拉-压及拉-扭载荷下6063合金的疲劳行为

对6063锻造铝合金进行了不同应力幅值下拉-压及拉-扭复合疲劳试验,并用透射电子显微镜观察了疲劳失效试样的位错结构。结果表明:在循环加载过程中,循环硬化占据主要地位,循环硬化的速率和程度对应力幅值和加载路径有依赖性;相同应力幅值下,试样在拉-扭复合加载下失效比拉-压失效形成更为复杂的位错结构,且位错密度更高;位错之间及位错与析出相的交互作用是材料发生循环硬化的主要原因,循环硬化程度越高,疲劳寿命越短。     

Field Emissions Caused by Fracture and Yielding

Two effects, magnetic and electric emission, have been observed and used in fracture tests to detect and analyse onset and progress of plastic deformation and fracture. While magnetic emission (ME) occurs only in ferromagnetic materials, electric emission (EE) can be found with any material, metals as well as plastics, ceramics, glass, and others. In ferromagnetic materials the two effects supplement each other. In steel, for example, electric emission is produced by plastic deformation but not by fracture; magnetic emission, on the other hand, is produced by fracture but not by plastic deformation. This paper describes the effects, demonstrates their application in Charpy tests, and shows how to derive fracture parameters, for instance the instants of the onset of plastic deformation and fracture. ME and EE, therefore, yield an attractive, inexpensive and fast supplement to conventional fracture test methods.     

77MnA盘条断裂原因分析

通过宏观观察,夹杂物评级和显微组织观察,分析了77MnA盘条拉拔过程中断裂的原因.实验结果表明:马氏体的出现和基体为索氏体、屈氏体和少量的贝氏体的混合组织是断裂的主要原因;基体组织为混合组织和马氏体的出现是由于轧件出精轧机组进斯太尔摩控制冷却线时冷却温度过低造成的;同一直径钢丝心部的马氏体含量相对边部的马氏体含量多表明存在C、Mn、Cr元素的偏析,并提出改进措施.     

Mechanical Behavior and Evaluation of Dented Pipe Caused by Cylindrical Indenter

Journal of Failure Analysis and Prevention - The defect of pipe dent has a great threat to the safety of oil and gas pipe transportation. In this paper, the numerical calculation model of pipe...     

受扭圆柱壳的大范围皱曲

本文给出受扭圆柱壳大范围皱曲的一种变形模式,从而求出不同转角时壳体的轴向缩短量以及壳体所吸收的塑性变形能的变化,计算结果和实验相符较好。     

Analysis of Fracture Surfaces of Ceramic Rods under Short-Term and Long-Term Bending

Analysis of fracture surfaces under short-term and long-term bending shows an increasing role of tangential stresses in fracture under conditions of long-term loading. It has been found that when developing rapid methods of life prediction, it is expedient to investigate the kinetics of shear cracks rather than mode I cracks.     

Dynamical Tension of a Hollow Cylindrical Specimen with Circular Crack

We propose a method for determining dynamic stress intensity factors for hollow cylindrical specimens with circular crack of finite length under impact tension. The solution of this problem is reduced to the numerical solution of an eigenvalue problem with the use of experimental loading–time diagrams. Simple formulas are obtained for determining dynamic stress intensity factors, depending on the history of loading of a specimen. The comparison of the values of dynamic stress intensity factors calculated by the formulas indicated and by the finite-element method testifies to the reliability of the results obtained.     

客车直拉杆断裂分析

通过宏观检验、化学成分分析、断口分析以及金相检验,对客车直拉杆断裂的原因进行了分析。结果表明:直拉杆的146°弯处在使用前存在微裂纹,在客车行驶过程中该直拉杆受交变应力的作用,微裂纹以疲劳的方式扩展,当扩展到一定程度,剩余截面不能承受外力作用时发生断裂。     

Fracture Assessment of U-Notched Graphite Plates Under Tension

The experimentally obtained tensile load-bearing capacity of fifteen U-notched polycrystalline graphite plates reported in literature was theoretically estimated by means of two well-known brittle fracture models, namely the mean stress (MS) and the point stress (PS) criteria. The results showed that while the mean discrepancies between the experimental and the theoretical results for both the models are very good and approximately equal, the discrepancies are significantly different for various notch tip radii. Meanwhile, the results of MS and PS criteria were compared with the results of the strain energy density (SED) criterion reported in literature. Relatively similar value of mean discrepancy was also obtained for the SED model. It was demonstrated in this research that for small values of the notch tip radius, the MS model is the most appropriate failure criterion while the PS and SED criteria are much better models for medium radii. Moreover, for large notch tip radii, the MS and PS criteria are better choices for tensile fracture assessment of U-notched graphite plates than the SED criterion.     

