基于SWT方法的钢绞线索微动疲劳特性分析

为得到钢绞线索丝间接触区的应力场分布并预测微动疲劳裂纹萌生位置和微动疲劳寿命,本文利用参数化方法建立了精细化的钢绞线拉索有限元模型,包括整索模型和不同层丝间接触区域的局部精细化子模型．分析了钢绞线索在两种交变荷载工况下的应力场变化情况,并基于多轴疲劳SWT(Smith-Watson-Topper)临界平面法进行了疲劳特性分析和疲劳寿命预测．主要结论如下：钢绞线索内接触区边缘处的微动幅值较大,中心处几乎没有相对滑动,微动疲劳的初始裂纹萌生点位于接触区域边缘;经不同区域子模型分析比较,在轴向循环荷载作用下,外层钢丝的接触区域比内层钢丝更易发生微动疲劳损伤;在横向位移循环荷载作用下,同层钢丝因位置角度不同而产生了较大的疲劳特性差异,且相比轴向循环拉伸,该工况下最不利单丝的微动疲劳寿命更低;与非接触区域相比,接触区的疲劳寿命大幅降低,微动现象对钢绞线索的抗疲劳性能有明显降低作用．     

Cumulative damage evaluation of steel using infrared thermography

Following the first damage models proposed by Palmgren and Miner, numerous researchers have focused on the problem of predicting the residual life of a material from its load history. Every component dynamically loaded, particularly over the fatigue limit, shows an increase in temperature. The higher the temperature, the higher the load applied. Therefore, in an undamaged material or mechanical component, it is possible to associate each loading stress over the fatigue limit with a temperature value at the hottest point of the surface during the first phase of the test trough a thermo-mechanical characterization (TMC) map. Using the thermoanalysis of steel specimen data, this paper shows that the energetic effect (as a different temperature increments for equal loading uniaxial stress) can be used to evaluate the cumulative damage caused by previous loading. The tests were performed using C40 steel for which traditional fatigue curves in literature are reported.     

Fatigue Loading and Life Prediction in Three Fretting Fatigue Fixtures

Three fixtures for conducting laboratory fretting fatigue tests are described and their respective testing methods and the results of the analysis are compared. Each of these fixtures has been used to investigate the effects of various parameters of interest in fretting fatigue. These fixtures include a unique apparatus in which all load applied to the specimen is transferred to the fretting pads, an apparatus similar to many found in the literature where partial load transfer occurs across the pads, and a simplified dovetail fixture in which the clamping load, P, and the shear load, Q, are varied in phase. Select test conditions from prior experiments performed on identical material and resulting in similar lives ranging from one to ten million cycles from these fixtures are identified. The various testing conditions were used to compute the unique stress field for each case. The resulting contact stresses were used to calculate crack initiation based criteria, and to calculate stress intensity factors. The three fixtures were shown to be able to accommodate a range of loads, fretting pad contours, and specimen geometries that produced a variety of stress fields. A crack-initiation-based criterion was shown to predict the failure lives of thinner specimens accurately. The stress intensity factor calculations showed the possibility of a crack arresting for a stress field that decays rapidly and the possibility of a local minimum for K as a function of depth. The fixtures are shown to be complementary in generating data for development of robust fretting fatigue models that use these criteria.     

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。     

Fatigue and fretting fatigue of ion-nitrided 34CrNiMo6 steel

This work is concerned with the surface treatment (ion nitriding) of fretting fatigue and fatigue resistance of 34CrNiMo6. Tests are made on a servo-hydraulic machine under tension for both treated and non-treated specimens. The test parameters involve the applied displacements δ±80–±170 μm; fretting pressure σ_n=1000–1400 MPa; fatigue stress amplitude σ_a=380–680 MPa and stress ratio R=−1. The ion nitriding process improves both fatigue and fretting fatigue lives. Subsurface crack initiation from internal discontinuities was found for ion-nitrided specimens.     

