A study on chevron crack formation and evolution in a cold extrusion

A numerical algorithm based on the element deletion method and rigid-viscoplastic finite element approach depending on Cockcroft-Latham and specific plastic work fracture criteria was applied to predict formation and evolution of possible cracking in a cold extrusion of aluminum and steel alloys. The Cockcroft-Latham fracture criterion induced an internal crack while an external crack occurred owing to the specific plastic work criterion in simulations. As a result, the Cockcroft-Latham criterion was found to be valid for predicting chevron cracking in comparison with the experimental observation available in the literature. Using the Cockcroft-Latham criterion, cracking was carefully investigated in terms of the size of the crack and gap distance between cracks depending on the number of elements and boundary condition at the punch interface. The critical damage values for the Cockcroft-Latham fracture criterion were also calculated based on the tensile instability and fracture conditions to investigate their effect on possible cracking. Finally, a processing map based on the Cockcroft-Latham fracture criterion for preventing chevron cracking in the cold extrusion of commercially available steel alloy was developed by considering processing parameters such as reduction in area and semicone angle. According to this investigation, the developed element deletion method with the Cockcroft-Latham fracture criterion was reasonably accurate for carrying out chevron cracking analyses in the cold extrusion with proper selection of a critical damage value.     

Indentation of Polytetrafluoroethylene (PTFE) Thin Film: Simulations by Using Continuum Damage Mechanics

Thin compliant films on relatively hard substrates have a wide range of applications. In this work, continuum damage mechanics is used to simulate indentation of a 10-μm-thick polytetrafluoroethylene (PTFE) film deposited on glass for different load levels by finite element analysis. The results, compared to experiments, are useful in investigating the mechanics of wear and friction of soft thin films. The material model is elastic–plastic before damage initiation and includes linear damage progression thereafter. The effects of ductile and shear damage criteria and two parameters pertinent to the damage model, the equivalent plastic strain for damage initiation and the bulk fracture toughness, on the indentation are investigated. It is shown that the shear damage model is more suitable to characterize the indentation of the PTFE thin film. The bulk fracture toughness has greater significance with regard to damage compared to equivalent plastic strain at the onset of damage initiation. Comparison of simulation and experimental results shows that bulk fracture toughness of the thin PTFE film is approximately 20 J/m². This value is lower than that for the bulk PTFE, and the difference is attributed to the thin-film nature of the case considered here.     

Ti-6Al-4V燕尾榫结构微动疲劳裂纹萌生及扩展行为研究

针对Ti-6Al-4V钛合金燕尾榫连接结构在不同载荷下的微动疲劳现象,采用榫形微动疲劳试验进行研究,并对裂纹萌生扩展、微动磨损及断口进行分析。结果表明,微动疲劳使构件疲劳寿命显著降低约70%;疲劳载荷对微动裂纹扩展的影响比对裂纹萌生的影响更大;微动疲劳裂纹起始于接触面边缘,与接触表面约成45°角,裂纹扩展到60~150μm后转向与接触表面垂直;微动疲劳断口形貌表面在微动磨损区具有多个裂纹源点,但只有一个主裂纹形成。     

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.     

焊缝金属中氢、氮含量与氢裂敏感性的关系

试验和分析结果表明．铁素体中氢致裂纹敏感性的大小取决于氢原子在裂纹尖端局部区域扩散聚集的速度。原始氢含量的提高对断裂应力的影响是由于其降低裂纹尖端局部区域材料的断裂韧性，含氢量越高，Ｋ下降的速度越快。含氮量较低时，随含氮量提高，铁素体的氢裂敏感性增加，含氮量超过一定值后，提高含氮量会降低铁素体的氢裂敏感性。     

铝合金管材无芯模冲切过程的有限元模拟

对铝合金管的冲切过程进行了有限元模拟,刀具采用刚体模型,工件采用弹塑性模型,材料断裂破坏采用Normalized Cockcroft&Latham破坏准则,分析了冲切过程中冲切力、等效应力的变化,得出了弹性变形、塑性变形、塑性变形至裂纹扩展、断裂始终贯穿于整个冲切过程之中。通过模拟切口与实际切口的对比以及模拟切屑形状与实际切屑形状的对比,说明了模拟过程的有效性。     

