A non-local fatigue approach to quantify Ti-10V-2Fe-3Al fretting cracking process: Application to grinding and shot peening

This paper presents an experimental study on the tribological behaviour and cracking response of a Ti-10V-2Fe-3Al titanium alloy under fretting loading with a cylinder on plane configuration. Three types of surfaces were investigated: a polished one considered as the reference, a ground one and a shot peened surface. Surfaces were compared with respect to residual stress, hardness and roughness. The first step of this study was to determine sliding conditions and coefficient of friction of the three contact types. Next, fretting tests under stabilized partial slip regime were carried out to investigate crack nucleation and propagation. Results show that whatever surface roughness or residual stress in the material, tribological behaviour is the same. These latter confirm that sliding condition and coefficient of friction in partial slip regime is due to material effect and not to roughness or surface hardness. Then, residual stress induced by grinding or shot peening have no influence on the crack nucleation threshold under fretting solicitation because crack nucleation is only induced by a sufficient tangential loading. The crack nucleation threshold is formalized by applying the Crossland criterion taking into account the stress gradient and the ensuing “size effect”. As expected, cracks propagation is influenced by residual stress under the surface. Compared to the reference case, for a same loading parameters set, residual stress induced by grinding is not sufficient to decrease the crack length reached whereas effects of shot peening decrease highly these latter. So, there is a threshold of residual stress from which residual stresses are useful against cracking.     

Identification of fretting fatigue crack propagation mechanisms using acoustic emission

In this study, a fretting fatigue test has been equipped with an acoustic emission (AE) device in order to identify the successive crack propagation mechanisms. The fretting fatigue crack nucleation and propagation is a complicated process. Cracks initiate and propagate firstly due to shearing (mode II) and then by tension (mode I). The crack propagation generates mechanical energy emission. Elastic waves appear and can be detected through AE. A complete analysis of the AE signals (multi-parameter analysis, location of the AE in the loading cycle and a statistical analysis) led to an identification of three different steps in the crack propagation process. The evolution of the shearing and the tension influences in the crack propagation process is recognizable separately. Therefore, the three crack propagation steps have been identified as (a) crack propagation in mode II, (b) mixed mode crack propagation and (c) pure mode I crack propagation.     

A new methodology for predicting crack initiation life for rolling contact fatigue based on dislocation and crack propagation

A new methodology for predicting crack initiation life is presented and validated experimentally. The methodology considers that the total fatigue life is the summation of crack initiation life and crack propagation life, since fatigue failures are due to crack initiation and crack propagation. It has been established that the crack propagation life can be estimated based on a modified Paris’ law when the size of crack is larger than a certain value. However, there has been no verified method for estimating the crack initiation life with good accuracy. The proposed methodology for predicting the crack initiation life is based on a dislocation model, and the constants for the model are determined by the crack initiation lives obtained by a new approach. This new approach determines the crack initiation life by subtracting the predicted crack propagation life from the experimentally obtained total fatigue life. The developed crack initiation life model is combined with a crack propagation life model for the prediction of fatigue life. It is noted that the standard deviation in the ratios of experimental life to predicted life by the developed fatigue life model is only 14% of that by the International Standard.     

声发射技术在疲劳裂纹检测中的应用

主要讨论了声发射技术应用于疲劳裂纹的检测.根据疲劳裂纹的信号特征及测试场合,选用合适的疲劳裂纹检测方法,结合板簧疲劳裂纹试验机上的实验分析,得出声发射技术应用于疲劳裂纹检测的优势.     

Fretting fatigue analysis using a fracture mechanics based small crack growth prediction method

A complete life model for the nucleation and growth of a fretting fatigue crack has been developed. The nucleation of a fretting crack is predicted by superimposing the crack growth rate experienced under fretting conditions onto S–N fatigue data for the alloy. The growth model utilizes small crack growth rate data and a fretting fatigue stress intensity factor to account for the small crack sizes and higher stresses experienced under fretting fatigue conditions. The development of the propagation model within the established fatigue crack growth code AFGROW allows this approach to be readily used by members of the aerospace industry.     

一种确定有效应力强度因子ΔK_(eff)的解析模型

通过对疲劳裂纹尖端应力－应变的分析，提出了一种新的ΔＫｅｆｆ的定义，给出了确定ΔＫｅｆｆ的解析模型。将其应用于不同材料数据的分析，结果表明，该模型能够很好地描述疲劳裂纹的闭合效应，并能够给出裂尖附近的应力－应变响应，从而为更加合理地分析疲劳裂纹的扩展提供了重要的依据。     

含缺口结构压疲劳失效研究

结构的疲劳失效与内部的应力水平有着紧密的联系。这里阐述了压应力作用下结构内部裂纹的萌生和扩展机理，并介绍了循环压应力下的裂纹扩展计算方程，结合现代疲劳研究方法的新成果新方法对压应力条件下疲劳破坏研究的未来提出展望。     

16Mn钢焊接件疲劳裂纹扩展的微观特征研究

研究了在循环载荷作用下,16Mn钢焊接件疲劳裂纹的扩展情况.通过光学显微镜观察了裂纹扩展的微观特征,发现了裂纹在焊接结构不同部位的扩展路径方式并分析了显微组织对裂纹扩展的影响.裂纹在母材处为穿晶扩展,在热影响区和焊缝金属中的扩展为穿晶和沿晶混合型.     

