考虑分级加载的连续非线性Chaboche-Paris全寿命模型

结构疲劳全寿命可分为裂纹萌生和裂纹扩展两个阶段,裂纹萌生寿命和裂纹扩展寿命的预测通常分开进行,很少有理论能将两者合二为一。结合CHABOCHE提出的非线性损伤理论,对Paris公式进行修正,将其扩展至全寿命阶段;建立损伤累积与裂纹长度关系模型,分析分级加载对损伤累积的影响。计算结果表明,提出的Chaboche-Paris全寿命模型对无初始裂纹结构的寿命预测结果与其S-N疲劳试验数据结果一致,对具有宏观可见裂纹结构的寿命预测结果与Paris公式计算结果一致,验证提出的全寿命模型在全寿命预测和裂纹扩展寿命预测两个阶段的可用性和正确性;分级加载时,Chaboche-Paris模型可以体现出加载顺序对疲劳损伤累积的影响,当外载为低-高加载时,循环比之和大于1,当外载为高-低加载时,循环比之和小于1,与试验结果吻合。     

超声疲劳裂纹扩展与摩擦生热研究

在高频循环载荷作用下,材料疲劳裂纹的萌生与扩展过程伴随着明显的温度变化,该温度变化反映材料内部结构的损伤特征。通过20 kHz的超声疲劳试验,研究一种碳锰钢在超高周疲劳加载条件下的内部疲劳裂纹萌生与扩展过程中温度的演化过程。通过对该材料在疲劳损伤过程中,内部裂纹间的摩擦生热机理分析,从微观角度出发,结合分形理论,建立内部裂纹微观结构的摩擦模型,数值模拟超声疲劳过程中材料内部疲劳裂纹面间的摩擦生热情况,并定量地计算该过程中由裂纹间摩擦所导致的温度上升,将模拟结果与试验结果进行比较。探究高频疲劳载荷下微裂纹扩展与摩擦生热的关系,并结合超高周疲劳裂纹扩展公式,建立超声疲劳过程中的裂纹扩展与裂纹面温度演化关系的模型。     

摩擦热和夹杂对高速微型球轴承接触亚表面裂纹萌生的影响

为探究高速工况下微型球轴承摩擦生热和材料夹杂对接触亚表面裂纹萌生的影响，基于赫兹接触理论与内聚力单元的本构关系建立了微观接触数值计算模型，设置具有热力耦合的自由单元，引入摩擦热和夹杂，研究在联合载荷作用下温升和夹杂对接触亚表面裂纹萌生的影响。结果表明：夹杂与温升均会促进裂纹的萌生且材料较硬的夹杂更易萌生裂纹；夹杂边界出现应力集中，摩擦生热使夹杂边界更易形成裂纹且裂纹大多在夹杂的左右两端。通过轴承温升的宏观试验结果与微观仿真结果对比验证了模型的正确性。     

非金属夹杂物对滚动接触疲劳裂纹萌生及扩展的影响

为提高机械设备结构部件的疲劳寿命,研究材料中非金属夹杂物对滚动接触疲劳的影响,综述了非金属夹杂物的类型、形状及尺度,分析了非金属夹杂物相对表面深度、应力集中与热膨胀系数对裂纹产生的影响。阐述了疲劳裂纹的扩展和疲劳失效,分别介绍了基于动态剪应力、Mises应力和裂纹尖端应力的裂纹扩展模型。介绍了疲劳剥落失效的有限元法和基于弹塑性力学的理论分析法,以及试验技术如磁场检测、超声波检测、声发射、X射线和辐射分层成像技术等,分析了这些方法的优缺点,最后,提出了需要进一步研究的问题,以期为滚动轴承的疲劳寿命预测与提高提供基础。     

