacted composite laminates determined by an inverse method

This paper presents use of an inverse method and non-contact optical measurements for determining the apparent compressive stiffness reduction in impact damage zones in composite laminates. The tensile stiffness distribution and nonlinearity is also briefly covered. The method is based on iterative updating of the material properties in a finite element model with the objective to match the predicted displacement fields to those measured optically in impacted specimens under load. To examine the effect of the damage on initial imperfections, strain and buckling, the displacement fields obtained experimentally by digital image correlation are demonstrated and discussed. Finally, the method is applied to the obtained full-field measurements and the influence of applied strain on the nonlinear tensile stiffness and apparent compressive stiffness of real impact damage zones is evaluated. Material nonlinearity in tension is found to increase towards the damage centre where fibre damage is more severe. Stiffness in compression can only be represented by a uniform apparent material nonlinearity, which is strongly linked to local buckling.     

Small punch and uniaxial creep fracture behaviours of modified SUS304H steel at various temperatures

Commercial austenitic stainless steel SUS304H with small amount of vanadium addition was used in this study. Small punch (SP) creep and uniaxial tensile creep tests were conducted at 650, 700, and 750 °C to measure creep lives and the minimum displacement rates or the minimum creep strain rates. The measured parameters were compared between the two test methods, seeking empirical relationships among the parameters using Larson-Miller Parameter and Monkman-Grant relation. Magnitude of the applied stress (MPa) in the uniaxial tensile creep test was approximately equal to the applied load value (N) in the SP creep test at all test temperatures. It was shown that during the creep deformation of the SP creep specimen, crack initiation and accompanying crack growth occur simultaneously. Competing failure mechanisms of creep deformation and crack growth may affect the SP creep life and consequently determine the proportionality function, α, in the relation between the SP load and the tensile creep rupture stress in creep tests.     

Forming and fracture limits of aluminium alloy

Abstract

Forming and fracture limits of an AA 3104-H19 aluminium alloy sheet were studied by hydraulic bulging and Marciniak type deep drawing and tensile tests. The alloy appeared to be highly anisotropic, exhibiting distinctly different fracture patterns in the rolling and transverse directions. The preferred fracture direction was transverse to the rolling direction. In the tensile test, samples loaded in the rolling direction failed transverse to the rolling direction, but in the transverse direction, the fracture was inclined at ～55° to the tensile axis. In some cases, two such competing fractures in the characteristic directions could be observed. Scanning electron microscopy studies revealed a typical ductile fracture pattern. The fracture occurred by shearing in the through thickness direction, and typical alternating shear lips in a direction inclined at ～45° to the through thickness direction could be observed. Forming limit diagrams for both rolling and transverse directions were determined from the experiments. The measured limit strains in uniaxial tension were predicted well by the modified Rice–Tracey theory, but in equibiaxial tension, the theory overestimated the fracture limit strains.     

高密度聚乙烯材料在大变形条件下的数值模拟研究

针对高密度聚乙烯(HDPE)在大变形条件下有限元模型不易收敛、本构关系较为复杂的问题,对HDPE片材进行了单轴拉伸试验和数值模拟研究。通过对比试验结果和模型计算结果发现:非线性粘弹性本构模型与小变形条件下HDPE的单轴拉伸试验结果较为吻合,但与大变形条件下的试验结果相差较大;而Kwon模型的计算结果与大变形和小变形条件下的试验结果均较为吻合。同时,对Kwon模型的参数选择进行了优化,得到了100mm/min和150 mm/min拉伸速率下的优化参数,对大变形下片材的数值模拟具有较好的参考价值。此外,通过对条带单元的应力应变分析,可知HDPE条带在单轴拉伸下的应力应变呈不均匀分布,中心点区域是片材最大应力应变的集中点,这也解释了HDPE条带断裂多出现在中心区域的原因。     

结构钢开裂准则及断裂试验分析

为探寻结构钢开裂机理及抗断设防,给出了结构钢在应力三轴空间的广义屈服轨迹方程。结合古典强度理论和现代损伤力学对金属屈服和断裂解释的力学原理,基于三向等拉伸应力状态下结构钢屈服和宏观脆断重合性假设,导出了结构钢在应力三轴空间的开裂准则。在结构钢开孔板的单向等速拉伸断裂试验中,较为精确地测量了初始开裂时的加载位移及全程载荷-位移曲线。试验结果显示,初始裂纹位于孔边且扩展迅速。通过对结构钢开孔板断裂试验的数值模拟分析,对比验证了该开裂准则的普适性及精度。最后,给出了结构钢广义屈服和开裂模型的物理解释及抗断设防。     

Simulation of crack propagation resistance in brittle media of high porosity

Summary The crack propagation resistance through a porous or microstructurally heterogeneous brittle solid with local variability in strength and stiffness has been simulated. Specifically, the simulation probes the behavior of porous brittle materials in the range of porosity less than those of cellular materials and greater than those of microstructures that are in the category of dilute porosity. The simulation plane consists of a triangular network of points interacting with each other through both linear central force springs and bond angle springs, incorporating an appropriate element of a noncentral force contribution. Explicit microstructural details were incorporated into the model and the simulation was first carried out under conditions of uniaxial tensile strain in order to investigate the mechanisms of subcritical damage evolution, leading to quasi-homogeneous fracture. In order to investigate material strength and stiffness variability on the scale of a representative volume element for coherent fracture events in a crack tip stress gradient, the explicit microstructural results were incorporated into a simulation with boundary conditions characteristic of the displacement field of an infinite Mode I crack. To impart some 3D realism to the primarily 2D simulations a special 2D super-element was devised, which incorporated variability information as might be sampled by a crack front in three dimensions. For a given porosity, in general, only small differences were found between nominally diverse microstructures in terms of their tensile toughness, maximum strength and elastic moduli. The strongest dependence of the overall fracture toughness was found to come from the average porosity. The variability in local element strength and stiffness on the scale of the porosity produced highly tortuous crack paths, roughly on the scale of the chosen representative volume element. The tortuosity of the crack was largest where local variability of strength and stiffness was uncorrelated. Examples of microcrack toughening and crack bridging were observed.     

