CCCD-SHPB动态断裂试验系统原理及数值分析

提出在分离式Hopkinson压杆(split Hopkinson pressure bar,SHPB)上用中心裂纹圆盘(central cracked circular disk,CCCD)型试件形成CCCD-SHPB试验系统来实施脆性材料的动态断裂试验.该系统的基本思想是基于SHPB的一维试验原理,得到中心裂纹圆盘型试件两端载荷的平均载荷;推广准静态的应力强度因子公式,并以此平均载荷代入以获得动态应力强度因子.对此动态断裂试验系统的三维动力学数值分析表明,CCCD-SHPB动态断裂试验方法是有效的,用此动态应力强度因子表征和测试脆性材料动态断裂韧度的方法也是有效可靠的.     

Modeling of ductile fracture using the dynamic punch test

Many models have been proposed for simulating events at quasi-static and dynamic rates. However, the Johnson–Cook constitutive model had been the workhorse for engineers and analysts when simulating such events, and it is readily available in the increasingly widespread numerical packages. Johnson and Cook also suggested a generalized failure model to be used with their constitutive model, but the challenge has always been in easily determining the correct parameters for these models. In this study, a procedure for determining the Johnson–Cook constitutive and, simplified, failure model parameters for three materials with specific importance in the aerospace industry, aluminum 6061-T6, titanium Ti-6Al-4 V and austenitic, nitrogen-strengthened, stainless steel (Nitronic 33) is proposed. Quasi-static and split Hopkinson pressure bar (SHPB) shear punch experiments were used to assess the materials’ ductility, while the quasi-static and SHPB compression experiments were used to determine the materials’ response. The laboratory results coupled with ABAQUS/Explicit simulations provided a tool for determination and validation of the models’ parameters for each material. While the use of the Johnson–Cook model with the proposed simplified failure model proved adequate for aluminum and titanium, the same could not be concluded for nitronic partly due to the intrinsic characteristics of this material and the multiplicative form of the Johnson–Cook model.     

数控冲床加工过程中工件变形的问题分析和研究

分析了数控冲床加工典型工件的冲裁过程,阐述了冲裁变形的产生机理.通过改进编程方法和冲压过程中压紧板料的方法来减小冲裁变形量,并在生产中取得了良好的效果.     

The design of a semi-additive manufacturing shape using metal 3D printing for a partially strengthened mold based on a high-alloy tool steel powder

In this paper, describe the fabrication of high strength punch molds that can be applied to ultra-high strength sheet materials after processing. A method for improving the strength of the punching die by additive manufacturing (AM) of a high strength powder material using a metal 3D printer was proposed. Furthermore, a semi-additive technique was proposed to increase the punch strength through partial AM of specific parts of the punch that require high strength. A preprocessing process for predicting the semi-additive shape for the punch function portion is proposed for application of the AM technology of a metal 3D printer to this semi-additive technique. The preprocessing for determining the semi-additive shape consists of the predicting step of the punch strength based on the shear process of the sheet material, analyzing step the stress distribution of the punch, defining step the semi-additive range, designing step the semi-additive shape, and verifying step the additive interface strength. Based on this simulation, the range of shapes for the semi-additive was 1.21 mm and 2.62 mm for sheet material CP1180 and 1.3 mm and 3.2 mm for sheet material 22MnB5. The shape and range determined in the simulation process defines a semi-additive area (volume) for the 3D printing AM technique using a high-strength powder material, and a semi-additive punch was manufactured according to the defined area. The semi-additive punch (HWS powder material) fabricated in this study was performed a durability test for validity verification in the piercing process of high-strength sheet material (CR980). This validation test compared the state of the punch after 1000 piercing processes with a typical cold piercing punch (SKD11 solid material). From this test, the feasibility of the semi-additive punch was confirmed by showing a similar state of scratches and abrasion from the two punches. The simulation analysis processor for the additive shape and the additive range prediction for the semi-additive punch manufacturing presented in this paper can be useful for the additive manufacture of cutting and trimming punch mold.