滚动轴承接触疲劳内部裂纹扩展有限元分析

滚动轴承滚道在工作过程中承受较大的接触应力,接触疲劳是滚动轴承失效的主要形式。为了探究轴承的接触疲劳以及接触疲劳引起的内部裂纹,将损伤力学带入Voronoi有限元方法中,仿真轴承材料拓扑随机性和材料的劣化过程。建立轴承接触疲劳裂纹扩展模型,有效仿真出轴承内部裂纹的萌生、生长、相交、扩展至表面的过程,得到轴承表面裂纹出现的寿命和内部裂纹扩展路径。计算了裂纹扩展过程中所释放的能量,研究内容能够为轴承接触疲劳损伤研究提供新的思路和工具。     

疲劳载荷下复合材料分层损伤扩展的数值模拟方法

为了研究复合材料层合结构在疲劳载荷下的分层扩展行为,开发了复合材料层间疲劳损伤数值计算模型。定义层间材料模型包括层间材料本构关系、损伤判据以及疲劳损伤演化法则。疲劳损伤通过定义内聚力单元损伤变量的演化规律来表示。通过有限元商用软件,考虑到疲劳问题的特点,运用加速算法和疲劳载荷等效原理,并结合自定义材料子程序实现了疲劳损伤的模拟。最后,运用该模型模拟了层状复合材料双悬臂梁的疲劳分层扩展,并将数值模拟结果与实验数据对比,表明该模型在模拟层间裂纹的形成和扩展的有效性。     

典型构件疲劳微裂纹全寿命预测方法研究

基于损伤力学理论建立金属疲劳裂纹损伤模型,结合有限元法预估疲劳微裂纹萌生及扩展全寿命。引入附加载荷法,实现了对刚度矩阵的连续计算,应用Matlab编程计算,通过对单个单元的损伤计算,得到了单元从无损到破坏过程中等效应力的变化,进而给出疲劳裂纹损伤模型,并对不同规格金属试件的疲劳寿命预估。将计算结果与试验结果进行对比,得到的相对误差在接受范围内,验证了该模型的准确性,对于研究金属试件微观损伤以及疲劳寿命预估具有重要的参考意义。     

2.25CrMoV钢于夹杂物和晶界处开裂低周疲劳裂纹扩展的原位观测

对汽轮机转子用2.25CrMoV钢在真空常温条件下进行高应力低周疲劳试验,对试样中起源于夹杂物和晶界等典型缺陷处的裂纹扩展进行了原位连续观测,研究了典型缺陷对疲劳裂纹萌生和初期扩展的影响。结果表明:在高应力循环载荷作用下,夹杂物的形状和尺寸对疲劳裂纹的扩展无明显影响,夹杂物处萌生裂纹的扩展速率小于晶界处萌生裂纹的扩展速率;短裂纹群体效应和滑移线快速增殖的共同作用是导致试样疲劳破坏的主要原因。     

16MnR钢在不同应力比下的疲劳裂纹扩展的试验研究及模拟

采用3.8 mm厚带有圆形缺口的CT试样,研究了16MnR钢在不同应力比的恒幅循环载荷作用下的疲劳裂纹扩展。开发了一种基于疲劳损伤的方法来模拟疲劳裂纹扩展速率。将16MnR钢的循环塑性本构模型通过用户材料子程序UMAT嵌入到ABAQUS中。把有限元计算得到的疲劳裂纹尖端附近区域的弹塑性循环应力应变结果,代入到疲劳损伤模型中,得到每个加载循环在裂尖各点产生的疲劳损伤值。通过疲劳损伤准则,导出疲劳裂纹稳定扩展速率的计算公式。疲劳裂纹扩展试验验证了模拟结果。实验结果和模拟结果都表明,该试样厚度下,应力比对裂纹扩展速率几乎没有影响。     

磨损对钢轨滚动接触疲劳损伤的影响

通过对广深铁路钢轨疲劳斜裂纹与钢轨侧磨的现场调查及钢轨滚动疲劳模拟试验,研究了钢轨滚动接触疲劳损伤与磨损之间的作用关系,分析了轮、轨滚动疲劳过程中钢轨磨损率对疲劳裂纹形成及裂纹扩展寿命的影响。结果表明:钢轨磨损严重时,疲劳斜裂纹损伤就表现轻微,钢轨滚动疲劳损伤与磨损之间表现为相互竞争与制约的耦合作用关系;通过增加磨损率能减轻钢轨的滚动疲劳损伤,有利于延长钢轨的疲劳裂纹扩展寿命。     

