拉—拉疲劳下15MnMoVN多裂纹扩展研究

为分析单裂纹或多裂纹在裂纹面承受疲劳拉伸载荷作用下尖端应力强度因子变化规律和裂纹形貌变化以及疲劳寿命情况,以含不同初始长深比的半椭圆单裂纹或双裂纹的薄片试样为研究对象,对试样在应力比R=0.1的疲劳拉伸载荷下单裂纹或双裂纹情况进行了仿真分析。建立含裂纹试样的有限元模型,仿真分析了裂纹在扩展过程中尖端应力强度因子的分布情况,并将单裂纹扩展结果与双裂纹相互作用影响下的结果进行了对比研究;进行含裂纹试样的疲劳实验,分析了含单裂纹或双裂纹的试样的断裂面的形成原因,并验证仿真结果正确性。结果表明,裂纹面之间的相互作用会逐渐影响裂纹的扩展方向、扩展速率以及在扩展过程中尖端应力强度因子的变化趋势;而且初始形貌为半椭圆形的双裂纹在相互作用影响下会逐渐过渡到半圆形。     

7055铝合金T型型材的应力腐蚀行为研究

通过拉伸性能测试、C环应力腐蚀试验、金相分析、扫描电镜和透射电镜观察等研究了7055铝合金T型型材的应力腐蚀开裂(SCC)行为。结果表明:7055铝合金T型型材纵向试样的抗拉强度、屈服强度、伸长率及断面收缩率均大于横向试样的;在间浸腐蚀和恒温恒湿环境下,纵向C环试样的开裂时间均长于横向试样的。型材纵向截面晶粒变形特征明显,主要呈拉长状,横向试样的裂纹沿纵向扩展,裂纹一旦形成,沿着拉长晶界迅速扩展,裂纹平直,开裂时间短;型材横向截面晶粒呈等轴状,纵向试样的裂纹沿横向扩展,裂纹扩展过程中会沿着晶界发生偏折,开裂时间长。7055型材应力腐蚀机理为氢脆和电化学理论的共同作用。      

40CrNiMoA钢轮缘堆焊层裂纹产生原因分析

40CrNiMoA钢轮缘堆焊后,通过磁粉探伤发现堆焊层下存在大量裂纹。通过宏观检验、化学成分分析、金相检验、硬度测试、扫描电镜及能谱分析的方法,对轮缘堆焊层下裂纹的产生原因进行了分析。结果表明:由于堆焊过程中预热温度偏低,在熔合线附近形成塑性、韧度极差的上贝氏体组织,在焊接冷却过程中收缩应力的作用下产生裂纹。     

铍青铜棒材中裂纹产生的原因分析

铍青铜棒在交货复验中发现有裂纹.采用化学成分分析、金相检验和断口宏、微观检验等方法对其进行了分析,并采用验证试样与其进行了对比分析.结果表明:裂纹是热处理不当造成晶粒长大所致.     

Q235B厚钢板断后伸长率不合格的原因分析

采用金相检验和化学成分分析等方法对热轧Q235B厚钢板出现的断后伸长率偏低的情况进行了分析.结果表明,Q235B厚钢板断后伸长率偏低的原因是由于带状组织和夹杂物引起的,因此必须严格控制带状组织和夹杂物.     

SS400钢卷冷弯不合格原因分析

通过宏观检验、力学性能测试、化学成分分析和金相检验对SS400钢卷冷弯不合格的原因进行了分析。结果表明:钢板中氧化铝夹杂偏多是冷弯试样出现裂纹的主要原因,同时提出了减少冷弯裂纹出现的措施。     

预制裂纹角度对橡胶拉伸断裂影响的有限元分析

初始裂纹的形态影响着裂纹尖端的应力场和扩展方向,进而决定着橡胶材料的使用寿命。目前人们关于预制裂纹试样拉伸断裂的研究主要集中在直裂纹,很少涉及预制裂纹角度的改变对橡胶拉伸断裂的影响。文中应用ANSYS有限元分析软件计算拉伸状态下含不同裂纹角度橡胶试样裂纹尖端的等效应力值和撕裂能的大小,判断裂纹是否扩展及扩展方向,并对橡胶试样进行拉伸验证试验测试。结果表明,在拉伸断裂过程中,裂纹尖端的应力值和撕裂能随着初始预制裂纹角度的增大而增大,裂纹尖端形状均由初始的尖点变成圆弧状;含不同裂纹角度橡胶试样的拉伸断裂形貌与裂纹预测扩展方向基本一致,验证了有限元分析的正确性。     