组织特征对粉末高温合金FGH96疲劳裂纹扩展速率的影响

通过对不同热处理后获得3种典型显微组织的粉末高温合金FGH96合金试样在650 进行保载90 s和5 s并恒载荷循环应力作用下的疲劳试验,研究组织特征对疲劳裂纹扩展速率的影响规律,确定不同载荷条件下孕育期、萌生期、扩展期及瞬断期所占的比率。结果表明,γ相特征显著影响疲劳裂纹扩展速率;疲劳行为对保载时间存在敏感性,随保载时间的延长,在疲劳-蠕变的交互作用下会明显加快合金疲劳裂纹扩展;在整个疲劳破坏过程中,萌生比率均高于扩展比率,说明该合金抗裂纹萌生的能力要高于抗裂纹扩展的能力;此外,该合金疲劳裂纹扩展四个阶段在整个断裂周次所占的比率分配上,孕育期比率一般较小,萌生期和扩展期比率较大,瞬断期的比率很小,表明该合金裂纹扩展一旦失稳将高速扩展并迅速断裂。     

2.25Cr—1Mo复杂应力状态下低周疲劳裂纹扩展规律研究

通过对2.25Cr-1Mo材料进行不同温度下的低周疲劳试验,并应用NHRDS有限元程序进行裂纹尖端各应力应变分量,各当量弹、塑性应变范围的计算,利用当量J积分范围,研究了该材料在复杂应力状态下的低周疲劳裂纹扩展规律,结果表明,在复杂应力状态下2.25Cr-1Mo材料的低周疲劳裂纹扩展速率可采用当量J积分范围进行表征。     

Life estimation of Ti-6Al-4V specimens subjected to fretting fatigue and effect of surface treatments

Suitability of different multi-axial parameters in predicting fretting fatigue life of Ti-6Al-4V specimens has been investigated. Ameliorating effect of surface treatments on fretting fatigue has been studied. In simple uni-axial/multi-axial fatigue tests, nucleation as well as propagation of cracks occur under the influence of identical stresses. Hence nucleation accounts for most of the total life. Fretting fatigue crack nucleation occurs due to very large contact stresses, effect of which is felt only close to the surface (due to steep gradients). Propagation mostly occurs due to lower stresses in the bulk of the material (negligible influence of contact tractions) and forms a significant portion of total life. Total life has to be taken as sum of initiation life calculated from different multi-axial fatigue parameters and propagation life from conventional fracture mechanics approach. Steep stress gradients necessitate the adoption of a statistics based approach to predict the crack initiation life, based on an assumed distribution of flaws. The quality of comparison between predicted and experimentally observed failure lives provides confidence in the notion that conventional fatigue life prediction tools can be used to assess fretting fatigue failure. Effect of surface treatments like shot-peening with or without additional surface coatings on total life of the specimen and on friction coefficient has been studied.     

Fatigue life prediction by statistical approach under constant amplitude loading

In general, the experimental data of fatigue crack growth rates scatter very much even under identical experimental condition such as a constant amplitude loading condition. It is, thus, essential to take into account the data scatter of crack growth rates by using statistical approach for a reliable fatigue crack, propagation analysis. In this study, fatigue crack propagation tests were conducted on a 1.02 mm-thick 2024-T3 aluminum alloy under a constant amplitude loading condition. The distribution of the fatigue crack propagation life is estimated by using the stochastic Markov chain model based on a modified Paris-Erdogan equation to consider the variability of the fatigue crack growth. The fatigue lives estimated by using the Markov chain model are found to be agreed well with the experimental results.     

Surface and Subsurface Cracks in Rolling Contact Fatigue of Hardened Components

The competitive aspect of surface and subsurface fatigue crack propagation in hardened components subjected to rolling contact fatigue is highlighted, the former being greatly affected by the working conditions (in particular the presence of tangential stresses and lubricant), the latter depending mainly on the inclusions content and on the hardness profile. In order to determine which one of these kinds of damage is favoured, initial data consisting of contact load, rolling and sliding speed, theological properties of the lubricant, material hardness and inclusions content are necessary.

The concurrent role of asperities and Hertzian stress field in determining surface crack propagation is explained with the approach of the “quiescent zone.” calculating the stress intensity factors range in a contact cycle and considering the pumping effect of the fluid possibly present on the contact surface.

