Resonant fatigue testing of riveted joints

A special test facility was constructed for evaluating the fatigue strength of turbine-blade components under steady pullout and vibratory bending loadings. The use of mechanical resonance for these applications was particularly attractive in that failure in inaccessible areas could be detected by the change in natural frequency of the specimen. The sensitivity of the method allowed determination of a fatigue crack as early in life as possible. The application of this facility to the determination of the fatigue strength of a type of riveted connection employed on certain types of blades is described in this paper. A steady load representing the centrifugal force was applied hydraulically through a long flexible pullrod which in combination with the test specimen was excited laterally at its resonant frequency by an electromagnetic shaker. An accelerometer provided closed-loop control on both vibration amplitude and resonant frequency through special circuitry. Fatigue-strength reduction factors at 10⁷ cycles were determined for three types of riveted test specimens in air at room temperature. These values exceeded the elastic stress-concentration factors estimated from earlier photoelastic tests for all three cases.     

缺口件疲劳寿命分布预测的有效应力法

本文提出了一种由光滑件疲劳寿命试验数据预测缺口件疲劳寿命分布的有效应力法。该方法中缺口件的裂纹可能萌生表面被分解成一个个微元,整个表面可看成是这些微元组成的一个串联模型,按照串联概率失效模型,缺口件的疲劳强度失效概率就可以由各微元的疲劳强度失效概率计算得到,其中微元的疲劳强度失效概率是由光滑件的疲劳强度失效概率通过最弱环节理论计算得到的。在缺口件的疲劳强度失效概率表达式中,引入了有效应力的概念,用它查取光滑件的疲劳寿命试验数据就可以直接得到缺口件的疲劳寿命分布。该方法可以同时考虑到应力梯度和试件尺寸对缺口件疲劳寿命分布的影响。进行了材料LY12CZ的带中心孔缺口件的寿命算例分析,预测结果和试验结果吻合良好,表明该方法是有效的。     

两种激光熔覆涂层对轮轨材料磨损与损伤性能的影响

选用钴基合金粉末和铁基合金粉末,利用CO₂多模激光器对轮轨材料进行激光熔覆处理. 分析了钴基合金涂层和铁基合金涂层的微观组织、成分、硬度与应力状态. 未处理试样表面残余应力为拉应力,激光熔覆处理后,涂层表面残余应力为压应力. 利用MJP-30A滚动接触疲劳试验机对激光熔覆处理前后轮轨试样进行滚动摩擦磨损试验. 结果表明:激光熔覆处理后轮轨试样磨损率明显降低,其中激光熔覆钴基合金后,轮轨试样磨损率分别降低96.7%和98.9%,激光熔覆铁基合金后,轮轨试样磨损率分别降低81.7%和93.5%. 未处理轮轨试样表面损伤为疲劳损伤;钴基合金涂层表面损伤最轻微,磨痕表面光滑,出现轻微的小块剥落;铁基合金涂层表面出现细小裂纹和犁沟.      

New equipment for cyclic biaxial testing

Methods of achieving biaxial-stress states in fatigue tests are reviewed. A new design of equipment that produces five distinct biaxial-stress states by simultaneous direct pressurization and axial loading of thinwalled cylindrical specimens is described. Four variations of stress state are obtained by the use of two sizes of specimen and by reversal of the pressurizing connections. The fifth state is obtained by direct pressurization without axial load. The actual magnitudes of stress in the specimen are computed from the output of the load cell in the reaction frame in which the biaxial-testing device is mounted. Additional stress ratios are obtained using standard uniaxial and torsional cyclic-testing arrangements. The initial program using this equipment was to investigate the effects of biaxiality on the cyclic properties and low-cycle fatigue behavior of normalized 1018 mild steel, under fully reversed constant-amplitude strain control. Control was achieved using a servo-controlled, electrohydraulic testing system with one of the two clipon strain transducers, that were mounted on the specimen, providing the controlling electrical signal. The system allowed direct recording of the stress-strain hysteresis loops in both principal directions. The behavior of the equipment and the modes of failure of the specimens are described and some test data are presented. The range of application and limitations of the equipment for further cyclic biaxial testing are discussed.     

An Experimental Rig to Investigate Fatigue Crack Growth Under Dynamic Loading

A novel experimental rig capable of generating a versatile dynamic loading has been designed and tested to overcome the shortcomings of conventional fatigue testing machines such as the difficulty in providing zero crossing aperiodic loading. The main principle of this new design is based on two, single degree of freedom based excited oscillators, where inertial forces act on a specially designed specimen. By changing the natural frequency of the oscillator, the extent of the preloads and pattern of the excitation signal on the shaker, the rig provides a new and robust means of fatigue testing, particularly for aperiodic loading.     

