Low-cycle fatigue properties of an ultrafine-grained magnesium alloy processed by equal-channel angular pressing

Magnesium alloy Mg-3%Al-1%Zn (AZ31) billets prepared from equal channel angular pressing (ECAP) were utilized in low-cycle fatigue tests in order to investigate their fatigue life. Fully reversed strain-controlled tension-compression fatigue tests were conducted at the frequency of 1 Hz in ambient air. The microstructures were examined by optical microscopy (OM) and scanning electron microscopy (SEM). The hysteresis loops of the ECAP processed and conventionally extruded samples display obviously different shapes in the total strain amplitude range from 0.2% to 0.6%. Accordingly, the low cycle fatigue lives of ECAP processed samples are found to be longer than those of extruded samples, which can be attributed to the different in the hysteresis energy incorporating tensile strain energy.     

AZ91D应变状态下低周疲劳行为的研究

对AZ91D镁合金进行了应变控制单轴加载的低周疲劳试验研究,结果表明：AZ91D镁合金在低周载荷下,当循环应变幅值小于0．4％,材料表现出循环软化．随着循环应变幅值的增加,材料表现出明显的循环硬化现象;且随着应变幅值的增加材料的附加强化程度增大．当总应变幅大于1．0％,曲线的斜率基本不变,即循环应变硬化率大致相同．应变幅值对材料低周疲劳寿命有较大影响,随着应变幅值的提高,疲劳寿命降低．运用Coffin-Manson公式进行寿命预测,在高应变幅值下有较好的一致性．     

锈蚀对钢材低周疲劳性能的影响分析

以我国钢桥结构中使用较多的Q345钢材为例,对人工加速锈蚀试样进行2种应变幅下的低周疲劳试验研究.应用精细的有限元模型分析,基于微观断裂判据--循环空穴扩张模型（CVGM）,对锈蚀钢材的低周疲劳断裂机理进行分析,讨论蚀坑对钢材低周疲劳（LDF）寿命的影响机理,并验证方法的有效性.结果表明,质量损失率约为6%的锈蚀Q345钢材试样的低周疲劳寿命比无锈蚀钢材减少约30%;完好试样与锈蚀试样的稳定滞回环形状几乎相同,锈蚀对钢材承载能力影响较小;在1%应变幅作用下,试样破坏现象为脆性的低周疲劳破坏,而在2.5%应变幅作用的循环荷载下,试样发生延性的超低周疲劳破坏.锈蚀引起钢材超低周疲劳寿命下降程度与蚀坑位置的应变集中有关,随着应变集中程度的增加,疲劳寿命的下降增大.     

矿渣聚丙烯纤维混凝土抗弯疲劳性能

为了研究聚丙烯纤维和磨细粒化高炉矿渣（GGBFS）在不同应力水平和频率下对混凝土抗弯疲劳性能的影响,将4个配比的聚丙烯纤维和5个配比的矿渣分别掺入混凝土中,当应力水平为0.49、0.59、0.69,频率为20 Hz时以及应力水平为0.59,频率为30、40、50、60 Hz时测试抗弯疲劳极限强度和疲劳寿命.研究表明：累积抗弯疲劳强度能够更准确地评价混凝土抗弯疲劳性能;聚丙烯纤维可以提高混凝土累积抗弯疲劳强度和抗疲劳寿命;矿渣及其水化物使得混凝土结构密实,改善了界面过渡区（ITZ）的结构,可以提高混凝土抗弯疲劳性能;抗弯疲劳性能随着应力水平提高而下降,S N数学模型可以用于预测20 Hz频率动疲劳荷载下的矿渣聚丙烯纤维混凝土工程寿命;在一定的应力水平下,测试频率越高,抗弯疲劳性能越差,f N数学模型可以用于预测变频率动疲劳荷载下的矿渣聚丙烯纤维混凝土工程寿命.     