     

基于Dynaform的圆筒拉深件数值模拟

以Dynaform为模拟分析平台,对典型圆筒件拉深成形过程进行了模拟。用正交实验的方法评定了压边力、冲压速度、凸凹模间隙三个因素对零件成形质量的影响,并得到一组优化的参数值。圆筒件拉深成形相关参数的最优值压边力为20kN,冲压速度为2000mm/s,凸凹模间隙为1.1mm。     

Indirect measure of shale shear strength parameters by means of rock index tests through an optimized artificial neural network

Shear strength is one of the most important features in engineering design of geotechnical structures such as embankments, earth dams, tunnels and foundations. Shear strength parameters describe how rock material resists deformation induced by shear stress. Rock shear strength parameters are usually measured through laboratory tests, and these methods are destructive, time consuming and expensive. In addition, providing good-quality core samples is difficult especially in highly fractured and weathered rocks. This paper presents an indirect measure of shear strength parameters of shale by means of rock index tests. In this regard, 230 shale samples were collected from an excavation site in Malaysia and shear strength parameters of samples were obtained using triaxial compression test. Furthermore, rock index tests including dry density, point load index, Brazilian tensile strength, ultrasonic velocity, and Schmidt hammer test were conducted for each sample. A particle swarm optimization-artificial neural network (PSO-ANN) integrated model was developed by setting the results of rock index tests as inputs and shear strength parameters as outputs of the model. The obtained correlation of determination of 0.966 and 0.944 for training and testing datasets show the applicability of the proposed model to predict shale shear strength parameters with high accuracy.     

The collapse response of sandwich beams with a Y-frame core subjected to distributed and local loading

Sandwich beams comprising a Y-frame core have been manufactured by assembling and brazing together pre-folded sheets made from AISI type 304 stainless steel. The collapse responses of the Y-frame core have been measured in out-of-plane compression, longitudinal shear and transverse shear; and the measurements have been compared with finite element predictions. Experiments and calculations both indicate that the compressive response is governed by bending of the constituent struts of the Y-frame and is sensitive to the choice of lateral boundary conditions: the energy absorption for a no-sliding boundary condition exceeds that for free-sliding. Under longitudinal shear, the leg of the Y-frame undergoes uniform shear prior to the onset of plastic buckling. Consequently, the longitudinal shear strength of the Y-frame much exceeds its compressive strength and transverse shear strength. Sandwich beams were also indented by a flat bottomed punch, and a relatively high indentation strength was observed. It is argued that this is due to the high longitudinal shear strength of the Y-frame. While finite element calculations capture the measurements to reasonable accuracy, a simple analytical model over-predicts the indentation strength. Finally, the finite element method was used to investigate the energy absorption capacity of the sandwich beams under indentation loading. The calculations reveal that for a given tensile failure strain of the face-sheet material, a sandwich beam with Y-frame core has a comparable performance to that of a sandwich beam with a metal foam core. The relative performance is, however, sensitive to the choice of design parameter: when the indentation depth is taken as the design constraint, the sandwich beam with a Y-frame core outperforms the sandwich beam with the metal foam core.     

Joint strength measurements of individual fiber-fiber bonds: An atomic force microscopy based method

We are introducing a method to measure tensile strength of individual fiber-fiber bonds within a breaking force range of 0.01 mN-1 mN as well as the energy consumed during breaking. Until now, such a method was not available. Using a conventional atomic force microscope and a specifically designed sample holder, the desired force and the breaking behavior can be analyzed by two different approaches. First, dynamic loading can be applied, where force-versus-distance curves are employed to determine the proportions of elastic energy and energy dissipated in the bond. Second, static loading is utilized to study viscoelastic behavior and calculate viscoelastic energy contributions. To demonstrate the capability of the proposed method, we are presenting results for breaking strength of kraft pulp fiber-fiber bonds in tensile opening mode. The procedure is by no means restricted to cellulose fibers, it has the potential to quantify joint strength of micrometer-sized fibers in general.     