Q345低周疲劳性能与疲劳寿命预测分析

本文对结构用钢Q345的低周疲劳性能进行了试验研究。试验在常温下岛津电液伺服疲劳试验机上进行,采用轴向应变控制方法,恒定应变速率为0.005s-1,应变比为-1。试验结果表明,初始阶段,Q345在高应变幅值(0.6%)循环作用下出现循环硬化效应,而在低应变幅值(0.6%)作用下出现循环软化效应;随着加载应变幅的增加,硬化和软化率呈直线上升趋势。Q345疲劳裂纹萌生阶段占其整个寿命的60%以上,其裂纹萌生寿命与应变幅存在幂函数关系。根据Coffin-Manson公式得到了Q345的应变-寿命关系公式;采用能量预测法得到了材料的塑性应变能与疲劳寿命的关系表达式。上述结果对钢结构的设计、评估具有重要的工程应用参考价值。     

Freeting fatigue strength of mechanical joints

An analytical approach to the calculation of fretting fatigue strength of mechanical joints based on a stress model of the fretting fatigue damage mechanism has been presented. The fretting fatigue limit has been defined as a stress (expressed by the threshold stress intensity factor) required for further propagation of the non-propagating main fatigue crack appearing in a cyclically loaded member. Some practical recommendations resulting from the theory have been suggested.     

Limitations of the Coulomb friction assumption in fretting fatigue analysis

A significant factor in the fretting process, both experimentally and analytically, is the Coulomb friction. Most analyses of fretting fatigue consider a constant coefficient of friction (CCOF) in modeling a contact geometry. This work reevaluates the constant assumption of the Coulomb friction coefficient, and develops a Coulomb friction model based on a non-classical model allowing the coefficient of friction (COF) to be a function of local contact pressure and local slip magnitude. Here, the Coulomb COF varies locally along the contact surface. Results of computations using this model are applied to fretting fatigue experiments utilizing several specific contact geometries, which have nominally identical fatigue lives experimentally. The analysis shows that certain combinations of parameters in the variable coefficient of friction model can produce nominally identical stress states. Such results cannot be obtained using a CCOF.     

预应变对中碳钢扭转疲劳特性的影响

应用扭转疲劳试验机对退火后的非预应变45#钢及其3种预应变材料进行 实验. 实验结果表明：预应变量小于5%时,与非预应变材料相比疲劳极限降低10\%; 预应变量大于5{%}时,随着预应变量的增加,其疲劳极限有恢复的趋势,当预应变量达到 8%时,预应变材料与非预应变材料的疲劳极限几乎相等;当循环应力达到1*10^7$时,各种试件表面都有明显的停留裂纹存在.     

TC21钛合金显微组织对超长寿命疲劳行为的影响

本文主要研究钛合金的网篮尺寸对于超声疲劳行为的影响。研究发现,对于TC21钛合金,高应力下疲劳裂纹萌生于试样表面;低应力下疲劳裂纹萌生于试样亚表面。最大应力-循环次数(S-N)曲线显示,在两段连续下降的曲线之间有一个表面萌生向内部萌生的转变平台。对于网篮尺寸为60μm的钛合金,转变平台对应的应力幅为540MPa,疲劳极限为430MPa。网篮尺寸为40μm的转变平台应力幅为600MPa,疲劳极限为530MPa。分析得知网篮尺寸为60μm的TC21钛合金疲劳性能低于网篮尺寸为40μm的TC21钛合金。     

