Influence of thickness and initial groove angle in M–K model on limit strain of 7B04 by considering through-thickness stress

Marciniack–Kuczinski (M–K) model is widely used to predict material’s forming limit curve (FLC). The prediction of FLC traditionally neglected through-thickness normal stress. However, it cannot be neglected in some forming processes. Much work has been done to study the effect of through-thickness normal stress on FLC with constant through-thickness normal stress or constant ratio of through-thickness normal stress and maximum principal stress. In addition, based on Nakazima test process, the ratio of through-thickness normal stress and maximum principal stress has been derived, which was a function of instantaneous thickness and loading path. Here, initial groove angle in M–K model was not considered. In this paper, uniaxial tension tests and Nakazima tests were performed on 7B04 aluminum alloy. Based on Hill 48 yield criterion and M–K model, the prediction model of FLC was established. The increase of thickness can enhance FLC. Meanwhile, it is necessary to consider through-thickness normal stress and initial groove angle in prediction model. On the left side of FLC, the effect of initial groove angle on FLC is weakened by increasing sheet thickness. On the right side of FLC, the effect of initial groove angle on FLC is strengthened by increasing sheet thickness. On the right side of FLC, the relation between limit strain points with different thicknesses is linear under one certain loading path. Thickness has decisive effect on through-thickness normal stress level and the changing trendy of through-thickness normal stress during calculation is different under different stress condition.     

超高温NiAl合金锥形薄壳件制备成形一体化新工艺

针对NiAl合金板坯制备及板坯成形锥形薄壳件存在的材料流动及组织性能控制困难的问题,提出了一种制备成形一体化新工艺。该工艺是将塑性成形和反应合成在同一工步中,即先将Ni/Al叠层箔置于模具中进行塑性成形,随后对成形的Ni/Al叠层箔原位加热加压反应合成NiAl合金薄壳件。采用三维扫描仪及Geomagic Studio/Qualify对锥形薄壳件的形状精度进行了分析,采用扫描电子显微镜( Scanning Electron Microscope, SEM )和电子背散射衍射( Electron Backscatter Diffraction, EBSD )技术对其微观组织进行了表征,并对构件的高温力学性能进行了测试。结果表明,采用该新工艺制备的锥形薄壳件成形效果良好,整体型面偏差尺寸在±0.1 mm以内;构件沿轴截面壁厚平均偏差为0.012 mm,沿横截面壁厚平均偏差为0.072 mm,构件轴截面壁厚分布均匀性及组织成分均匀性高于横截面。构件高温力学性能稳定,1000 °C时平均屈服强度为77.8 MPa,平均抗拉强度为82.6 MPa。NiAl合金构件的微观晶粒形貌与Ni箔的初始热处理状态及Ni/Al箔的初始厚度有关,未退火的Ni箔将延缓粗晶区晶粒的长大,减小初始箔材厚度可实现晶粒的进一步细化。     

Influence of uniaxial tensile pre-strain on forming limit curve by using biaxial tensile test

This paper focused on the effect of pre-strain on forming limit curves (FLC) of 5754-O aluminum alloy sheet through utilizing biaxial tensile approach. Based on Swift model and Yld2000-2d yield criterion, the dimensions of cruciform specimen was optimized through applying finite element method for increasing the strain at specimen center. After that, with the recommended specimen size, the cruciform specimen was tested under various stroke ratios to experimentally characterize the limit strains under different pre-strain levels. Subsequently, the biaxial tensile tests were simulated by Abaqus to obtain the limit strains and validate the material models. It can be observed in both experiments and simulations that the pre-strained uniaxial tension followed by plane tension or equi-biaxial tension can improve the formability of sheet metals. Besides, the strain path change affects the trend of first derivative of strain rate difference between neighboring points with respect to time. An early increase occurred and then fell back to the stable value, the steady evolution continued until to a new increase reaching the critical value. The M–K prediction approach was simulated to verify the influence of pre-strain on FLC. It can be found that the early increase peaks of the major strain incremental ratio rose with the amplitude of pre-strain. Finally, the phenomenon of pseudolocalization caused by the strain path change was explained through evolution of stress state inside the groove.     