A1-Si过共晶合金拉伸断裂过程的研究

以铸造Al-Si过共晶合金为研究对象,采用现场金相显微镜对Al-Si过共晶合金的断裂过程进行观察,探讨了两相合金的断裂机理。试验发现:Al-Si过共晶合金的断裂首先发生在先析Si相的凹角处,随着外力的增加,裂纹向先析Si相内部进行扩展,达到相界处时改变方向,沿着Al-Si两相的相界进行扩展。分析认为:Al-Si两相合金中,相界的结合强度最弱,裂纹的扩展是择弱进行的。     

Longitudinal Fracture Analysis of Nonlinear Elastic Circular Shafts Loaded in Torsion

Strength of Materials - The longitudinal fracture of circular shafts loaded in torsion is analytically investigated. It is assumed that a longitudinal crack with a circular crack front is...     

On the Determination of the Stress State of a Solid Cylinder in Tension with Torsion

The solution of problems on the determination of the stress state within the framework of flow theory with isotropic hardening has been obtained for a solid cylinder made of a material sensitive to the strain rate under conditions of proportional loading in tension combined with torsion. An analysis of the obtained expressions has been performed. The results of tensile tests with simultaneous torsion under conditions of superplasticity were used to plot a stress-strain diagram for titanium alloy VT9 with the use of the expressions for the stress intensity.     

Fracture Analysis on a Cylindrical Shell Section of the Low-Pressure Absorption Tower

Cracking of a cylindrical shell section from an absorption tower occurred during the hydraulic pressure testing. In order to find out the cause of failure, the cracked cylindrical shell section was inspected and destructively analyzed. Optical microscopy was performed to evaluate the basic microstructure of the material used to fabricate the cylinder and the effect of welding on the microstructure. The fracture surface was examined in a scanning electron microscope, and the effects of the heat treatment temperature on the structure and properties of 410S/16MnR explosively clad plate were investigated. Detailed metallographic studies indicated that bainite existed in the base layer of the explosively welded material. The weld-induced residual stress in the crack origin was investigated by a three-dimensional (3D) finite element model. The numerical result was consistent with the fracture analysis supporting the conclusion that cracking was caused by the poor mechanical properties of the explosively clad plate and that the poor properties were induced by improper heat treatment after explosive welding. The welding residual stresses also contributed to the failure process.     

Measurement of Specific Fracture Energy and Surface Tension of Brittle Materials in Powder Form

Studies of the influence of specimen geometry and size–effect on the K _R –curves and the related fracture parameters were carried out by the authors (Kumar and Barai 2008b). The present paper is a supplementary contribution and reports interesting results related to the effect of the loading condition and size–effect studies on the K _R –curves associated with the cohesive stress distribution for complete fracture process, the double–K fracture parameters, the CTOD–curves and the process zone length using two different loading conditions (i.e., three–point bending test and four–point bending test). The laboratory size specimen with initial–notch length/depth ratios 0.3 and 0.5 are considered in the work. The load–crack opening displacement curves for these loading conditions are obtained using well known version of fictitious crack model.     

等温锻造TiAl合金超塑拉伸断裂机理的研究

通过对等温锻造ＴｉＡｌ合金的超塑拉伸断口和组织的扫描电镜观察，研究了其超塑拉伸断裂的机理。发现等温锻造ＴｉＡｌ合金在超塑拉伸过程中，不同形态的孔洞的产生，长大和连是导致试样断裂的主要机制。     

The Influence of Inertial Effects on the Fracture of Rapidly Loaded Compact Tension Specimens Part A: Loading and Fracture Initiation

Compact tension specimens were loaded at 1 and 7 m/s with two different accelerations imposed. Experimental photoelastic patterns recorded by a high speed camera were compared with those simulated by transient dynamic elastic Finite Element (F.E.) analysis. The positive results of this comparison validated the F.E. analysis, and gave credit to a comparison of the experimental results as well. It is demonstrated that when the initial acceleration of the specimen is reduced, using a damper in the loading device, static analysis can be applied for both testing speeds used. In high accelerations tests (without use of a damper), a transient dynamic stress state in the specimens is found, and therefore a dynamic analysis should be used. Furthermore, in high acceleration tests a transient mixed mode of loading at the crack tip occurs. This revised version was published online in August 2006 with corrections to the Cover Date.     

Influence of Hydrogenation on the Character of Fracture of Nitinol Alloy in Tension

Materials Science - We establish the macro- and microfractographic features of the tensile fracture of cylindrical specimens made of nitinol (Ni–Ti alloy) at room temperature and at 0°C...     

60Si2MnA铁轨铺设用弹条疲劳试验断裂分析

某60Si2MnA铁轨铺设用弹条疲劳试验寿命未达到规定值,通过对断裂弹条进行断口宏观和微观形貌观察、显微组织检查、硬度测试和化学成分分析,找出了弹条的断裂性质和早期断裂原因.结果表明:该弹条为弯曲-扭转疲劳断裂;弹条早期断裂的主要原因是其原材料碳含量偏低,导致热处理后材料硬度偏低,降低了弹条的疲劳性能;弹条表层脱碳形成...