高温长期运行Cr-Mo钢断裂性能微试样测试实验研究

开展采用小冲杆微试样试验技术评定材料断裂性能的研究.针对2.25Cr-1Mo钢(脆化态和脱脆态)及1.25Cr-0.5Mo钢,将小冲杆试验测试结果与常规冲击韧度及断裂韧度试验结果相关联,得到小冲杆试样变形过程中消耗的总变形能与常规夏比冲击功间的对应关系经验公式,以及小冲杆试样等效断裂应变与材料延性断裂韧度间的关联式,说明可以直接采用小冲杆试验结果估算材料的冲击及断裂韧度值,为无法取标准试样进行材料抗断裂性能测试场合提供了一种可行的间接测试技术.     

Investigation of deformation and collapse mechanism for magnesium tube in axial crushing test

This paper demonstrates the quasi-static axial compression and high-speed axial compression tests of extruded magnesium alloy circular tubes for evaluating the crash and fracture behavior of mg parts. To capture the buckling and fracture behavior of Mg tube during the axial compression tests, FE simulation adopts different types of flow curves depending on the deformation mode such as tension and compression with LS-DYNA software. The Mg tube undergoes compressive plastic strain prior to buckling while according to the model based on Hill yield criterion only bulging along the radial direction is predicted. Due to the tension-compression asymmetry of Mg alloys, diameter of Mg tube expands largely at the initial plastic strain before having bulging or folding while only a bulging mode typical for materials with cubic crystal structure can be predicted. Simulation results such as punch load and deformation mode are compared with experimental results in the axial crushing test with AZ61 alloy.     

Failure in single point incremental forming

This paper revisits the formability limits of single point incremental forming (SPIF) in the light of fundamental concepts of plasticity and ductile fracture mechanics. The paper has a twofold objective of investigating the limiting strain pairs at fracture in parts showing and not showing signs of necking before cracking and of demonstrating that failure by fracture occurs by tension in crack opening mode I. The overall methodology is based on the combination of circle grid analysis, measurement of the ‘gauge length’ strains at fracture and determination of fracture toughness from experimental tests performed with truncated conical SPIF parts and double edge notched test specimens loaded in tension. The work is performed in aluminium AA1050-H111 and is a step towards comprehension of the circumstances under which failure will occur in SPIF. It is shown that fracture strain pairs of truncated conical parts, fracture forming limit lines (FFLs) determined from conventional sheet formability tests and fracture toughness in crack opening mode I can be merged to create a new understanding of plastic flow and failure by fracture above the onset of necking.     

A numerical/experimental study of contact damage in non-planar porcelain/metal bilayers

In this report, we investigate and visualize the effect of shape irregularity on contact damage in a brittle coating on a stiff metal substrate. Hertzian contact damage in a dental porcelain layer of thickness between 0.25 and 0.75 mm, fused onto a Ni–Cr alloy substrate in both curved and planar geometries was studied with the aid of the finite element method and experimental investigation. Three failure modes were examined with varying porcelain layer thickness: cone cracking at the upper surface of the porcelain, median or interface cracking at the layer/substrate interface and plastic deformation below the contact area in the substrate. It is shown that curvature has very little effect on the initiation of surface cone cracks in this system, but substantial effect on the initiation of interface radial cracks. In particular, curvature reduces the critical load for the onset of interface cracks.     

Fracture prediction of thin plates under hemi-spherical punch with calibration and experimental verification

The response of thin clamped plates subjected to static punch indentation is investigated experimentally, analytically and numerically to determine the onset of fracture. The accumulated equivalent plastic strain with stress triaxiality as a weighing function is introduced as ductile fracture criterion in the finite-element simulation and analytical prediction. The fracture criterion was calibrated by finite-element simulations of uniaxial tensile tests. Based on the calibration, and calculated distributions and histories of stress and strain, the critical location, and penetration to fracture were predicted within 5–10% accuracy for three punch radii.The plots of force–penetration and locations of fracture initiation in the static punch indentation tests were compared with finite-element simulations and analytical approximations showing good agreement. The transverse deflection profiles of the plates at the point of fracture obtained numerically were shown to agree well with the closed-form solution derived by taking into account a variable stress ratio and varying stress triaxiality. The strain distribution along the plate radius is influenced by the friction between the interfaces of punch and plate. By changing the friction coefficient, the fracture-forming limit diagram was constructed numerically. The present procedure can replace the time-consuming experimental technique in which the strain path is controlled by changing the radius of a cut off.     