QT500-7球墨铸铁拉伸试样断口异常分析

拉伸试验中QT500-7球墨铸铁拉伸试样断裂后断口存在颜色较深的异常区。通过宏观观察、化学成分分析、金相检验、力学性能测试等方法对异常区形成原因进行了分析。结果表明:球墨铸铁铸造过程中局部石墨球化不良或球化衰退造成了该区域显微组织中有大量的蠕虫状石墨,导致球墨铸铁拉伸试样断口异常,这严重降低了球墨铸铁材料的屈服强度、抗拉强度和断后伸长率等力学性能。     

ER90S-G焊丝接头力学性能出现差异的原因

ER90S-G低合金钢焊丝作为第三代压水堆核电机组AP1000钢制安全壳拼装焊材,对其质量需要严格控制。某批次ER90S-G焊丝在入厂验收过程中,检测发现焊接试样在室温条件下的抗拉强度为740MPa,超过ASMEⅡSFA 5.28:2013要求的上限值(735 MPa),而在后续复验时测得抗拉强度分别为650MPa和640MPa。通过力学性能试验、化学成分分析、金相检验和焊接参数对比分析等方法,对造成力学性能前后测试结果存在差异的原因进行了分析。结果表明:焊接过程中层间温度偏低是造成性能差异的主要原因,其造成接头的晶粒尺寸变小,最终导致抗拉强度和屈服强度升高,断后伸长率降低。     

不同拘束条件下P92钢高温蠕变裂纹扩展速率的有限元模拟

基于蠕变损伤力学,仿真了蠕变裂纹扩展过程。用ABAQUS软件模拟研究了试样几何形状和宽度不同的两种面内拘束条件对P92钢在650℃时蠕变裂纹扩展(CCG)速率的影响,并进行了试验验证。仿真结果表明:试样几何形状和宽度对CCG速率的影响与载荷水平(C*)有关;在低载荷水平区内,C(T)试样的拘束水平大于SEN(T)试样,并且对应的CCG速率较大;在中载荷水平区,随着试样宽度的增大,裂纹尖端拘束和CCG速率增大;在高载荷水平区,CCG速率基本不受面内拘束(试样几何形状、宽度)条件的影响;在条件相同的情况下,试样宽度拘束水平大于试样几何形状。试验结果表明,试验与有限元分析(FEA)的蠕变裂纹扩展结果相符合。     

Design of Over-Stress Life-Test Experiments When Failure Times Have the Two-Parameter Weibull Distribution

This paper deals with estimation of reliability parameters when life testing is conducted at stress levels above that which would normally be applied in standard usage. A range of stress for testing is prescribed, and a two-parameter Weibull model for failure times is assumed. The logarithm of the Weibull scale parameter is assumed to be a polynomial function of known degree k of the reciprocal of stress level. The Weibull shape parameter is assumed to be independent of stress level.

The problem considered is that of determining the design for obtaining the least-squares-curve intercept with minimum variance at the nominal testing level. The design obtained specifies the number and location of stress levels in the prescribed range at which the life tests will be conducted and proportion of the total sample of specified size to be randomly allocated to each testing level.     

曲轴疲劳试样表面磁痕分析

采用宏观和微观检验等手段对曲轴疲劳试样不同部位磁痕产生的原因进行了分析。结果表明:磁痕是由于带状组织、疲劳裂纹和磨削裂纹等原因造成的。磨削裂纹主要是由于磨削时摩擦应力过大造成的。原材料中存在较严重的带状组织,导致淬火组织和残余应力不均匀,对磨削裂纹的产生起到了促进作用。     

汽车起重机臂梁声发射检测技术初探

汽车起重机臂梁在长期的使用过程中，受交变负荷、压力等载荷作用，疲劳破坏导致各种缺陷，使其安全性下降。传统的无损检测方法费时、费力，而且在汽车起重机臂梁的检测中难于施行。声发射检测可以预测到由未知不连续活动缺陷引起系统的灾难性失效，通过加载、放置探头在起重机臂梁上，可对其整体进行快速的探伤检测。重点分析了汽车起重机臂梁现状，以及将声发射检技术应用于QY8C汽车起重机臂梁检测的方法及其试验结果，并指出在汽车起重机臂梁声发射检测存在的问题。     

Interfacial shear stress in SiC fibre-reinforced cordierite

An analytical model based on a consistent shear-lag theory was developed to predict the interfacial shear stress in single fibre pull-out tests. The calculations show that the stress is highly dependent on the specimen thickness and the method of testing. Data for the debond stress and the interfacial shear stress were measured for single SiC fibres embedded in a magnesium aluminium silicate (cordierite) matrix. The effect of fibre embedded length, processing schedule, and matrix toughening were investigated. For a fixed sample support configuration during testing, good agreement was obtained between the model predictions and experimental data.     