Fatigue properties of RPC under cyclic loads of single-stage and multi-level amplitude

Fatigue properties of Reactive Powder Concrete (RPC) under axial compression of single-stage and multi-level amplitude in cycles were studied. The tests reveal the fatigue life, the strain and residual life of the RPC samples. Through the analysis of the test results under cyclic loads of single amplitude, the S-N curve of RPC and the evolution rule of macro-damage of RPC were presented, which can be divided into latency stage, stable development stage and instability development stage according to the evolution pattern of the fatigue crack. Accordingly, the development of longitudinal deformation presents the similar three-stage-model, and the proportion of each stage is 15%, 75%, and 10%. According to test results, the fatigue strength reduction factor is 0.564. We brought forward an empirical formula to predict the life of RPC via total longitudinal strain and got the evolving rule for the residual strength of the RPC. The analysis of the test results under cyclic loads of multi-level amplitude shows that the strain under this loading pattern experiences three stages. The characteristic value for the residual strain was obtained. The irreversible damage and non-linear evolution of RPC was described by the development of the residual plastic strain.     

Low cycle fatigue behavior of high strength gun steels

The low cycle fatigue (LCF) behavior of two high strength steels, with nominal chemical compositions (mass fraction, %) of 0.40(2-1.5Cr-3Ni-0.4Mo-0.2V (PCrNi3MoV) and 0.25C-3Cr-3Mo-0.8Ni-0.1Nb (25Cr3Mo3NiNb), was investigated by using the smooth bar specimens subjected to strained-controlled push-pull loading. It is found that both steels show cyclic softening, but 25Cr3Mo3NiNb steel has a lower tendency to cyclic softening. 25Cr3Mo3NiNb steel has higher fatigue ductility, and its transition fatigue life is almost three times that of PCrNi3MoV. 25Cr3Mo3NiNb steel also shows higher LCF life either at a given total strain amplitude above 0.5% or at any given plastic strain amplitude, despite its lower monotonic tensile strength than that of PCrNi3MoV. It also means that 25Cr3Mo3NiNb steel can endure higher total strain amplitude and plastic strain amplitude at a given number of reversals to failure within 10^4. 25Cr3Mo3NiNb steel is expected to be a good gun steel with high LCF properties because only several thousand firings are required for gun barrel in most cases.     

PBL剪力连接件的疲劳试验与分析

为了研究PBL (Perfobond Leiste)这种新型剪力连接件的抗疲劳性能,基于深圳南山大桥剪力连接件的尺寸和要求,设计并制作了6个PBL剪力连接件的推出试件进行疲劳试验.试验在MTS815机器上进行,采用常幅正弦波荷载,加载频率为3 Hz,试验应力比为0.1.通过对试件进行常幅荷载的循环加载,观察试件中钢与混凝土块的相对滑移、连接件的破坏模式、疲劳寿命以及应变的特征等.试验结果表明：主要开裂方向为连接件底部斜向下呈45°,裂缝的宽度同裂缝与竖向的夹角成反比;疲劳荷载作用时,PBL连接件的抗疲劳性能良好,孔径越大试件的疲劳寿命越长;试验从一定程度上说明腹板越厚和疲劳荷载幅值越小,滑移量的增长速度越慢,同时孔径越大和疲劳幅值越小,滑移量的增长速度也越慢.     

AA2524-T34铝合金的微观结构与疲劳裂纹萌生机制

通过对新型铝合金AA2524-T34的四点弯曲疲劳实验,结合光学显微镜和电子扫描电镜（SEM）,对疲劳试样表面和疲劳断口进行了微观观测,研究了材料的微观结构及在不同相对湿度下材料的疲劳裂纹萌生机制。实验应力比0．1,加载频率15Hz,实验相对湿度15％和95％。结果显示实验材料呈现了完全再结晶的层状晶粒结构,晶粒沿着轧制方向被拉长,并较为平坦。不同相对湿度下,多数疲劳裂纹萌生于材料中含Fe的粗大的β相粒子,同时伴有少量的滑移带裂纹形核、材料缺陷裂纹形核等;高相对湿度下,可见裂纹沿晶界启裂。     