一种韧性断裂准则中材料常数的计算模型及其应用

为确定符合板材变形规律的韧性断裂准则中的材料常数,基于传统M-K模型框架并进行修正,结合单向拉伸和平面应变试验数据,提出一种新的韧性断裂准则材料常数计算模型。利用MATLAB软件编写该计算模型的算法程序,得到应用于铝镁合金5A06-O板材的不同韧性断裂准则材料常数。同时将CL韧性断裂准则嵌入Abaqus/Explicit显示模块的用户材料子程序VUMAT。在200℃的条件下,对铝镁合金5A06-O板材在热介质胀形和充液热拉深中的断裂行为进行数值模拟,并与相同工艺参数下的试验所得结果作对比。结果表明,热介质胀形高度误差为6.2%,充液热拉深深度误差为8.5%,验证了韧性断裂准则材料常数计算模型的正确性,表明了CL韧性断裂准则在板材充液热成形中的适用性。     

一种改进的Lemaitre韧性断裂准则及其在旋压成形中的应用

采用Lemaitre韧性断裂准则与有限元模拟相结合来预测材料在塑性成形过程中的开裂是广泛应用的一种方法,其中的Lemaitre准则是以损伤值与塑性应变呈线性关系为基础的,而实际一些材料的损伤值与塑性应变间却是非线性的.为此,本文将原Lemaitre准则中反映损伤值与塑性应变间的线性关系修正为非线性关系,得到了改进的Le...     

Prediction of forming limit curve （FLC） for AI-Li alloy 2198-T3 sheet using different yield functions

The Forming Limit Curve （FLC） of the third generation aluminum-lithium （Al-Li） alloy 2198-T3 is measured by conducting a hemispherical dome test with specimens of different widths. The theoretical prediction of the FLC of 2198-T3 is based on the M-K theory utilizing respectively the von Mises, Hill＇48, Hosford and Barlat 89 yield functions, and the different predicted curves due to different yield functions are compared with the experimentally measured FLC of 2198-T3. The results show that though there are differences among the four predicted curves, yet they all agree well with the experimentally measured curve. In the area near the planar strain state, the predicted curves and experimentally measured curve are very close. The predicted curve based on the Hosford yield function is more accurate under the tension-compression strain states described in the left part of the FLC, while the accuracy is better for the predicted curve based on Hill＇48 yield function under the tension-tension strain states shown in the right part.     

含多处损伤结构的剩余强度研究

系统地回顾了含多处损伤MSD结构的损伤连通准则,指出采用Dugdale的片状塑性区尺寸的Swift韧带屈服准则对一般工程材料最为合适.对具有潜在危险性的典型的广泛MSD结构作了剩余强度分析,分析结果表明:广泛MSD大大地降低了结构的剩余强度.     

Cross-sectional deformation of H96 brass double-ridged rectangular tube in rotary draw bending process with different yield criteria

Different yield criterion has great difference in predicting the deformation of tube with different material. In order to improve the prediction accuracy of the cross-sectional deformation of the double-ridged rectangular tube (DRRT) during rotary draw bending (RDB) process, Mises isotropic yield criterion, Hill’48 and Barlat/Lian anisotropic yield criteria commonly used in practical engineering are introduced to simulate RDB of DRRT. The inverse method combining uniaxial tensile test of whole tube and response surface methodology was proposed to identify the parameters of Hill’48 and Barlat/Lian yield criteria of small-sized H96 brass extrusion DRRT as well. Then based on ABAQUS/Explicit platform, the FE models of RDB process of DRRT considering Mises, Hill’48 and Barlat/Lian yield criteria were built. The results show that: The variation trend of cross-sectional deformation ratio is same when using different yield criteria. The cross-sectional deformation ratio by using Mises yield criterion is close to that by using Hill’48 yield criterion. However, there is a quite difference between by using Barlat/Lian yield criterion and by using Mises or Hill’48 yield criteria. The prediction values of cross-sectional height deformation by using three yield criteria all underestimate the experiment ones, and the prediction values of cross-sectional width deformation overestimate the experiment ones. By comparing the simulation results of cross-sectional deformation of the DRRT with different yield criteria and experiment ones, Barlat/Lian yield criterion is found to be suitable for describing the RDB process of DRRT.     

基于原位拉伸测定GTN损伤模型参数的5052铝合金成形极限应力图研究(英文)

传统成形极限图(FLD)由第一主应变与第二主应变的比值构成。但FLD依赖于成形历史和应变路径。因此,本文采用成形极限应力图(FLSD)来预测铝合金5052-O1(AA5052-O1)的成形极限。将修订的Gurson-Tvergaard-Needleman(GTN)塑性势模型用来模拟Nakazima凸模胀形实验。采用带扫描电镜(SEM)的原位拉伸试验观察记录损伤演化过程以确定GTN模型中的材料参数。通过计算AA5052-O1三个损伤阶段的孔洞分数获得GTN模型参数。根据实验结果,原生孔洞体积分数、最大形核孔洞体积分数、临界孔洞体积分数,最终断裂孔洞体积分数分别为0.002918,0.0249,0.030103,0.04854。在断裂前最后一个载荷步获得的应力应变值用来绘制AA5052-O1的FLSD和FLD。与Nakazima凸模胀形试验和单轴拉伸试验结果相比较,预测结果与试验结果较为吻合。由原位拉伸试验确定的模型参数可用于研究韧性金属的成形极限。     