Design and Simulation of a Novel Multi-Axial Fatigue Test Rig

This research covers the design and simulation of a novel experimental concept for multi-axial fatigue analysis of cylindrical specimens. The resulting design allows a combination of bending and torsional stress to test specimens with critical diameters ranging from 5 to 15 mm at test frequencies up to 50 Hz. Furthermore, the amplitude and frequency of both loadcases can be controlled independently. The test rig will be used to study and validate fatigue criteria for synchronous and asynchronous loading conditions and to analyze the effect of size on the fatigue life of metal and plastic components. The test setup consists of a closed mechanical loop. The primary shaft contains the cylindrical test specimen and is modified to impose rotating bending loads. The secondary shaft is adjusted to introduce fluctuating torque in the transmission loop. Both shafts are connected by means of two double link mechanisms to minimize the clearance and the inertia of the system. The time-varying multi-axial stress state in the cylindrical specimen is analyzed as a function of the amplitudes and frequencies of both bending and torsional loadcases. This is verified by a numerical fatigue analysis in MSC-Patran and MSC-Fatigue. Finally, the dynamical behavior of the test system is studied using a 5 DOF torsional mass-spring representation and the Lagrangian method. A more complex model with 20 DOF is implemented in SimDriveLine and solved via Matlab to analyze the kinematical and dynamical properties more accurately. Both studies take the mechanical properties of steel and plastic test specimens of different sizes into account.     

Experimental modal analysis for microelectromechanical systems

The structural dynamics behavior of microelectromechanical systems (MEMS), which include moving, overhung, and compliant subcomponents, plays a pivotal role in determining their performance and reliability. Traditionally, experimental modal analysis is used to characterize the dynamic behavior of structures, as well as to derive, validate, update, and correct analytical and numerical models. Due to their small size, however, conventional modal testing methods cannot be directly applied to microstructures. In this paper we provide an overview of modal testing techniques for microsystems. A particular experimental modal analysis methodology that includes base excitation via a piezoelectric shaker and measurement through a laser interferometer is then described and evaluated. A distinguishing characteristic of the methodology is its simplicity, including its simple setup and off-the-shelf components. The modal model is derived for the base excitation of microcantilever beams. The effectiveness of the methodology is illustrated through various experiments on polysilicon microcantilevers for different geometries and ambient pressures. Analysis of the damping data for different pressures has confirmed the well-documented fact that the structural damping in microsystems can be considerably less than damping arising from interaction with the ambient gases.     

Rapid Determination of Fretting Fatigue Limit by Infrared Thermography

This paper demonstrates the feasibility of infrared thermography to determine the so-called fretting fatigue limit. Fretting fatigue tests are performed on aluminum and steel specimens. The coupled fatigue and tangential loads are sequentially increased (block loading) whilst the normal load is kept constant for all blocks. The temperature data is processed and analyzed using a Fast Fourier Transform (FFT) algorithm implemented in the commercial software Matlab. It is demonstrated that the second harmonic of the temperature signal can be linked to the specific loading block below which no or negligible damage is generated in the specimen. The stress amplitude of this block is considered to be a best estimate of the fretting fatigue limit. A constant amplitude fretting fatigue test with this stress amplitude confirmed that the specimen remains intact at 10⁷ cycles.     

A Uni-Axial Nano-Displacement Micro-Tensile Test of Individual Constituents from Bulk Material

For both single-phase and multiphase metallic materials, it is necessary to understand the mechanical behavior on the grain-size scale in detail to get information that is not obtainable from macro-scale mechanical characterizations. This paper presents a methodology for uniaxial tensile testing of micro-specimens isolated from a bulk material. The proposed concept of multiple parallel micro-tensile specimens at the tip of a macro-sized wedge reduces the alignment work and offers an easy way for specimen handling. The selection of site-specific specimens is based on detailed microstructural and crystallographic characterization. Three kinds of representative specimens are presented to illustrate the wide range of application of the methodology for a variety of materials. Accurate, reproducible measurement of force (2.5 μN resolution) and displacement (~10 nm resolution) is demonstrated, while accurate alignment (in-plane rotational and out-of-plane tilt misalignment of <0.2°) limits the stress due to bending to <0.2% of the imposed uni-axial stress. Combined with detailed material characterization on both sides of the micro-specimens, this method yields detailed insights into the micro-mechanics of bulk materials which is hard to obtain from traditional macro-mechanical tests.     