Fracture toughness of CFRP laminated plates according to resin content

CFRP(carbon fiber reinforced plastic) has recently found wide use in different industries. The material, however, is very prone to damage from collision with foreign objects. This study aims at finding Ĵ-integral in mode II for CFRP laminated plates based on classical bar theory for dynamic conditions in consideration of inertia forces and eventually to finding dynamic inter-layer fracture toughness. Dynamic inter-layer fracture toughness was observed using an in-house ENF (End Notched Flexure) experimental facility using Split Hopkinson’s Bar (SHPB). Also the variation of the fracture toughness depending on different resin contents and fiber arrangement in the CFRP specimen ([0°3/90°3/0°6/90°3/0°3], [0°20], [0°5/90°10/0°5]) was observed. It was established that under both quasi-static and dynamic load conditions, the critical load and the inter-layer fracture toughness increased sharply following the extension of the resin content. Thus, it may be concluded that the resin content is the major factor determining the inter-layer fracture toughness in the CFRP laminated plate.     

脆性剖分类零件启裂数值分析

裂解槽作为初始裂解源其尖端的应力状态尤其是厚度方向的应力分布直接影响着剖分类零件的裂解性能,是裂解加工成败的关键。文中利用ABAQUS有限元软件,对脆性剖分类零件(轴承座)启裂进行了分析。分析结果表明:裂解槽应力集中效果明显,随着载荷的增加,裂解槽根部的最大正应力逐渐增加,趋近于材料的断裂强度;最大正应力从裂解槽的中间部位向两侧均匀递减;对于脆性材料,最大正应力出现在裂解槽顶端,启裂发生在工件中心对称面裂尖附近,这种启裂位置的唯一性有利于获得高质量的断裂面,减少裂解缺陷。     

Experimental and numerical investigations of mode I delamination behaviors of woven fabric composites with carbon, Kevlar and their hybrid fibers

This paper studied the effect of the hybridization of carbon and Kevlar fibers on mode I interlaminar fracture toughness and crack propagation behaviors with double cantilever beam (DCB) tests. The crack propagation characteristics, crack growth trend and rate, and fracture surfaces were observed using an optical microscope and SEM micrographs for the three different types of materials. Moreover, details of the stress distribution around the crack tip and the crack propagation pattern across the width of the DCB specimen were investigated using the finite element method, including a cohesive element. The mode I interlaminar fracture toughness of carbon-Kevlar hybrid/epoxy was nearly average for carbon/epoxy and Kevlar/epoxy. The maximum load predicted by the numerical method showed good agreement within an error of 5% with the experimental results.     

Evaluation of interfacial friction condition by boss and rib test based on backward extrusion

Proper consideration of tribological problems at the contact interface between the tool and workpiece is crucial in metal forming, since interfacial friction condition plays an important role in metal forming by influencing the metal flow, forming load, die wear, etc. In order to quantitatively estimate such friction condition, a new friction testing method “Boss and Rib Test” based on the backward extrusion process is proposed in this work. In boss and rib test, a key design is to use a tube-shaped punch so that the boss and rib at the deforming workpiece along the inner and outer surfaces of the punch are formed during backward extrusion. It was experimentally and numerically revealed that the heights of the boss and rib vary according to the friction condition applied. It was also found that the height of the boss is higher than that of the rib when the friction condition at the contact interface is severe. From this finding, the shear friction factor can be evaluated according to lubricant characteristics assigned. In addition, simulation results revealed that calibration curve demonstrating deformation pattern of the workpiece is affected by strain-hardening exponent of the workpiece.     

Evaluation of surface cracking in micron and sub-micron scale scratch tests for optical glass BK7

In order to obtain the fundamental information on the deformation and fracture behavior of brittle materials during precision and ultra-precision grinding, micron and sub-micron scale scratch tests were conducted on optical glass BK7 using Vickers indenters. Three types of surface cracking were observed around the scratch grooves. They are lateral cracking, radial cracking and cracking in front of the moving indenter. It is found that lateral cracking is the main damage type due to its large damage size and low crack initiation load. The effect of surface cracking on the relationship between the normal load and the square of scratch depth was studied. The plastic zone size as well as the sliding blister field strength was expressed as a function of the contact zone size of the indenter. A prediction model for the size of damage zone induced by lateral cracking was established and was compared with experimental results.     