高温多轴载荷下低周疲劳寿命预测

针对Mod.9Cr-1Mo铁素体钢进行550℃下多轴低周疲劳试验,定义主应变比用以考虑不同的比例程度对多轴疲劳寿命的影响.针对各路径和各幅值下的循环软化特性进行分析,对比结果表明：路径对循环软化特性的影响大于应变幅值对循环软化特性的影响,其软化程度随扭转载荷所占比例增加而增加.采用最大正应变方法、最大剪应变方法和von Mises等效应变法进行疲劳寿命预测,结果表明：最大正应变方法针对扭转路径得到的预测结果偏于安全;最大剪应变方法预测扭转路径较好,但对于单轴路径给出的预测结果过于不安全;等效应变法得到的预测结果偏于保守.     

热处理对挤压态6061铝合金低周疲劳行为的影响

为了确定固溶处理、时效处理及固溶+时效处理对挤压变形6061铝合金的疲劳变形行为和疲劳寿命的影响规律,针对不同处理状态的挤压变形6061铝合金进行了低周疲劳实验研究.结果表明,低周疲劳加载条件下,不同处理状态的挤压变形6061铝合金均呈现明显的循环应变硬化,且随着外加总应变幅的增加,循环应变硬化效果愈明显;在较高的外加总应变幅下,固溶态6061铝合金的疲劳寿命最长,而时效态6061铝合金的疲劳寿命最短,当外加总应变幅较低时,挤压态6061铝合金的疲劳寿命高于经过热处理的合金的疲劳寿命;此外,固溶处理可明显降低挤压变形6061铝合金的循环应变硬化指数和循环强度系数.     

Properties and mechanism on flexural fatigue of polypropylene fiber reinforced concrete containing slag

Properties and mechanism were investigated on flexural fatigue of concrete containing polypropylene fibers and ground granulated blast furnace slag(GGBFS).Four polypropylene fibers’volume fractions and five slag proportions were considered.An experiment was conducted to obtain the fatigue lives at three stress levels in 20 Hz frequency and at a constant stress level of 0.59 in four frequency respectively.Mechanism and evaluation were investigated based on the experimental data.Fatigue life span models were established.The results show that the addition of polypropylene fibers improves the flexural fatigue cumulative strength and fatigue life span.It is proposed that the slag particles and hydrated products improve Interfacial Transition Zone(ITZ)structure and benefit flexural fatigue performance.A composite reinforce effect is found with the incorporation of slag and polypropylene fibers.The optimum mixture contents 55%slag with 0.6%polypropylene fiber for the cumulative fatigue stress.Fatigue properties are decreased as the stress level increasing,the higher frequency reduces the fatigue strength more than lower frequency at a constant stress level.     

三角形通道内对称三角翼的脉动传热特性

该文研究了三角形管道内加入对称三角翼扰流元件在脉动流作用下的对流换热特性,工质是体积分数为1%的Water-SiO₂纳米流体,雷诺数为500~1 600,利用CFD软件进行数值仿真。结果表明,当雷诺数和振幅定常时,即当雷诺数为1 500,振幅为0.75时,脉动流作用下存在强化与弱化换热的临界频率约为1 Hz。当临界频率大于1 Hz时,强化换热,反之则会弱化换热。当频率为10 Hz时,换热最大增强了3%;当频率为0.001 Hz时,换热最大弱化了5%。同时利用场协同原理研究了传热机理,结果表明场协同角越小,换热效果越好,并且距离三角翼片下游越近,场协同角越小。当振幅为0.75,频率为10 Hz,雷诺数为1 500时在一个周期内的平均场协同角最小,换热效果最好。     