Al2O3陶瓷蠕动进给超声磨削特性试验研究

在改装成的超声磨削机床上进行了陶瓷超声磨削与普通磨削的对比试验,以揭示蠕动进给超声磨削加工的磨削特性及其材料去除机理。结果表明:当超声振动方向与磨削进给方向相同时,超声振动所引起的抛磨作用非常显著,超声磨削的表面粗糙度比机械磨削的低;Al2O3陶瓷在一般条件下超声磨削的去除方式仍属于脆性断裂,以晶粒粉碎、穿晶断裂为主,但伴随有沿晶断裂、弹性划擦、塑性流动等现象。     

直角切削6061-T6铝合金剪切区力学行为及微观结构演化预测

力学行为是塑性变形微观过程的宏观表现,早期的金属切削理论模型没有考虑微观结构对切削力的影响。在考虑热力耦合效应的基础上建立了基于位错密度材料模型的6061-T6铝合金直角切削力预测模型,分析了不同切削参数下基于位错运动的塑性变形机制对切削力的影响。结合等分剪切区和非等分剪切区模型,构建了第一变形区多物理场计算方法,提出一种切屑形成过程中由塑性变形引起的微观结构演化解析模型。通过测量切削力和切屑内晶粒尺寸对模型的可行性进行了初步验证。结果表明：剪切区长度变长引起参与位错滑移的材料增多是切削深度增大导致切削力增大的主要原因。增大切削速度导致切削力的降低不是单一变量影响的结果,而是应变降低引起位错增殖数量减少和温度升高引起位错湮灭作用增加的共同作用结果。非等分剪切区模型正确反映了第一变形区温度和应力的分布特征,且与二维有限元模型分布相一致,建立的第一变形区微观结构演化解析模型能够预测切屑内位错密度和晶粒尺寸。     

定应变下固体火箭发动机药柱概率贮存寿命预估研究

为研究定应变对固体火箭发动机药柱概率贮存寿命的影响,对推进剂高温加速老化力学性能数据进行了统计分析,利用随机有限元法分析了发动机药柱在内压和过载的联合作用下Von Mises应变的均值和标准差,采用应力-强度干涉模型计算了药柱结构可靠性随应变敏感系数的变化趋势,据此分析了定应变对发动机药柱概率贮存寿命的影响。结果显示,定应变对发动机药柱概率贮存寿命影响显著,以0.97为可靠性下限,当应变敏感系数为2.94时,其寿命约为30.98年,应变敏感系数为-2.94时,其寿命约为0.92年,在此范围,药柱概率贮存寿命随应变敏感系数的增大而延长。     

断裂问题研究中裂尖塑性区的影响及处理原则

通过对K判据的适用区与裂纹尖端塑性区的比较,在塑性区一定要小于K判据适用区的条件下,给出了K判据的适用边界线和判断脆性断裂的判断式;确定了K判据在结构静强度计算中塑性修正的基本原则,并在此基础上讨论得出了塑性区尺寸小于裂纹长度,裂纹扩展寿命的估算方法仍适用,塑性对裂纹扩展寿命估算影响不大,而选用不同的经验公式估算裂纹扩展寿命所出现的误差却很大的结论。     

TiB2/7050颗粒增强铝基复合材料热变形行为与热压工艺优化

分析TiB₂/7050颗粒增强铝基复合材料的热变形行为及工艺参数的影响规律，对其热变形后的组织设计和获得理想性能参数至关重要。基于此，针对TiB₂/7050颗粒增强铝基复合材料开展了相关研究。用Gleeble-3500热模拟机进行热压缩试验，研究了TiB₂/7050颗粒增强铝基复合材料在变形温度300~450 ℃、应变速率0.001~1 s-1时的热变形行为，建立了材料的双曲正弦本构方程；根据动态材料模型计算得出热加工图，优选了材料的热加工工艺窗口；对原始挤压成型坯料和优化工艺的热压成型坯料进行了力学性能测试和微观组织形貌分析。结果表明:两种成型工艺相比，热压件强度指标略有提高，但塑性大幅提高，其长横向断后延伸率提高400%；热压工艺件晶粒更加细小、且无明显择优取向；拉伸断裂机制均为准解理断裂，热压件断口韧窝更深、撕裂棱更粗大，表明塑性撕裂持续时间更长。      

A new method to predict fatigue crack growth rate of materials based on average cyclic plasticity strain damage accumulation

By introducing a fatigue blunting factor, the cyclic elasto-plastic Hutchinson-Rice-Rosengren (HRR) field near the crack tip under the cyclic loading is modified. And, an average damage per loading-cycle in the cyclic plastic deformation region is defined due to Manson-Coffin law. Then, according to the linear damage accumulation theory-Miner law, a new model for predicting the fatigue crack growth (FCG) of the opening mode crack based on the low cycle fatigue (LCF) damage is set up. The step length of crack propagation is assumed to be the size of cyclic plastic zone. It is clear that every parameter of the new model has clearly physical meaning which does not need any human debugging. Based on the LCF test data, the FCG predictions given by the new model are consistent with the FCG test results of Cr2Ni2MoV and X12CrMoWVNbN 10-1-1. What’s more, referring to the relative researches, the good predictability of the new model is also proved on six kinds of materials.     