经不同表面改性处理的钛合金的微动疲劳和微动磨损行为对比研究

对比研究了钛合金微动疲劳(FF)和微动磨损(FW)失效行为,考察了表面喷丸强化和氮化等表面处理对钛合金微动疲劳和微动磨损性能的影响,探讨了钛合金微动磨损和微动疲劳性能的相关性.结果表明,钛合金微动疲劳和微动磨损损伤表面形貌特征相似;当微动位移幅较大、微动区发生整体滑动时,微动接触区磨损有利于延缓微动疲劳裂纹萌生;而在小位移幅、部分滑移情况下,局部磨损促进微动疲劳裂纹萌生.利用喷丸强化在钛合金表面引入残余压应力,可以在降低摩擦系数的同时,提高钛合金抗微动疲劳和微动磨损失效的能力;氮化处理后钛合金表面硬度提高,有利于改善其微动磨损性能,但表面韧性降低导致抗微动疲劳能力降低.因此,在提高表面硬度的同时,不应忽视表层韧性的降低对钛合金微动疲劳性能的不利影响.     

Influence of load amplitude and uniaxial tensile properties on fatigue crack growth

The rate at which energy is accumulated within a unit volume of material in fatigue is assumed to depend not only on load-time history but also on the specimen size and geometry in addition to material type. A threshold level for the hysteresis strain energy density function accumulated in the material is used for predicting macrocrack growth. This is accomplished by application of the incremental theory of plasticity for each increment of crack growth. The accumulated hysteresis strain energy density factor ΔS to crack growth increment Δa ratio is found to be constant for fixed specimen size and loading, i.e., . Results are obtained for the cylindrical bar specimens with a penny-shaped defect at the center subjected to a constant amplitude and frequency loading. The resistance curves in the ΔS versus Δa plot are parallel lines as specimen size is altered. This information provides a rational means for predicting the influence of specimen size on fatigue lifetime.The results are also compared with those found for geometrically similar plate specimens with line cracks. Cylinder bar specimens of the same material are found to sustain more load cycles prior to catastrophic failure.     

Low-endurance fatigue behavior under combined bending and torsion

Specimens made of thin-walled low-carbon steel tubes are subjected to combined bending and twisting moments of such magnitudes as to lead to fatigue failure within some one-thousand loading cycles. Tests are run, at room temperature, under conditions of controlled angle of twist and constant rate of loading. Experimental results show that the fatigue life is best expressed in terms of the amplitude of equivalent stress and ultimate strength of the material in potential form. For same maximum stress amplitude, test results show, within the scope of present investigations, that conditions of combined bending and torsion lead to lower fatigue life than those of reversed torsion.     

Estimation of Fatigue Limits from Temperature Data Measured by IR Thermography

To determine the fatigue limits of materials, fatigue testing, which is time-consuming and very expensive, is required. In order to overcome these shortcomings, estimating methods for fatigue limits based on temperature changes measured by IR camera have been proposed, and research and development of such methods have been widely conducted. In the current paper, a non-lock-in type, inexpensive IR camera was used for rotational bending fatigue testing to estimate the fatigue limit from temperature changes independent from loading signals. The results indicated that it is possible to estimate the fatigue limit from the time-temperature change curves measured under various stress conditions and converted stress amplitude-temperature change curves. The estimated results were sufficiently accurate and thus we confirmed that it is promising to estimate the fatigue limit by a non-lock-in type, inexpensive IR camera. The inflection point of a stress amplitude-temperature change curve can be determined by approximating a curve by two straight lines and finding the combination of the lines for which the sum of the residuals between the curve and the lines is the smallest. Although the temperature changes depended on the loading history (the number of loading cycles), the results of fatigue limit estimation changed little. Therefore, the proposed method is practically accurate as a simple estimation method. We also measured the behavior of stress-stroke for each loading history (the number of loading cycles) in tension-compression fatigue testing and confirmed that temperature changes during fatigue testing are associated with plastic strain energy.     

