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.     

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。     

一种消除AFM中悬臂梁横向扭转串扰误差的修正技术

串扰效应始终存在于原子力显微镜(AFM)的测量过程中,并影响其测量精度.在对串扰效应的产生机理进行分析,并建立了悬臂梁形变与光斑位置检测器(PSPD)输出信号之间的数学关系的基础上,提出了一种可消除横向扭转串扰影响的修正方法.在该方法中AFM系统可由输出信号直接获得悬臂梁的实际形变量,进而获得原子间作用力的实际值,并通过保持作用力的恒定对样品进行测量.该方法可有效地将悬臂梁扭转导致的纵向信号的变化从系统测得的纵向信号中剔除出去.仿真结果表明,在恒力接触模式下,串扰效应严重影响测量结果,导致测量误差,且针尖.样品夹角导致的串扰效应比摩擦力导致的串扰效应更严重.为验证本文方法消除串扰效应的效果,在应用该修正方法前后分别对标准梯形样品进行测量.实验对比结果表明,在普通恒力接触模式下的样品表面最大测量误差范围为21.2 nm～32.0 nm;对串扰效应进行修正后,最大测量误差范围降低至5.5 nm～10.2 nm.这表明该方法可有效降低AFM中串扰效应导致的测量误差,提高其测量精度.     

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。     

50CrVA扭杆断裂原因分析

对断裂的50CrVA扭杆进行断口宏微观观察、金相组织检查、硬度测试及疲劳强度校核.疲劳强度校核结果说明扭转载荷过大是扭杆疲劳断裂的主要原因.材质分析显示扭杆中含有大尺寸富Zn夹杂物,它促进了疲劳裂纹的萌生,缩短了扭杆的疲劳寿命.     

A Model of Quasibrittle Fracture Caused by Pulsed Loading

We study the stress-strain state formed in a plate containing a stationary crack of finite length subjected to pulsed loading, develop a model of quasibrittle fracture caused by loads of this type, and deduce analytic expressions for the stress-strain state near the tip of the stationary crack in the plate. The proposed model admits rigorous physical justification and enables one to describe the development of brittle and quasibrittle fracture processes in materials with cracklike defects under dynamic and static loads within the framework of the same approach. The results of numerical simulation are in good qualitative agreement with the experimentally established regularities of the behavior of the critical parameters of fracture processes.     

Fracture Analysis of Composites in High-speed Tension

In this paper the results of a high-speedtension experiment of the SiC_w/Al compositehave been reported and a simplified theoreticalmodel has been developed to study the fracturemechanism of composites in high-speed tension.This theoretical model provides a new explanationfor the increase of dynamic fracture strengthof composites in high-speed tension.     

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.     

镍基单晶合金紧凑拉伸试样高温断裂特性的实验研究

采用紧凑拉伸（CT）试样对三种晶体取向[001]，[011]，[111]的镍基单晶合金DD3在950，850℃和760℃下的拉伸以及950℃时的蠕变和疲劳性能进行了实验研究。应用光学显微镜（OM）和扫描电子显微镜（SEM）分别对单晶体裂纹扩展路径和断口表面进行了观察和分析。拉伸实验结果表明：单晶体裂纹扩展路径沿着特定的晶体学矢量方向扩展而在试样自由表明呈现Z字型外观，其扩展方向及形状取决于晶体取向。温度对试样的断裂形式影响较为显著，760℃时的断裂特征为剪切型脆性断裂，断裂平面为沿滑移面的光滑斜断口；950℃下试样断裂逐渐转变为微孔聚集型韧性断裂，断口由粗糙的剪切唇和纤维区构成。蠕变和疲劳实验结果表明：镍基单晶具有明显的蠕变和疲劳性能各向异性，蠕变寿命以[011]，[111]，[001]顺序依次减小，疲劳寿命以[111]，[011]，[001]顺序依次减小；蠕变及疲劳裂纹扩展路径均与加载方向垂直，断裂表面均为光滑的平断口。试样断口细观分析显示，蠕变试样断口由杯锥状韧窝组成，而疲劳试样断口则由疲劳裂纹组成。疲劳损伤比蠕变损伤更有利于裂纹的扩展。     

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...     

温度梯度下薄壁圆管试件拉伸性能的试验与理论研究

开发了一个温度梯度试验系统,在不同温度以及温度梯度下对薄壁圆管试件进行拉伸试验.采用有限元软件模拟得到不同温度下材料的真实应力应变关系,采用计算流体软件对冷却薄壁圆管试件进行流固耦合传热分析,并将得到的温度载荷通过温度插值程序传递到有限元模型中.根据拟合得到的不同温度的材料性能,模拟温度梯度下拉伸过程,模拟结果与试验结果相吻合,结果表明:在目前的温度梯度下,温度梯度对材料的拉伸性能有一定的影响,但影响较小,可以采用通用的数值方法研究存在温度梯度的拉伸性能.