Numerical determination of stretch zone width (SZW) using tensile test data

There is an increasing effort to use critical stretch zone width (SZW) for the evaluation of initiation fracture toughness that is considered as geometry independent material property. The existing numerical SZW evaluation method is based on crack tip opening displacement measurement, which is evaluated using various definitions. This numerical method also does not define the critical stage for critical SZW measurement. This work attempts to establish a procedure for numerical determination of SZW, its critical value (SZW_c) and initiation fracture toughness using tensile test data. The proposed methodology also tries to explain the mechanism involved in the creation of stretched zone and thus defines the stage to calculate critical SZW. Numerical analyses have also been carried out to understand the role of crack tip constraint in standard fracture specimen during the blunting process and the tensile test sample.     

含孔天然纤维织物复合材料力学性能

研究了含孔天然苎麻纤维织物/异氰酸酯复合板在双轴向拉伸载荷下的力学行为。对0.5、1.0、2.0、4.0mm 4种孔径板进行了单向和双轴向载荷拉伸试验, 同时采用数字散斑相关方法对全场位移及孔径大小对应变的影响进行了表征。结果表明: 当载荷线性变化时孔周围的位移场分布较为均匀; 随着载荷接近破坏值, 位移场呈非线性分布并出现高应变值点, 破坏以极快的速度沿孔边在这些点首先发生。随着孔径的增大, 在1000~2000N双轴向载荷下孔周围相同面积内x、y方向正应变的平均值减小, 应变值波动小但范围增大。材料在单向和双轴向拉伸时表现出不同的力学特征: 双轴向载荷下失效强度要比单向拉伸时低, 降低比例为14%～27%, 且随着孔径的增加而增加。      

The Manufacture of Resorbable Suture Material from Magnesium

Magnesium alloys present an alternative to medical cases in which polymeric sutures may not be ideal. To date, no efficient and low‐cost manufacturing process for the fabrication of magnesium‐based sutures can be found. To obtain an alternative, the previously characterised magnesium alloys ZEK100 (98% Mg), AX30 (96.2% Mg), AL36 (91% Mg) and MgCa0.8 (99.2% Mg) were cast, extruded into 30 mm diameter bars and extruded into 0.5 mm diameter wires. To determine the mechanical properties of the wires, grain size measurements, tensile tests as well as qualitative bending tests were carried out. The ZEK100 alloy's wires showed the finest microstructure having grains of 1.2 µm in diameter. Coarser microstructures were observed for MgCa0.8, AX30 and AL36. The alloy ZEK100 had the highest tensile stress (367 MPa) also revealing a brittle behaviour due to its fine microstructure. The tensile test, carried out for AX30 as well as AL36, resulted in comparable high fracture strains (10.6%) and tensile stresses (300 MPa). However, MgCa0.8 showed a tensile strength of 315 MPa and a low strain of 1.6%. Within the qualitative bending test wires made of the alloy AL36 were able to form tight knots, which is a key feature for suture applications. The comparison of the magnesium sutures with commercially available polymeric sutures revealed lower strength and elongation for the magnesium alloys. Yet, the wires exhibited mechanical properties that can meet the requirements of a suture material.     

激光选区熔化成形铝合金板与CFRP复合材料层合板螺栓连接结构失效分析方法评估

对激光选区熔化成形(SLM)铝合金板与碳纤维增强树脂基(CFRP)复合材料层合板两列四排沉头螺栓单剪连接件在拉伸载荷作用下进行了数值分析和试验研究。基于渐进损伤法的三维有限元模型准确地预测了连接件材料损伤萌生和演变,对比试验和三维有限元所得钉载比例、极限载荷及失效模式,可以发现,通过拟合SLM铝合金板断裂应变和应力三轴度曲线,编写UMAT子程序引入韧性准则和Hashin失效准则的三维有限元模型预测的连接件失效载荷与试验值误差仅为1.9%,且失效模式均为净截面拉断,两者吻合,此方法可以满足工程精度要求。利用经过验证的数值模型,分别预测了SLM铝合金板和CFRP层合板损伤演变过程,并分析了SLM铝合金板刚度对连接结构失效模式的影响,当SLM铝合金板厚度增大到4mm时,连接结构失效模式由SLM铝合金板净截面拉断转移到CFRP层合板上。     

浮空器蒙皮膜复合材料单轴拉伸力学性能及弹性常数

对新型飞艇蒙皮材料在0°、15°、30°、45°、60°、75°、90°七个偏轴方向单轴拉伸循环试验结果进行了分析,给出了残余应变和弹性模量随循环次数的变化规律;得出了由单层板理论推导出的关于弹性模量的本构关系对各功能膜层压合成的平纹织物膜复合材料适用性较差。使用VIC-2D数字散斑测量系统测出膜材在拉伸过程中的位移场和应变场,通过位移场求膜材的泊松比和通过应变场验证分析膜材拉伸破坏机制,并可以预测断口形态和位置。采用两种不同规格的试样测试膜材的单轴拉伸强度,通过对比发现采用试样3更能反应材料Uretek5876实际强度。本文工作对该材料应用于飞艇结构设计和分析具有参考价值。