高强铝合金晶界偏析与氢致断裂机理的研究

提出了一个三元合金晶界偏析与沿晶断裂模型，并将该模型与准化学理论相结合，研究了高强铝合金Mg、H晶界偏析对沿晶断裂功的影响。结果表明，Mg、H偏析均使沿晶断裂功下降，且H比Mg脆化晶界严重。这与7075铝合金的应力腐蚀实验结果相一致。     

考虑几何限制的航向道模式设计

民机自动飞行系统在截获LOC信标切入跑道中心线时，由于空间几何限制导致较大超调，会对飞行安全产生不利影响。针对这一问题，设计了一种考虑几何限制的航向道模式。为了解决LOC线性区外信号不能用于自动飞行指令计算的问题，利用飞行管理系统的现有功能计算出飞机偏离跑道中心线的水平偏差；使用切入点位置和最大滚转角的几何限制作为机动判断条件，同时以进入线性区作为LOC模式的激活条件，使得飞机在达到几何限制时能提前机动。最后，通过仿真对比了采用三种不同LOC截获方法时飞机的响应，仿真结果表明所提出的方法能够有效解决因几何限制导致的LOC截获超调问题。     

基于库伦摩擦效应的箔片轴承极限承载力理论与试验

基于有限单元法建立了考虑库伦摩擦的波箔型径向气体箔片轴承的箔片结构模型,采用有限差分法和有限单元法耦合求解Reynolds方程和气膜厚度方程,通过求解轴颈达到极限偏心率时的轴承极限承载力,研究了箔片结构库伦摩擦效应对轴承极限承载力的影响规律,并搭建了轴承极限承载力测试试验台,利用温度法测量了两个具有不同轴承壳内表面粗糙度的波箔型径向气体箔片轴承的轴承极限承载力.通过对比分析仿真结果与试验结果表明:轴承壳圆柱孔内表面粗糙度为0.4μm的轴承在10000r/min和20000r/min下,轴承极限承载力分别为15.5N和42.3N;而表面粗糙度为1.6μm的轴承极限承载力为10.9N和29.6N,这是由于波纹箔片和轴承壳体之间的库伦摩擦力增大了波纹箔片的刚度,因此增大箔片结构摩擦因数使得轴承极限承载力降低,并且仿真结果变化趋势和试验结果变化趋势吻合.      

箔片型红外面源诱饵扩散规律

目前红外面源诱饵已成为对抗红外成像制导导弹的重要方法。鉴于此,以箔片型红外面源诱饵为研究对象,利用计算流体力学（CFD）方法得到单个箔片和两平行箔片在不同迎角下的气动力系数。通过对箔片空中运动分析,特别是考虑到燃烧力作用的特殊性,将箔片的运动分为两个阶段：起燃阶段和完全燃烧阶段,并建立了两个阶段的运动模型。在单个箔片运动模型的基础上,设置上千个箔片初始姿态和旋转角速度的概率分布情况,同时对上千个箔片求解运动方程,即可得到整个红外面源诱饵的空间运动扩散规律。仿真结果表明：在高速运动平台上垂直向上发射红外面源诱饵,其扩散形状近似为前部稀疏后部密集的锥形云团;仿真得到的面源诱饵扩散尺寸和运动位置与实测数据吻合度较高,特别是面源诱饵在x轴方向扩散尺寸误差小于5%。     

激光点焊工艺参数对止推箔片轴承接头组织与性能的影响

止推箔片轴承是航空陀螺仪中的重要支撑元件,为了提高支承面的耐磨性及耐腐蚀性,通常会在支承面涂覆特氟龙涂层,该涂层的添加给后续的制造工艺带来不确定性。研究激光点焊工艺参数对止推箔片轴承接头组织与性能的影响,利用光学显微镜、扫描电镜、电子精密拉伸仪、超声检测等分析方法,对焊缝表面成形、接头显微组织及力学性能进行分析。结果表明:焊点表面直径随脉冲功率的增加而增大,而抗拉剪力随热输入量的增加先增大后减小;激光点焊制造的优化工艺参数为脉冲功率百分比22%、脉冲宽度5.0ms,抗拉剪力达最大值63.0N;超声波浸液检测能够精确测定搭接界面的尺寸,但抗拉剪力与界面尺寸不完全成正比关系,接头的抗拉剪力主要与接头的断裂机制有关。