数控转塔冲床机身动态特性分析

为了研究数控转塔冲床机身的振动情况,以江苏亚威机床股份有限公司的HPR-3048数控转塔冲床为例,对机床机身进行有限元模态分析和实验模态分析,建立准确的有限元模型,同时通过塑性成形软件DEFORM计算板料冲孔时模具冲裁力的时间历程曲线,并以此作为动态载荷,通过模态叠加法计算机床的动态响应,得出机身冲裁过程中的最大动应力、最大动态位移以及几个关键点动态位移的时间历程曲线和模态参与因子随时间变化的三维曲面,为减小机床的振动提供实际指导。     

平纹编织C/SiC材料复杂应力状态的力学行为试验研究

利用Arcan夹具,实现对2D平纹编织C/SiC复合材料拉剪、压剪加载试验,研究材料在复杂应力状态下的力学行为,分析材料的损伤演化和失效模式。试验结果表明,材料在拉剪复杂应力状态下不同角度的应力—应变曲线呈现非线性关系;在压剪复杂应力下,随着加载角度增大,应力—应变曲线非线性程度增大。断口分析表明纯剪切形成平齐断口,受拉应力影响材料在拉剪复杂应力状态下形成斜断口。试验分析表明,适当的压应力能提高抗剪切破坏能力。偏移型椭圆失效判据与试验值吻合较好,该失效判据能较好地描述材料在复杂应力状态下的破坏行为,能较好反映材料的强度特征。     

汽车副车架安装螺栓碰撞断裂的数值模拟研究

针对10.9级M14高强螺栓碰撞断裂,设计了三种不同受力状态的螺栓强度试验,采用有限元逆向参数方法获得不同应力三轴度下的断裂极限应变,建立了螺栓材料的CrachFEM剪切断裂模型。基于副车架总成试验的数值模拟研究了变直径螺栓的抗剪切强度,为考虑碰撞要求的设计提供了可靠的模拟方法,同时也表明CrachFEM模型能够很好地预测高强螺栓的断裂失效行为。     

灯座三角盘硬质合金多工位级进模设计

分析了灯座三角盘的工艺特点,确定了排样方案及模具的结构,采用了快换式、分体式凸模结构和防误送的检测装置,冲压制件精度高,模具使用寿命高,便于维修和维护。     

剪切修正GTN模型在小冲杆试验中的应用研究

小冲杆试验作为一种非标准的微试样测试技术,能有效地获取薄板结构的材料参数。而选用合适的损伤模型对准确表征材料变形到断裂的整个过程有着重要影响。基于NAHSHON提出的含剪切修正项的Gurson-Tvergaard-Needleman(GTN)模型,通过有限元软件ABAQUS及用户自定义子程序VUMAT考察不同应力三轴度对断裂失效的影响。采用有限元模拟和拉伸试验获得冷轧硅钢材料的无损伤弹塑性力学参数以及GTN损伤演化模型中的形核参数和临界断裂参数,通过纯剪切试验和数值模拟的对比确定出材料中微孔洞的剪切变形对材料损伤演化的贡献。运用剪切修正的GTN模型对小冲杆试验进行模拟,结果表明,由于修正GTN模型考虑了微孔洞剪切畸变的对材料损伤影响,模拟结果比原GTN模型更接近于试验数据,可更好地应用于小冲杆试验的研究。     

基于微小试样法的国产A508-Ⅲ钢力学性能测试研究

小冲杆试验法是利用冲杆冲压试样薄片,并记录试片从变形到失效的全过程的载荷一位移曲线,并借此分析得到材料屈服强度、抗拉强度等力学性能的试验方法。该方法所需材料少,特别适合于试验材料缺乏或在役设备的无损取样检测的情况。采用小冲杆试验法测试国产A508-Ⅲ钢的力学性能（包括屈服强度、抗拉强度）,并且与传统的标准试样测试的拉伸性能进行了比较,结果表明采用小冲杆试验方法能够有效测试材料的屈服强度及抗拉强度。     

