Investigation of the viscous and thermal effects on ductile fracture in sheet metal blanking process

In this paper, a methodology is proposed to predict the ductile damage in the sheet metal blanking process using a coupled thermomechanical finite-element method. A constitutive material model combined with the ductile fracture criteria was used. The effect of material softening due to the heat generated during plastic work in a specimen was considered in blanking simulations. To verify the validity of the proposed model, several blanking simulations are performed and the results compared with those obtained from an experimental study. The interaction of fracture initiation and temperature distribution in the sheet metal during the process was studied. The effect of velocity and the clearance on the product shape were examined. It was seen that at high punch speeds the viscous and thermal effects have significant effects on product quality.     

On the tearing energy of a ductile thin plate

The tearing energy of a ductile thin plate has long been considered as a material constant. This concept of energy is regarded useful for design of energy absorption components. However, it has been noticed that various existing test data for the tearing energy are not consistent. This indicates that the tearing energy may not be a constant determined by material only. Other factors, such as loading mechanisms and geometry may be influential. In the present study, we propose a “mode” concept for in-plane tearing tests by introducing a mixity parameter γ, for in-plane tearing. Through the finite element modelling, the mixity parameter is calculated from the principal stresses in front of tear tips. Laboratory tests were also conducted on samples with two different geometric configurations. The first set of the tests, conducted on the double-notch-tensile specimen, provided the tearing energy, while the second set of tests on the unnotched uni-axial tensile specimen served the purpose of verification.     

A micromechanical model for inelastic ductile damage prediction in polycrystalline metals for metal forming

This paper deals with micromechanical modeling of ductile damage and its effects (coupling) on the plastic behavior of FCC polycrystalline metallic materials. The ‘fully coupled’ constitutive equations are written in the framework of rate-dependent polycrystalline plasticity where a ‘ductile’ damage variable has been introduced at a crystallographic slip system (CSS) scale in order to describe the material degradation by initiation, growth and coalescence of microdefects inside the aggregate. Both, theoretical and numerical (FEA) aspects of the proposed micromechanical coupled model are presented. The ability of the obtained model to predict the plastic strain localization, due to the ductile damage effect, in the classical tensile test is carefully analyzed. Application is also made to the fracture prediction in deep drawing of a cylindrical cup using a thin sheet. Finally, some concluding remarks and perspectives are pointed out.     

Experiments and numerical approaches to ductile tearing in an 2024-T351 aluminium alloy

Ductile fracture of an 2024-T351 aluminium alloy has been investigated using a central crack panel (CCP). In order to predict rupture, two models using the local approach to fracture mechanics were verified: the uncoupled Rice and Tracey void growth model and the coupled Rousselier model based on continuum damage mechanics. A finite element analysis has been performed in order to verify the capability of these models to predict the crack extension.     

Prediction of optimum clearance in sheet metal blanking processes

The blanking process and structure of the blanked surface are influenced by both the tooling (clearance and tool geometry) and properties of the workpiece material (blank thickness, mechanical properties, microstructure, etc.). Therefore, for a given material, the clearance and tool geometry are the most important parameters. The objective of the present work is to develop a methodology to obtain the optimum punch-die clearance for a given sheet material by simulation of the blanking process. A damage model of the Lemaitre type is used in order to describe crack initiation and propagation into the sheet. A comparative study between numerical and experimental results shows good agreement.     

The effect of anisotropy in fracture toughness and yield strength in the tearing of ductile sheet materials

The dependence with orientation of the yield strength and tearing fracture toughness of cold-rolled 64/36 -bass and acetate sheets was determined. Parallel strips were cut into larger sheets of the same materials, rolled up on a slotted key, and converged to form pointed tongues which became detached from the larger sheets. The tongue length: width aspect ratio varied with direction in the plane of the sheet. A rigid-plastic fracture mechanics model of sheet tearing, which explained tear path convergence on the basis of minimum work rate [1] is extended to include ‘lift-off’ of the sheet under the slotted roller and gives agreement between the measured aspect ratios and the variation with direction of the independent determinations of yield strength and toughness.     

Size effects in micro blanking of metal foil with miniaturization

Miniaturization of micro blanking process caused changes in the relative contribution of relevant process parameters and the grain size of metal foil reaches the same order with the blanking clearance. As a result, the size effects occur, which is different from the other micro forming processes, such as micro upsetting, micro tensile, and micro bending. In the paper, the size effects of micro blanking of metal foil with miniaturization were investigated. Similar to the traditional blanking process, the micro blanking process is still composed of three stages of elastic deformation, plastic shearing deformation, and fracture separation. However, the maximum blanking force appeared at punch stroke of foil thickness, which is much larger than the transitional blanking process of 1/3 of work material thickness. The cross section of micro-hole is also composed of rollover, smoothly shearing zone, fraction zone, and burr. But the distribution of the fraction zone and burr is uneven, which is caused by the influence of the material anisotropy with the decreasing of the foil thickness. The results show that the ultimate shear strength of micro blanking is decreasing with the increasing of the length scale ??, not only for the brass foil, but also for the stainless steel foil. The fracture mechanism of micro blanking of brass foil is significantly changed from shear dimple to slip separation with the decreasing of length scale ??, but the fracture mechanism of micro blanking of stainless steel is not changed and kept ductile fracture with equiaxed dimples     