温度影响下基于应力波的钛合金试件疲劳裂纹监测实验研究

本文对温度影响下应用应力波监测钛合金CT试件中的疲劳裂纹进行了研究。建立了由MTS材料试验机和温控箱组成的实验装置,模拟温度变化对应力波在钛合金试件中传播的影响。采用PZT压电元件分别在试件中激励和接收应力波信号。通过连续小波分析,提取应力波信号的多种特征用于表征疲劳裂纹的发生与发展,并对信号特征进行了统计分析。为在温度变化下监测钛合金试件中的疲劳裂纹,对信号特征进行多元偏值分析,采用马氏平方距离融合多个信号特征,在概率统计意义下确定钛合金试件中是否存在疲劳裂纹。实验结果表明了所提出方法的可行性和有效性。     

Surface Strain Measurements Using a 3D Scanning Laser Vibrometer

Three-dimensional scanning laser vibrometers enable extremely accurate non-contact measurement of the three-dimensional displacements on the surface of structures. This paper looks at the feasibility of using such a scanning laser vibrometer for the non-contact measurement of dynamic strain fields across the surface of a planar structure subjected to in-plane loading. Issues such as laser head alignment and choice of differentiation filter parameters are discussed. Finally, experimental results of two test specimens are presented which clearly demonstrate the significant potential of this new experimental technique as well as highlighting several limitations.     

60Si2Mn螺旋弹簧的压缩应力松弛行为与贮存寿命预测

为评价60Si2Mn螺旋压缩弹簧的室温松弛特性,利用InstronE3000K8953型小吨位电子动静态疲劳试验机,对其在不同温度和初始应力水平条件下进行了高温压缩加速应力松弛试验,研究了环境温度、初始应力水平对松弛行为的影响.基于粘弹性体模型,揭示了应力松弛过程中弹性应变向塑性应变的转化特性与塑性应变随松弛时间的变化规律.在对应力松弛前后弹簧丝材金相和TEM微结构进行对比分析的基础上,探讨了应力松弛的微观机制.结果表明,环境温度与初始应力水平对松弛速率具有显著影响.基于应力松弛过程的热激活特性,建立了60Si2Mn螺旋压缩弹簧的贮存寿命预测方程,并对不同应力水平下弹簧的室温和高温贮存寿命进行了合理预测.     

Q235钢缺口试样拉压低循环疲劳的实验研究

以Q235钢制U型缺口板试样为研究对象,用有限元方法计算其缺口根部等效应变幅对应的试样标距段位移,以此控制试验机进行拉压循环疲劳试验。然后用局部应力应变法对试验测得的寿命结果进行分析。结果表明：无论用有限元还是修正Neuber公式计算缺口根部的应力应变,局部应力应变法的疲劳寿命评估只适用于缺口半径较大的试样;对缺口半径较小试样的估计寿命明显低于实测值,且有限元法比修正Neuber法更保守。进而又对试样缺口区域应变梯度的影响进行了探讨：参照有限元计算的应变梯度,利用Taylor模型估算了缺口根部的屈服应力和流动应力;在此基础上重新计算应变分布并估计试样的疲劳寿命,结果证实考虑应变梯度影响可改善缺口试样的疲劳寿命估计。     

Dynamic compressive responses of intact and damaged ceramics from a single split Hopkinson pressure bar experiment

A novel dynamic compressive experimental technique has been developed based on a split Hopkinson pressure bar. This new method dynamically loads the ceramic specimen by two consecutive stress pulses. The first pulse determines the dynamic response of the intact ceramic materiaal and then crushes the specimen, and the second pulse determines the dynamic compressive constitutive behavior of the ceramic rubble. Precise pulse shaping ensures that the specimen deforms at nearly constant strain rates under dynamic stress equilibrium during the loading by both stress pulses. Pulse shaping also controls the amplitudes of loading pulses, the values of strain rates, the maximum strains in the rubble specimens, and the proper separation time between the two loading pulses. The feasibility of the new technique is demonstrated by the experimental results obtained on an AD995 alumina.     

A forced-vibration technique for measurement of material damping

This article describes a technique for measuring material damping in specimens under forced flexural vibration. Although the method was developed for testing fiber-reinforced composite materials, it could be used for any structural material. The test specimen is a double-cantilever beam clamped at its midpoint and excited in resonant flexural vibration by an electromagnetic shaker. Under steady state conditions, material damping is defined in terms of the ratio of input energy to strain energy stored in the specimen. If external losses are negligible, the input energy must equal the energy dissipated in the specimen. Input energy and strain energy are found from measured specimen dimensions, resonant frequency, input acceleration and bending strain. Problems associated with minimization of external energy losses in the apparatus and verification of measurements are discussed in detail. Measured damping of aluminum-alloy calibration specimens shows good agreement with calculated thermoelastic damping. Examples of measured damping showing amplitude and frequency dependence in fiber-reinforced plastic materials are presented.     