Experience with the use of the acoustic emission technique for monitoring scale failure in wet and dry environments

Abstract

A brief survey of the acoustic emission technique for monitoring scale cracking and failure on 2.25–24% Cr steels in wet and dry environments is given. A number of acoustic emission test rigs are described. Some of the more simple test rigs are used for testing small oxidation coupons during isothermal oxidation. More sophisticated rigs have been used for testing full size heat exchanger tubes during thermal cycling.

Most acoustic emission measurements in a wet environment come from testing at temperatures below 650°C. There are examples from Alloy 800 and thermal barrier coatings that were tested at higher temperatures, 900°C and 1100°C, respectively. Through the years acoustic emission tests have been performed in dry air, dry air+10%H₂O, dry air+0.5%SO₂, and Ar+5%H₂+50%H₂O. Consequently, a wide variety of exposure temperatures and atmospheres can be investigated using acoustic emission techniques.

Qualitative acoustic emission results can detect when scale cracking occurs at exposure temperature, where such cracks are produced by growth stress. Acoustic emission signals have been measured during sample cooling, where the signal arises from scale cracking that is caused by the thermal expansion mismatch stress. Measured results have clearly shown that scale cracking caused by both growth stress and thermal expansion mismatch stress are affected by water vapor in the exposure environment. Post-test metallographic investigations show that crack orientation and the oxide scale phases are also affected by the gas composition in the test rig. Additionally the sample mass gain and scale thickness is affected by water vapor content.

Finally, acoustic emission techniques are helpful for understanding the phenomena of breakaway oxidation and spallation/exfoliation.     

20CrMnTi圆钢剪切开裂分析

采用理化检验及受力分析等方法，对20CrMnTi圆钢剪切开裂进行了分析。结果表明，原材料热轧后冷速过快，造成组织异常，硬度偏高，导致了剪切开裂，提出了改进措施。     

9FA燃机过渡段用螺栓断裂分析

某国产9FA燃机过渡段用螺栓在运行较短时间后发生断裂。采用化学成分分析、力学性能测试及金相检验等方法对螺栓断裂的原因进行了分析。结果表明：由于螺栓螺纹边缘部位凹凸不平造成大量的应力集中而形成微裂纹,在不断的热循环过程中,受热应力的作用微裂纹不断地扩展,直至剩余截面不能再承担负荷而导致螺栓的断裂。     

SiCf/TC17复合材料界面剪切强度测试与有限元分析

界面强度对钛基复合材料的性能有重要影响。采用纤维顶出实验(push-outtest)对连续SiC纤维增强TC17复合材料的界面剪切强度进行了测试,采用SEM观察了样品的形貌。以纤维/基体完全分离后的摩擦力为出发点,采用有限元方法确定了复合材料成型过程中残余应力的产生温度,并计算了残余应力的分布,比较了顶出实验样品制备前后残余应力的变化情况及样品厚度、体积分数对残余应力分布的影响;采用内聚力模型(CZM)分析了界面的化学结合强度。结果表明:SiCf/TC17复合材料高温成型后的冷却过程中开始产生残余应力的温度为775℃;顶出实验样品制备后界面处生成了残余剪切应力,其大小和分布与样品的体积分数和厚度相关,界面处的残余剪切应力造成了界面剪切强度的测试结果与界面化学结合强度的差异;室温下SiCf/TC17复合材料的界面化学结合强度约为450MPa。     

矫直辊断裂原因分析

采用断口分析、化学成分分析、力学性能测试、硬度检验和金相检验等方法分析了矫直辊断裂的原因。结果表明:断裂发生在非工艺要求的堆焊与未堆焊的交界部位,存在较大的焊接热应力和组织应力(主要是拉应力),最终导致矫直辊早期失稳断裂失效。     

In-situ observations of stress-induced thin film failures

In this work, the failure modes of thin films under thermo-mechanical treatments were observed via in-situ white beam X-ray topography. The in-situ experiments were carried out using an experimental setup on Beamline 2-2 at the Stanford Synchrotron Radiation Laboratory. Magnetron sputtered polycrystalline thin films of Ta and CrN on Si substrates were selected for the present study due to their disparate states of intrinsic residual stresses: the Ta film was anisotropically compressive and the CrN film was isotropically tensile. Under a similar heating-cooling cycle in air, the two types of films exhibited distinct failure modes, which were observed in-situ and in a quasi-real-time fashion. The failures of the samples have been interpreted based on their distinctive growth stress states, superimposed on the additional stress development associated with different forms of thermal instabilities upon heating. These included the formation of oxide for the Ta/Si sample, which led to an increase in compressive stress, and a phase change for the CrN/Si sample, which caused the isotropic stress in the film to become increasingly tensile.