薄板微小齿轮落料模的试制与探讨

在薄板微小齿轮落料模的试制过程中 ,分析了凸、凹模的加工方法及其间隙确定、研配等工艺方案的可实施性。采用上、下冲裁法 ,提高变形区静水压力 ,解决了冲裁中出现的齿尖塌陷、毛刺等缺陷。指出适当提高相对变形速度、提高静水压力 ,可有利于改善微小型制件的冲裁质量。     

A mechanistic model for the prediction of ductile erosion

Mechanisms operative in dust erosion of ductile materials were determined with the aid of scanning electron microscope studies. Dimensional analysis was employed in the development of a mathematical model for predicting the erosion of ductile materials. The basis of the model was an identified erosion mechanism (target melting) and the model was verified in an erosion testing program. The target materials in the testing program were three stainless steels, two aluminum alloys, a beryllium copper alloy and a titanium alloy. The erosive agents were three dusts with hard angular particles and one dust with spherical particles. Maximum particle velocities were 130 or 250 ms^?1.     

X射线成像波带片及制作

研究了X射线成像波带片的工作原理和制作工艺。从理论上分析了波带片的空间分辨率与最外环宽度的关系,以及波带片衍射效率与厚度和折射率的关系。利用国家同步辐射实验室发展的加工工艺,即电子束光刻技术和X射线光刻技术结合制作波带片。实验结果表明:波带片最外环宽度为150 nm,高宽比为4,基本满足高分辨X射线成像波带片的高空间分辨率、大高宽比、高精度等要求。     

铸造模具钢镶块刃口在板材冲裁过程中的动态显式有限元分析

应用ANSYS/LS-DYNA软件动态模拟了汽车覆盖件中厚度板材冲裁成形过程,得到了汽车覆盖件冲裁模镶块刀口应力场应变场的分布情况,进而分析了造成铸造模具钢镶块刃口崩刃、塌陷、横裂等失效现象的机理.     

Analysis of conical tool perforation of ductile metal sheets

The perforation of a ductile metal sheet with a conical tool is accompanied by elasto–plastic bending, stretching, plastic flow and perforation initiation and propagation and ultimately it results in material fracture in the form of petals. The number and size of petals depends on the sheet thickness, material properties, tool angle, anisotropy in the material and indentation speed. In this work the mathematical relations for the fracture mechanism has been developed to analyze different parameters’ response and evaluate fracture toughness of the metal sheets of various thickness using computer code based on this analysis.     

A phenomenological model of ductile fracture for API X65 steel

This paper presents a phenomenological model of ductile fracture for the API X65 steel using the Gurson–Tvergaard–Needleman (GTN) model. Experimental tests and FE damage simulations using the GTN model are performed for smooth and notched tensile bars, from which the parameters in the GTN model are calibrated. Comparison of experimental data of pre-strained, notched tensile and fracture toughness tests with finite element (FE) damage analyses show good agreements, suggesting the validity of the calibrated parameters. As application, the developed GTN model is applied to predict the pre-strain effect on deformation and fracture and the results are compared with experimental data.     

Numerical modelling of ductile plastic damage in bulk metal forming

This work addresses the computational aspects of a model for rigid–plastic damage. The model is a modification of a previous established model formulated by Perzyna (Recent Advances in Applied Mechanics, Academic Press: New York, 1966, p. 243–377 (Chapter 9)) which is here extended to include isotropic damage. Such an extension is obtained by incorporating the constitutive equations introduced by Lemaitre (J. Eng. Mater. Technol. 107 (1985) 83; Comput. Meth. Appl. Mech. Eng. 51 (1985) 31; A Course on Damage Mechanics, Springer, Berlin, Heidelberg, New York, 1996) for ductile plastic damage into the original model. In its original version (J. Eng. Mater. Technol. 107 (1985) 83; Comput. Meth. Appl. Mech. Eng. 51 (1985) 31) this model does not distinguish tension and compression in the damage evolution law, so it was necessary to introduce a refinement proposed by Ladevèze (in: J.P. Boehler, (Ed.), Proceedings of CNRS International Colloquium 351 Villars-de-Lans, France (Failure Criteria of Structured Media, 1983, p. 355) and Lemaitre (A Course on Damage Mechanics, Springer, Berlin, Heidelberg, New York, 1996) which takes into account the partial crack closure effect with isotropic damage. The accuracy of the computational model, developed for the analysis of the material degradation in bulk metal forming processes, is shown through the discussion of the results of two examples, allowing to compare the simulation results with experimental and numerical results obtained by other authors.     