Q345B母材及其焊接接头的低周疲劳行为

对Q345B及其光滑焊接接头的低周疲劳行为进行了试验研究.采用轴向对称应变控制方法,保持恒定应变速率为0.005 s-1,在0.3％～0.7％的应变范围内,在电液伺服疲劳试验机上测定了Q345B及其光滑焊接接头的低周循环应力响应特征、应变-寿命、应变-能量关系,并进行了对比分析.结果显示,Q345B母材在应变幅度低于和高于0.4％时分别呈现明显的循环软化和硬化现象,而焊接接头出现显著的硬化现象.与母材相比,焊接接头的低周转变寿命、疲劳强度和疲劳寿命明显减小.通过试验数据确定了Q345母材及其焊接接头Coflin-Manson关系的合理参数,建立了应变幅度-疲劳寿命及能量-寿命关系式.力学不均匀性引起的循环硬化趋势增加是导致焊接接头低周疲劳性能减低的主要原因.     

福斯罗W300扣件系统组装状态下刚度特性试验研究

利用高频疲劳动刚度试验平台,结合高速铁路实际运行状况,对福斯罗W300扣件系统组装状态下刚度特性进行研究,得到以下结果. 1)不同预压荷载下,静态和动态刚度有相同增长趋势,20 kN(40 kN)预压荷载下静刚度值比无预压荷载下增大17.7% (59.5%),40 kN下静刚度值明显超出扣件系统静刚度设计限值. 20 kN(40 kN)预压荷载下动态刚度比0 kN下增大4 dB(10 dB). 2)扣件系统静态和动态刚度均表现出低温敏感性,当温度低于20 ℃时,静刚度值随温度降低而上升,特别是在?30 ℃到?60 ℃时静刚度值急剧上升,?50 ℃(?60 ℃)时静刚度是20 ℃时的1.4(4.4)倍. ?30 ℃下,其动态刚度比30 ℃时增大8~10 dB,弹性单元体丧失弹性,直接影响扣件系统减振效果. 3)扣件系统刚度在5~1000 Hz时有明显频变特性,预载一定时,动态刚度随频率增大而增大,1000 Hz下动态刚度值比国家标准5 Hz下增大15 dB.     

挤压变形AZ81镁合金的疲劳变形与断裂行为

研究镁合金的疲劳行为，可为镁合金的抗疲劳设计和合理使用提供可靠的理论依据.通过总应变幅控制的疲劳实验和断口形貌分析，确定了挤压变形AZ81镁合金的循环应力响应行为、疲劳寿命行为和断裂机制.结果表明：在应变控制的疲劳加载条件下，挤压变形AZ81镁合金呈现明显的循环应变硬化，其弹性应变幅、塑性应变幅与断裂时的载荷反向周次之间的关系可分别用Basquin和Coffin-Manson公式来描述，得到了拉伸滞后能的理论计算值与应变疲劳寿命之间呈线性关系、疲劳裂纹的萌生和扩展均以穿晶模式进行的结论.     

合金钢的高温单轴棘轮效应实验研究1.25Cr0.5Mo

利用MTS810电液伺服试验机在300℃温度下对1.25Cr0.5Mo合金钢进行了单轴棘轮效应实验研究,探讨了温度、应力幅值、应力峰值、平均应力以及加载历史对材料棘轮效应的影响.以0.1%/s及0.2%/s两种不同的应变加载速率分别对材料进行单轴拉伸实验,结果表明1.25Cr0.5Mo合金钢在所研究的温度范围内的变形行为是率无关的;在0.4%应变幅值下进行了单轴应变循环实验,响应应力幅值随循环次数几乎没有变化,可视为循环稳定材料;单轴棘轮效应实验表明棘轮应变速率随平均应力、应力幅值及温度的升高而增大;棘轮变形依赖于加载历史,先前较大的循环应力对后继较小的循环应力下的棘轮变形起抑制作用.     