Inherent defects (especially oxides) are thought to be responsible for subsurface cracks origin and the Murakami formula for short cracks is extrapolated to describe their growth threshold, which also depends on the hardness and therefore on the depth in surface hardened components.

A crack propagation index is then defined as a ratio of applied to threshold stress intensity factor, both for surface and subsurface cracks. Evaluating this index for a general operating condition, it is possible to determine which damage mechanism is favoured, taking into account the decisive effect of the hardness profile.     

一种基于裂尖损伤区的裂纹扩展模型

提出了“裂尖损伤区”的概念,通过解析闭合模型对裂尖损伤区的应力应变历程进行了分析,应用累积损伤理论建立了一个用于分析变幅载荷下疲劳裂纹扩展寿命的模型。应用本模型计算了不同载荷谱下的裂纹扩展寿命,通过与试验结果的比较,表明本文的方法具有精度高、经济实用的特点,适合工程应用。     

铝锡轴瓦的疲劳裂纹分析

对AlSn20Cu轴瓦进行了疲劳试验,观察到疲劳裂纹的萌生及扩展过程,并以计算所得轴心轨迹、油膜压力和合金层应力分布为依据,分析了轴瓦的疲劳过程,与试验结果相吻合,从而为轴瓦的疲劳裂纹分析提供了新的研究方法。     

一种预测蚀坑腐蚀疲劳寿命的概率模型

蚀坑腐蚀引起的疲劳损伤过程包括七个阶段,蚀坑形成、蚀坑扩展、蚀坑转变为疲劳小裂纹、小裂纹扩展、小裂纹转变为长裂纹、长裂纹扩展和断裂.用一阶可靠性方法(first-order reliability method,FORM)和蒙特卡洛模拟法分别计算铝合金蚀坑腐蚀疲劳寿命,得到疲劳寿命的累积分布函数(cumulative distribution function,CDF),进行概率敏感性分析.同时研究几个随机变量及其变异系数(coefficient of variation,COV)对预测疲劳寿命的影响.     

Numerical Investigation into the Effect of Contact Geometry on Fretting Fatigue Crack Propagation Lifetime

Fretting fatigue is a phenomenon in which two contact surfaces undergo a small relative oscillatory motion due to cyclic loading. There is a need to analyze the effects of contact geometry on crack propagation under fretting fatigue conditions. In this investigation, a finite element modeling method was used to study the effects of different contact geometries along with crack–contact interaction on crack propagation lifetime. Different contacts geometries—that is, cylindrical on flat and flat on flat—along with different contact span widths were analyzed. In addition, the effects of different contact spans on stress distribution at the contact interface were investigated. The computed crack propagation life was compared with experimental results. It was found that the crack initiated near the contact trailing edge for all contact geometries, which agreed with experimental observations. In terms of crack propagation for different contact spans, the fretting fatigue life for a two-based cylindrical pad was shorter than that for a two-based flat pad. By increasing the contact span width for both flat and cylindrical pads, the crack propagation lifetime increased. A comparison between the experimental and numerical results demonstrated a difference of about 18% in crack propagation lifetime.     

航空发动机涡轮盘用GH4133B合金疲劳裂纹扩展行为研究

材料的疲劳寿命由裂纹形成寿命和扩展寿命两部分组成。针对航空发动机涡轮盘用GH4133B合金,进行室温下不同应力比的疲劳裂纹扩展试验,测试疲劳裂纹扩展门槛值。Paris公式回归分析结果表明,裂纹扩展速率随应力强度因子和应力比的增大而增大,含门槛值的修正Paris公式能精确描述疲劳裂纹扩展行为。利用光学显微镜在线观测裂纹扩展路径,并利用扫描电镜考察试样断口微观形貌。结果发现,随应力强度因子增大,裂纹扩展路径由平直变得曲折。在疲劳裂纹萌生区、稳定扩展区和快速扩展区,断裂表面依次呈现为解理断裂、疲劳条带和沿晶韧窝混合断裂模式。基于断口反推理论反推载荷和裂纹扩展方程,结果表明,利用反推方程预测疲劳裂纹的扩展,可有效防范疲劳断裂的发生。     

钛合金Ti-6Al-4V两种微观结构的裂纹扩展行为研究

介绍对两种均匀锻造的片晶结构的钛合金Ti-6Al-4V的疲劳裂纹扩展性能的研究情况。在材料准备时，注意避免材料的方向性等对性能的影响。采用标准试验方法进行各项力学性能试验，试验数据表明，两种片晶微观结构的钛合金的拉伸强度相差约5％，拉伸塑性相关13％，疲劳行为有较大不同，微观组织细密的钛合金材料的性能比较好。两种片晶的小裂纹扩展速率与长裂纹扩展速率相比有明显增加，但对于本研究的试验条件，在应力比R＝0和R＝－1的情况下，两种状态之间的裂纹扩展行为差别不大。试验数据说明，裂纹闭合是小裂纹效应的主要原因之一。