On the prediction of fatigue crack initiation in rolling/sliding contacts with provision for loading sequence effect

The principles of continuum damage mechanics are applied to predict the rolling/sliding contact fatigue crack initiation. The approach involves evaluating the subsurface stresses as well as the state of damage within the contact region. It is shown that the fatigue crack initiation life can be related to the scalar damage parameter D, which is a measure of micro-crack and voids density in the material. Comparison of the predicted results with the available experimental work shows good agreement. The effect of variable loading on the fatigue behavior of rolling contact with provision for non-linear damage evolution is also investigated.     

轧机油膜轴承锥套接触疲劳损伤的机理分析

研究了油膜轴承锥套与辊颈接触表面在轧制载荷作用下发生微动疲劳损伤的力学机理。在分析油膜轴承油膜压力分布特征的基础上,采用有限元法给出了锥套与辊颈接触区边缘产生接触应力集中和微滑移的分布规律,阐明了弹性结合连接的锥套与辊颈接触表面产生微动疲劳损伤的原因。利用断裂力学裂纹尖端应力场与接触边缘区域应力局部渐近场的等效原则,给出了疲劳裂纹自点蚀处萌生与扩展的力学条件。上述结果为锥套和辊颈的实际损伤状态及模拟试验所验证。     

Numerical Estimation of Fretting Fatigue Lifetime Using Damage and Fracture Mechanics

Fretting fatigue is a complex tribological phenomenon that can cause premature failure of connected components that have small relative oscillatory movement. The fraction of fretting fatigue lifetime spent in crack initiation and in crack propagation depends on many factors, e.g., contact stresses, amount of slip, frequency, environmental conditions, etc., and varies from one application to another. Therefore, both crack initiation and propagation phases are important in analysing fretting fatigue. In this investigation, a numerical approach is used to predict these two portions and estimate fretting fatigue failure lifetime under a conformal contact configuration. For this purpose, an uncoupled damage evolution law based on principles of continuum damage mechanics is developed for modelling crack initiation. The extended finite element method approach is used for calculating crack propagation lifetimes. The estimated results are validated with previously reported experimental data and compared with other available methods in the literature.     

钢轨滚动接触疲劳研究

介绍了滚动接触疲劳裂纹的萌生及扩展的形成机理、钢轨滚动接触疲劳的破坏分类、影响因素,从钢轨新材料的开发、轮轨接触几何型面的优化和铁路工况的改善等几个方面提出了减缓钢轨滚动接触疲劳的措施。     

In situ analysis and modeling of crack initiation and propagation within model fretting contacts using polymer materials

Crack initiation and growth under fretting-fatigue and fretting-wear contact conditions have been investigated using model epoxy materials which allowed an in situ visualization of damage processes within the contact zone. A fretting-fatigue predictive model has been used to analyze these experiments on the basis of an accurate determination of the contact conditions and knowledge of the bulk fatigue properties of the epoxy materials. A special emphasis has been directed towards crack nucleation and early growth during Stage I and Stage I-to-Stage II transition, Stage II crack growth. Photoelastic experiments especially showed that crack propagation under fretting-fatigue loading was strongly dependent upon the complex cyclic micro-sliding behaviour of the crack faces. Under a fretting-wear condition, the constancy of the local contact conditions within the gross slip regime allowed the derivation of realistic estimates of the crack initiation times from the theoretical model.     

Explicit finite element modeling of subsurface initiated spalling in rolling contacts

An explicit finite element model was developed to investigate crack initiation and spall formation in machine elements subject to rolling contact fatigue. The modeling approach utilizes continuum damage mechanics to capture the initiation and propagation of fatigue damage that leads to the formation of a surface spall. The material microstructure is modeled via a randomly generated Voronoi tessellation. The material parameters for the model were obtained independently from torsional fatigue life data for 52100 bearing steel. The life scatter (Weibull slope) and the spall geometry obtained from the model correlate well with experimental results available in the open literature.     