Inconel 625合金的室温低周疲劳与断裂行为

为了对Inconel 625合金构件的抗疲劳设计提供可靠的理论依据,在室温下对Inconel 625合金进行了轴向总应变幅控制的低周疲劳实验,分析了室温下合金的应变疲劳寿命和循环应力应变数据,进而给出了合金的应变疲劳参数.采用扫描电子显微镜和透射电子显微镜对Inconel 625合金试样进行了断口形貌分析和微观组织观察.结果表明,Inconel 625合金在室温下的弹性应变幅和塑性应变幅与载荷反向周次的关系可分别用Basquin和Coffin-Manson公式来描述.在室温疲劳变形过程中,Inconel 625合金会发生循环硬化和循环软化.合金的低周疲劳裂纹以穿晶方式萌生于疲劳试样的自由表面,并以穿晶方式扩展.     

In-flight observations of electromagnetic interferences emitted by satellite

Using the data from STAFF/TC-1, this paper for the first time analyzes the electromagnetic interferences of Chinese scientific satellite. The electromagnetic interference of satellite exists mainly below 30 Hz, but can extend to 190 Hz with an obviously decreasing power spectral density. The electromagnetic interferences at frequencies below 190 Hz have good correlation with the solar aspect angle. The electromagnetic interferences at frequencies between 190 and 830 Hz have also correlation with solar aspect angle. However, the electromagnetic interferences at frequencies above 830 Hz have no correlation with the solar aspect angle. The correlation coefficient between solar aspect angel and electromagnetic interferences is around 0.90. The larger the solar aspect angle, the stronger the satellite electromagnetic interference. When the solar aspect angle increases from 90.6° to 93.6°, the electromagnetic interferences at frequencies <10 Hz increase by 8 times and those at frequencies 190–830 Hz increase by 60%. This close association of electromagnetic interferences with the solar aspect angle indicates that the solar aspect angle is the main factor to determine the electromagnetic interferences. The electromagnetic interferences of satellite in sunlight are larger than those in eclipse. The electromagnetic interference produced by solar panel occupies about 87% in the low frequency band (<100 Hz) and 94% in the high frequency band (>100 Hz) of the total electromagnetic interference produced by satellite. These in flight observations of electromagnetic radiation of satellites will be very helpful to the designs of future satellites of space sciences or earthquake sciences. Supported by the National Hi-Tech Research and Development Program of China (“863” Project) (Grant No. 2008AA12A216), the National Science & Technology Supporting Program during the Eleventh Five-Year Plan, the National Natural Science Foundation of China (Grant No. 40523006), the National Basic Research Program of China (“973” Project) (Grant No. 2006CB806305), and the Specialized Research Fund for State Key Laboratories     

Fatigue Properties of RPC under Cyclic Loads of Single-stage and Multi-level Amplitude

Fatigue properties of Reactive Powder Concrete (RPC) under axial compression of single-stage and multi-level amplitude in cycles were studied. The tests reveal the fatigue life, the strain and residual life of the RPC samples. Through the analysis of the test results under cyclic loads of single amplitude, the S-N curve of RPC and the evolution rule of macro-damage of RPC were presented, which can be divided into latency stage, stable development stage and instability development stage according to the evol...     

Experimental study on durability fracture behavior and vibration modal sweep for vehicle rear axle

In order to study the fatigue fracture behavior of the rear axle of certain China-made car, the strain loading spectrum near the rear axle fracture location is collected, the modified Neuber rule and the cyclic stress-strain hysteresis loop curve equation are used to convert the nominal strain history into the local stress-strain response. The impact of mean stress on fatigue damage is corrected according to the Manson-Coffin model, and programming calculation of the fatigue damage of the fracture crack is conducted in the INFIELD software, the electromagnetic vibrators are used to sweep the vibration modal frequencies of the rear axle and car body. The enhancement test and frequency sweep results show that the rear axle fatigue damage mainly concentrates on the washboard road, and when the forced vibration excitation frequency is 24.07Hz, the vibration modal frequency of the rear axle is close to the excitation frequency of the washboard road, leading to resonance and making the rear axle subjected to large strain and fatigue damage, and then vibration fatigue fracture due to high stress concentration.