On deformation and tearing of stiffened and un-stiffened square plates subjected to underwater explosion—a numerical study

In modern combat operations warships can be subjected to underwater blast loads capable of causing considerable structural damage. Research in the field of underwater explosion effects on structures has seen systematic developments since world war-I (WW-I) with the increased awareness that the possible underwater explosions and threats from the hunter killer “U” boats could now be countered. Most of the earlier investigations were conducted by military and these were classified. Cole [1] established empirical relations to model the underwater explosion (UNDEX) loading, which were the outcome of numerous experimental investigations done by the military agencies. In the present study the UNDEX load profile has been modeled as an exponentially decaying shock wave, which varies spatially and transiently. The failure modes in high strength (HS) and WELDOX Steel rectangular plates was predicted using an elasto-plastic model with isotropic hardening, strain rate effects and fracture criterion under clamped edge conditions. These models were implemented in the nonlinear finite element code ABAQUS/Explicit. The present study predicts and establishes the failure modes using a 3D FE analysis.     

Elastoplastic buckling of rectangular plates in biaxial compression/tension

This paper examines the elastoplastic buckling of a rectangular plate, with various boundary conditions, under uniform compression combined with uniform tension (or compression) in the perpendicular direction. The analysis is based on the standard linear buckling equations and material behaviour is modelled by the small strain J₂ flow and deformation theories of plasticity. A detailed parametric study has been made for Al 7075 T6 over a range of plate geometries (a/b=0.25–4,a/h≈20–100) and with three sets of boundary conditions (four simply supported boundaries and the symmetric combinations of clamped/simply supported sides). For sufficiently thin plates we recover with both theories the classical elastic results. However, for thicker plates there is a remarkable difference in the buckling loads predicted by these two theories. Apart from the expected observation that deformation theory gives lower critical stresses than those obtained from the flow theory, we report on the existence of an optimal loading path for the deformation theory model. Buckling loads attained along the optimal path—specified by particular compression/tension ratios—are the highest possible over the entire space of loading histories. By contrast, no similar optimum has been found with the flow theory. This discrepancy in the buckling behaviour, obtained from the two competing plastic theories, provides a possibly new illustration of the plastic buckling paradox.     

Sliding wear of an unlubricated frictional combination of ductile metal and brittle abrasive

The concept of determining wear from hypotheses concerning shear effects in metal surface regions was applied to the sliding wear of a smooth elastoplastically deformable metal substance and a rough brittle abrasive. Interfacial layers or lubricating depositions on either surface were neglected. The form of the contact was modelled using random fields for the roughness profiles. The resulting intensities of wear may be compared with published experimental results. The wear intensity level was quantitatively determined. Dependence on the nominal pressure and properties of the rough abrasive is thus explained. Instead of hardness the compressive strength of abrasive peaks is the dominant characteristic parameter.     

Finite element analysis of the thermal effect in high-speed blanking of thick sheet metal

Investigating the shear zone temperature is one of the major challenges in the study of high-speed blanking in thick sheet metals. Several studies have shown that a clear temperature change occurs even at low speed and that this significantly influences the blanked edge quality and die life; however, the thermal effects of high-speed blanking of thick sheet metals have received comparably little attention. This study proposes a methodology to predict the temperature distribution in the blanking process using a coupled thermo-mechanical finite element method for thin phosphorous bronze. The finite element model of blanking was developed to characterize the quality of the blanked edge for different punch speeds. The effect of material softening due to the heat generated during plastic work was considered in the blanking simulations. To verify the validity of the proposed model, several simulations were performed and the results, such as the blanking force and proportion of sheared area, were compared with those obtained from experimental studies. It was found that the thermal effect in thin phosphorous bronze at high punch speeds does not significantly affect the product quality. Therefore, a higher productivity can be attained while maintaining a high quality of the finished product using high-speed blanking.     

纳米尺度金属薄膜在拉伸状态下的稳定性

将薄膜和基底作为一个基本结构来研究薄膜的变形和损坏可预测纳米薄膜器件的使用寿命.本文讨论和研究了薄膜基底结构在拉伸载荷下薄膜出现的分叉和断裂的过程.用在相同厚度的PET基底上沉积不同厚度铝膜的薄膜基底结构作为试件,分别对薄膜厚度为100,150和200 nm的3种不同试件进行了拉伸加载实验,并在OLYMPUS显微镜下观...     

基于SolidWorks的复杂相贯圆锥体钣金下料方法

本文针对传统钣金下料在实际工程应用中的缺点,以复杂相贯圆锥体为例,提出了一种基于SolldWorks钣金下料新方法,利用计算机三维设计分析软件SolidWorks的钣金下料功能,对钣金件进行实体造型,再进行抽壳,切割,转化为钣金件,展开,快速生成下料图,突破了传统作图法、计算法的束缚.其过程具有省时、计算量小、精确度高等优点,大大地提高了钣金下料的效率.