双向载荷下威布尔应力参量的研究

以往对双轴断裂的研究多采用线弹性断裂力学参量且限于弹性材料，局部法理论提出了威布尔应力参量，威布尔应力作为度量脆性断裂概率的参量已广泛采用。为研究双向载荷对威布尔应力的影响，文中开发了威布尔应力计算程序并进行考核验证，以威布尔应力为参量，以ASTM A36钢中心裂纹板为研究对象，结合有限元分析方法，研究二维裂纹问题在不同双向载荷比下威布尔应力参量的变化规律，从而得到双向载荷对脆断概率的影响。计算结果表明，平面应力状态下，双向载荷对威布尔应力和脆断概率基本上没有影响；而在平面应变状态下，威布尔应力和脆性断裂概率随双向载荷比增大而增大。     

Analysis of the elastic punch-substrate contact under fretting: Monotonic and cyclic loading of the punch

The object of this paper is to analyze the elastic behaviour of a 2-D contact problem between a right-angled flat punch and a semi-infinite substrate, subjected to a constant normal compressive load and a cyclic shear load using a finite element code. The knowledge of the stress and strain fields produced close to the corners of the punch under different loading conditions as a function of the friction coefficient will allow insight to be gained into the fretting fatigue problem associated to this cyclic loading. In order to better understand the behaviour and analyze the possibility of using fracture mechanics approaches to study the stress field close to the punch corners, two different models have been compared to each other: one without continuity solution between the punch and the substrate and the other with a couple of contact surfaces between them. Using the continuous model, a particularization of the general analytic solutions of Williams has been proposed. The complete stress field around the corners of the punch for this model has been obtained for any values of the punch size, normal and shear loads. Some general guides for understanding and systematizing the punch-substrate behaviour have been extracted from the above solution and that of a sliding wedge, provided by the literature, which enable a systematic numerical analysis of the problem. Further on, a more detailed study of the slip between punch and substrate, as well as of the stress field, has been accomplished using finite element analysis guided by the previous semi-analytical results. The study has been completed for the whole load process: compressive normal load, monotonic shear load, and cyclic shear load.     

Plastic bulging of sheet metals at high strain rates

The biaxial bulge test is a material test for sheet metals to evaluate formability and determine the flow stress diagram. Due to the biaxial state of stress induced in this test, the maximum achievable strain before fracture is much larger than in the uniaxial tensile test. A new dynamic bulge testing technique is simulated and analyzed in this study which can be performed on a conventional split Hopkinson pressure bar (SHPB) system to evaluate the strain-rate dependent strength of material at high impact velocities. Polyurethane rubber as pressure carrying medium is used to bulge the OFHC copper sheet. The use of hyperelastic rubber instead of fluid as a pressure medium makes the bulge test simple and easy to perform. The input bar of SHPB is used to apply and measure the bulging pressure. The finite element simulation using ABAQUS/explicit and analytical analysis are compared and show good correlation with each other. The results clearly show that as the strain-rate increases, the strength of the OFHC copper increases. From the study, a robust method to determine the material behavior under dynamically biaxial deformation conditions has been developed.     

A parameter study for static and dynamic denting

A parametric study of the factors controlling static and dynamic denting, as well as local stiffness, has been made on simplified panels of different sizes, curvatures, thicknesses and strengths. Analyses have been performed using the finite element method to predict dent resistance and panel stiffness. A parametric approach is used with finite element models of simplified panels. Two sizes of panels with square plan dimensions and a wide range of curvatures are analysed for several combinations of material thickness and strength, all representative of automotive closure panels. Analysis was performed using the implicit finite element code, LS-NIKE, and the explicit dynamic code, LS-DYNA for the static and dynamic cases, respectively. Panel dent resistance and stiffness behaviour are shown to be complex phenomena and strongly interrelated. Factors favouring improved dent resistance include increased yield strength and panel thickness. Panel stiffness also increases with thickness and with higher curvatures but decreases with size and very low curvatures. Conditions for best dynamic and static dent performance are shown to be inherently in conflict ; that is, panels with low stiffness tend to perform well under impact loading but demonstrate inferior static dent performance. Stiffer panels are prone to larger dynamic dents due to higher contact forces but exhibit good static performance through increased resistance to oil canning.