EFFECT OF SPECIMEN ASPECT RATIO ON FATIGUE LIFE OF CLOSED CELL Al-Si-Ca ALLOY FOAM

Quasi-static and compressive fatigue tests on the closed cell Al-Si-Ca alloy foam specimens with three different aspect ratios were performed. It turned out that the onset of cyclic shortening of foam with a lower aspect ratio took place earlier and the fatigue strength was lower compared with the specimen with a higher aspect ratio, although all the dimensions of specimen satisfied the seven times the cell size criterion, while the quasi-static stress-strain curves were almost same having same Young＇s modulus, yield stress and plateau stress. Therefore, the seven times the cell size criterion for the quasi-static compression behavior was not applicable to the fatigue analysis of Al-Si-Ca alloy foam.     

动、静裂纹作用偏置效应的动焦散冲击实验

为了研究冲击荷载作用下脆性材料中运动裂纹与静止裂纹的相互作用,选取动态载荷下断裂行为与岩石材料类似且本身光学特性较好的有机玻璃（PMMA）作为实验材料,试件尺寸为220 mm×50 mm×5 mm,采用激光切割制作长度5 mm的预制裂纹和长度10 mm的静止裂纹,预制裂纹位于试件的底部边缘中心,静止裂纹的中心位于试件水平轴线。将静止裂纹偏置距离作为单一变量,采用数字激光动态焦散实验系统对含不同缺陷的PMMA进行三点弯曲实验,并结合几何分形理论研究不同偏置距离下运动裂纹的分形规律。实验结果表明:存在预制裂纹与静止裂纹的临界偏置距离（6 mm）,该条件下裂纹轨迹对应的分形维数值最大,裂纹轨迹的规则程度最低,裂纹破坏形态最复杂。当预制裂纹与静止裂纹的偏置距离在0～6 mm时,裂纹Ⅰ起裂后垂直向上扩展一段距离,与静止裂纹交汇,并停滞一段时间后发生二次起裂,直至贯穿试件,偏置距离和交汇点竖向坐标值呈近似线性函数关系。偏置距离的存在不会影响裂纹Ⅰ的起裂时间和应力强度因子,但会显著减小裂纹Ⅱ的动态应力强度因子,且停滞时长随偏置距离的增大而逐渐缩短。当偏置距离大于临界偏置距离时,运动裂纹不再与静止裂纹交汇而是呈拱状向试件上边缘扩展直至贯穿,裂纹的起偏时间、起偏位置也会出现明显的滞后现象。     

谐振载荷作用下工程结构振动疲劳寿命预估的损伤力学-有限元法

建立了预估谐振载荷作用下结构振动疲劳寿命的损伤力学-有限元方法。首先根据损伤热力学原理,构建了损伤演化方程,建立光滑试件在恒幅应变交变载荷作用下寿命预估方法;进一步由损伤力学守恒积分原理,得到恒幅重复载荷作用下应力与寿命的关系式;然后根据标准件疲劳试验结果,拟合得到损伤演化方程中的材质参数;最后利用APDL语言编程对ANSYS软件进行了二次开发,借助ANSYS软件对谐振载荷作用下结构振动疲劳裂纹萌生寿命进行预估。作为算例,本文利用该方法预估了LC9CgS1铝合金梁谐振载荷作用下疲劳裂纹萌生寿命。     

一种改进的激光冲击强化效果实验验证方法

表面强化技术可以显著提高材料的疲劳性能,广泛应用于航空零部件的加工制造。在试验研究强化工艺对材料疲劳性能的增强效果时,常采用升降法来进行。针对升降法试验中存在的初始应力不易确定的问题,根据线性累积损伤理论和已知表面强化前材料疲劳特性的特点,提出一种快速确定升降法试验初始应力的方法,应用于K417材料激光冲击强化效果试验验证中。该方法仅需通过2～3个试件的实验即可找到适当的初始应力。通过对不同表面状态标准试片的振动疲劳试验,验证了激光冲击强化可较大幅度增强材料的疲劳性能。     

A Noninvasive, on Wafer Method to Determine the Intrinsic Stress of a Polymer Layer

The method described in this paper allows an investigator to determine the intrinsic stress of a polymer layer in a way that does not result in damage to devices or test structures. The method requires that a small area of the polymer be released from the substrate to form a diaphragm. The diaphragm is stimulated with acoustic white noise and the diaphragm movement is monitored with a laser vibrometer. The first few resonance frequencies of the diaphragm are obtained using a laser vibrometer and then those frequencies are used to calculate the membrane intrinsic bi-axial tension.