Investigation of sheet metal forming by bending—Part IV: Bending and wrinkling of circular sheets pressed by a hemispherical punch

This paper reports on an investigation of the bending forming processes in circular sheets pressed by a hemispherical punch into a conical die. Both theoretical and experimental considerations are provided. It is pointed out that the deformation mechanism of such a circular plate is different from that of the workpiece in the conical die cup test studied previously in part III of this series [Int. J. Mech. Sci.31, 327–333 (1989)]. The theoretical analysis shows that a central gap appears between the plate and the hemispherical punch at small deflections and disappears when the punch force increases beyond a certain magnitude. It is then shown that the previous approximate analytical treatment of the distribution of punch force is no longer appropriate to the wrinkling of the plate. It is shown that, as in the case of the conical die cup test, the circumferential wave number for the wrinkling mode corresponding to the critical wrinkling load of the present workpiece is four. In addition, the paper reveals that a doubling of the wrinkling wave may occur in the first stage of the forming process.     

Tribological analysis of fracture conditions in thin surface coatings by 3D FEM modelling and stress simulations

A tribological analysis of deformations and stresses generated and their influence on crack generation and surface fracture in a coated surface loaded by a sliding sphere in dry conditions is presented. A three-dimensional finite element method (3D FEM) model has been developed for calculating the first principal stress distribution in the scratch tester contact of a diamond spherical tip with 200 μm radius sliding with increasing load on a 2 μm thick titanium nitride coated steel surface. The model is comprehensive in that sense that it considers elastic, plastic and fracture behaviour of the surfaces. The hard coating will be stretched and accumulates high tensile stresses. At the same time, it is carrying part of the load and thus reducing the compressional stresses in the substrate under the sliding tip. The first crack is initiated at the top of the coating from bending and pulling actions and it grows down through the coating. The fracture toughness of the coating is calculated by identifying from a scratch test experiment the location of the first cracks and the crack density and using this as input data.     

Fabrication of clean cut surface on high strength steel using a new shaving die design

A shaving process is commonly applied to achieve a smooth cut surface thorough the workpiece thickness and a square cut-edge, also known as a finishing operation. However, this process is rarely successful for high-strength steel sheets, which is a major problem. In the present study, finite element method (FEM) simulation was used to clarify the main causes of this problem by comparing the shaving mechanisms between medium carbon steel grade SPCC (JIS) and high-strength steel grade SPFH 590 (JIS). Results show that in the case of SPFH 590 based on material flow, stress distribution, and strain distribution analyses, the shaved chip was difficult to form by sliding along the punch face. Moreover, the tensile stress generated in the shearing zone was increased and readily generated cracks. The shaving process was developed in the present study by generating the cutting-edge angle and rake radius on the punch. The cutting edge angle was designed to generate high compressive stress in the cutting-edge vicinity and shearing zone, and the rake radius was designed to tear a shaving allowance off and move it along the rake radius instead of moving downward along the punch movement direction, thereby decreasing the tensile stress in the shearing zone. Under these mechanisms, the increases in the generated tensile stress in the shearing zone could be delayed, and cracks could thus be prevented. The effect of the punch geometry on the cut surface characteristics and cutting forces were also investigated. Laboratory experiments were performed to validate the FEM simulation results. Experimental results agreed well with the FEM simulation results. Therefore, a smooth cut surface thorough the workpiece thickness of high-strength steel sheets could be successfully achieved by using the developed shaving process.

     

小冲杆试验评价材料的断裂韧度

为了评价材料断裂韧度，通过小冲杆试验对2．25Cr1Mo的回火脆态、脱脆态、焊缝区以及1．25Cr0．5Mo钢进行了试验研究，并对所得的小冲杆试验数据进行了分析。结果表明小冲杆等效断裂应变εe与小冲杆能量Esp之间存在重要的线性关系；同时发现断裂点的小冲杆能量值与材料的断裂韧度值线性相关。该方法为无法获取标准试样进行材料抗断裂性能评价场合提供了一种可行的测试技术。