Size effect of cylindrical specimens with fatigue cracks

The rate at which energy is accumulated within a unit volume of material in fatigue is assumed to depend not only on load-time history but also on the specimen size and geometry in addition to material type. A threshold level for the hysteresis strain energy density function accumulated in the material is used for predicting macrocrack growth. This is accomplished by application of the incremental theory of plasticity for each increment of crack growth. The accumulated hysteresis strain energy density factor ΔS to crack growth increment Δa ratio is found to be constant for fixed specimen size and loading, i.e., $\text{ΔS}\text{Δa}\text{=}\text{const}$. Results are obtained for the cylindrical bar specimens with a penny-shaped defect at the center subjected to a constant amplitude and frequency loading. The resistance curves in the ΔS versus Δa plot are parallel lines as specimen size is altered. This information provides a rational means for predicting the influence of specimen size on fatigue lifetime.The results are also compared with those found for geometrically similar plate specimens with line cracks. Cylinder bar specimens of the same material are found to sustain more load cycles prior to catastrophic failure.     

Extended finite element method for fretting fatigue crack propagation

In this paper, the extended finite element method (X-FEM) is considered for the analysis of fretting fatigue problems. A two-dimensional implementation of the X-FEM is carried out within the finite element software ABAQUS? by means of user subroutines, and crack propagation in fretting fatigue problems is investigated. On utilizing the non-linear contact capabilities of this code, the numerical technique is applied to a specimen-indenter model. The use of the X-FEM facilitates very accurate stress intensity factor computations on relatively coarse meshes, and furthermore, no remeshing is required for crack growth simulations. The implementation is applied to complete and incomplete contact fretting problems. A study of crack growth is conducted for several bulk loads applied to the specimen, and the influence of the initial crack angle is ascertained. The numerical simulations reveal the merits of applying the X-FEM to fretting fatigue problems.     

Fatigue crack opening stress based on the strip-yield model

The modified strip-yield model based on the Dugdale model and two-dimensional approximate weight function method were utilized to evaluate the effect of in-plane constraint, transverse stress, on the fatigue crack closure. The plastic zone sizes and the crack opening stresses considering transverse stress were calculated for four specimens: single edge-notched tension (SENT) specimen, single edge-notched bend (SENB) specimen, center-cracked tension (CCT) specimen, double edge-notched tension (DENT) specimen under uniaxial loading. And the crack opening behavior of the center-cracked specimen under biaxial loading was also evaluated. Normalized crack opening stresses σ_op/σ_max for four specimens were successfully described by the normalized plastic zone parameter Δω^′_rev/ω^′ considering transverse stress, where Δω^′_rev and ω^′ are the size of the reversed plastic zone at the moment of first crack tip closure and the size of the forward plastic zone for maximum stress, respectively. The normalized plastic zone parameter with transverse stress also was satisfactorily correlated with the behavior of crack closure for CCT specimen under biaxial loading.     

比例超载对铝合金圆孔薄板试件低周拉-拉疲劳寿命的影响

对铝合金圆孔薄板试件进行了超载和常辐的对比实验,取六种超载比,对每一超载比各进行了不同超载周次的实验,从这些实验结果看出,对每一超载比,超载试件的疲劳寿命和常辐疲劳试件的寿命比值先是随超载周数的增加而较快的增加,之后在一个相当大的超载周数范围内,超载寿命比的变化不大。当超载周数超过一定值时,超载寿命比随超载周数的增加而略有减少。本文的实验中,在超载60％的情况下,超载周数从20周到90周的范围内,超载试件的平均寿命比常辐疲劳试件的寿命约高3.5倍。     

Plasticity in polycrystalline fretting fatigue contacts

Plastic deformation at the scale of microstructure plays an important role in fretting fatigue failure of metals under cyclic loading. In this study, crystal viscoplasticity theory with a planar triple slip idealization is employed to represent crystallographic plasticity in two-dimensional fretting analyses of Ti-6Al-4V. Subsurface deformation maps, fretting maps, and shakedown maps are constructed based on application of J₂ plasticity theory for the polycrystalline substrate. Comparisons are then made with polycrystal viscoplasticity simulations, the latter suggesting that plastic ratchetting plays a significant role in the fretting fatigue process.