Effects of compressive stresses on torsional fatigue

Rolling contact fatigue (RCF) is the dominant failure mode in properly installed and maintained ball and roller element bearings. Lundberg and Palmgren in their seminal publication indicated that this failure is due to the alternating component of shear stress. Thus, torsional fatigue experiments have been used to predict the RCF behavior of bearing materials. In non-conformal contacts, due to Hertzian pressure the contact experiences large compressive stresses. Hence, it is critical to take into account the effect of these large compressive stresses in torsional fatigue to better simulate RCF conditions. This paper presents an investigation of torsional fatigue of bearing steels, while the effects of combined compressive stress and its relevance to material behavior in rolling contact fatigue is examined. An MTS test rig was used to investigate the fatigue life of several bearing steels and their failure mechanisms were evaluated through fractography. Then the effects of compressive stresses on torsional fatigue were investigated. A set of custom designed clamp fixtures were designed, developed and used to apply Hertzian pressures of up to 2.5 GPa on the torsion specimens. The experimental results indicate that at high cycle fatigue, a combination of shear and biaxial compression, by application of Hertzian contact, is more detrimental to fatigue life than shear alone; however, as expected it has little to negligible effects in the low cycle fatigue regime. Also the failure mode changes such that fracture planes form a cup and cone pair with multiple internal cracks as opposed to helical planes observed in pure torsion which are formed by a single crack. A 3D finite element model (using ABAQUS) was developed to investigate the fatigue damage accumulation, crack initiation, and propagation in the material. The topology of steel microstructure is modeled employing a randomly generated Voronoi tessellation wherein each Voronoi cell represents a material grain and the boundaries between the cells are assumed to represent the weak plane in the steel matrix. Continuum damage mechanics (CDM) was used to model material degradation during the fatigue process. A comprehensive damage evolution equation is developed to account for the effect of mean stress on fatigue. The model predicts the fatigue lives and crack patterns successfully both in presence and absence of compressive stresses.     

Evaluation of slant crack propagation under RCF in railway rail

Crack propagation due to rolling contact fatigue (RCF) could be a significant potential challenge to the integrity of railway rails because it may lead to a serious disaster. Fatigue cracks subjected to cyclic rolling contact force experience a complex non-proportional mixed loading and complicated boundary condition. In the present research, complex crack opening/closure/sliding/locked behaviors as the cyclic contact loading movement is analyzed considering liquid lubrication action on rail surfaces as well as crack faces. Based on a series of FE analyses, the calculations of the effective SIF ranges for RCF cracks under certain contact loading and boundary conditions are proposed in the form of polynomial functions which will be appropriately used to predict RCF crack growth rate.     

Service Loading of Rolling Bearings in Wind Turbine Gearboxes: X-ray Diffraction Material Response Analysis of White Etching Crack Premature Failures

Rolling bearings in wind turbine gearboxes occasionally fail prematurely due to white etching cracks. The appearance of preparatively opened fracture faces indicates damage initiation from the surface in most cases due to brittle spontaneous tensile stress–induced cracking. Under the influence of decomposition products of the penetrating lubricant, branching crack growth is subsequently driven by corrosion fatigue. The material response analysis of rolling bearings from all gearbox locations, based on X-ray diffraction residual stress measurements, reveals vibrations in service as the root cause of surface crack initiation. The occurrence of high local friction coefficients in the rolling contact is described by a tribological model. Depth profiles of the equivalent shear and normal stresses are respectively compared with the measured residual stress patterns and a relevant fracture strength. White etching crack failures are experimentally reproduced on a rolling contact fatigue test rig under increased mixed friction. Causative vibration loading is evident from material response analysis. The generation of compressive residual stresses by cold-working the surface is proven to be an effective countermeasure.