某型飞机单机寿命监控技术研究

用一类疲劳危险部位的寿命代替全机疲劳寿命的方法,对某型飞机的寿命做了定量分析,并基于考虑材料记忆特性的局部应力应变法,对某型飞机的疲劳危险部位进行了标准谱下的寿命计算,计算结果与试验结果较吻合。最后采用局部应力应变法及修正的Manson-Coffin公式,得到了基于飞参数据载荷谱的某型飞机危险部位的实际寿命,并与基于标准谱的疲劳寿命做了对比,为某型飞机的单机寿命监控奠定了基础。     

梁框三维实体单元与板壳单元的组合建模研究

在Patran全机有限元模型中将梁框的分析部位离散为三维实体单元,其余部分离散为板壳单元,这两种单元采用几种不同的方式连接。在全机有限元模型中施加109种疲劳载荷工况分析计算,提取出梁框分析部位的计算结果组成疲劳应力谱,并将疲劳应力谱转化为R=-1的等效应力谱,采用Miner累积损伤理论估计出分析部位的疲劳寿命,比较不同连接方式对疲劳寿命的影响。结果表明梁框三维实体单元与板壳单元的连接方式不同对各工况的疲劳应力值影响较小,对分析部位的疲劳寿命影响较大。     

某机型货舱连接接头的疲劳和损伤容限分析

根据某机型货舱连接接头的结构特点和应力有限元分析结果,针对危险点编制符合货舱门下部两梁之间的连接接头的细节应力谱,采用名义应力法计算得出货舱门下部梁1与梁2之间的连接接头的疲劳寿命。根据疲劳寿命计算结果,建立疲劳危险点区域的裂纹扩展分析模型,进行疲劳裂纹扩展寿命分析,最终验证了此连接接头的疲劳寿命和裂纹扩展寿命满足设计要求。     

15MnVN钢疲劳性能的实验研究

总结了15MnVN钢56mm厚正火板材的疲劳实验研究结果,给出了S-N曲线和疲劳裂纹起始寿命曲线的表达式以及疲劳极限和裂纹起始门槛值。研究结果表明,疲劳裂纹起始门槛值是在切口根部不造成疲劳损伤的上限当量应力幅值;拉伸超载对15MnVN钢的疲劳裂纹起始寿命没有影响。本文对上述实验研究结果的工程应用进行了讨论,并提出了在进行变幅载荷下疲劳裂纹起始寿命模拟试验和寿命估算时如何截除载荷谱中的小载荷的建议。     

某机中央翼下壁板疲劳裂纹扩展寿命估算

为评估某型飞机中央翼下壁板萌生疲劳裂纹后的使用安全性,进行结构模拟件疲劳试验,并利用AFGROW软件对其疲劳裂纹的扩展寿命进行估算,与试验结果进行对比,初步确定出该部位裂纹扩展速率,为裂纹故障部位的修理提供参考依据.     

基于等效裂纹法的桥(门)式起重机箱形梁疲劳寿命估算

针对起重机箱形梁结构及其受力情况复杂的特点,提出了一种以疲劳试验数据为基础的等效裂纹法来估算疲劳寿命。疲劳试验数据能够很好反映结构疲劳危险处的应力集中影响。箱形梁疲劳危险点处的应力集中影响可以大概等效为一未焊接平板上具有一等效裂纹,并用Paris公式计算等效裂纹的扩展寿命。通过将该方法的估算结果与文献中的疲劳试验结果进行对比分析,文中方法对于常幅载荷作用下的疲劳寿命估算是有效可行的,而对于变幅载荷作用下的疲劳寿命估算还有待进一步研究。     

振动环境下飞机薄壁结构紧固件疲劳寿命研究

列举了飞机薄壁结构紧固件损伤失效故障的典型实例,简要分析了其失效特点与危害性。针对某型飞机机翼前缘襟翼单面连接结构紧固件松动、脱落的故障问题,提出了3种改进方案,并采用拉-拉疲劳试验与振动疲劳试验方法对改进前后的紧固件进行了对比试验。为模拟紧固件在振动疲劳载荷作用下的失效模式与失效过程,研制出了冲击振动试验台;初步建立了飞机结构紧固件振动疲劳试验方法。结果表明,在振动疲劳载荷与传统疲劳载荷作用下飞机结构紧固件的失效模式、损伤机理及寿命分布规律有显著差别。依据试验结果,给出了某型飞机机翼前缘襟翼单面连接结构改进的首选和备选方案。     

主起落架疲劳断口定量分析

对某型主起落架全尺寸疲劳试验的裂纹断口进行了分析,利用疲劳扩展理论和迟滞模型导出了谱载荷下裂纹扩展寿命的两个估算公式。利用它们对断口的宏观弧线和微观条纹进行最佳拟合,两者结果相当吻合,并由此给出了主起落架的首翻期。     

用当量S—N曲线估算疲劳裂纹形成寿命

介绍了一种利用相对Ｍｉｎｅｒ法则估算构件在变幅载荷谱下裂纹形成寿命的当量Ｓ－Ｎ曲线方法。与常规的名义应力法相比，此方法不用常幅试验下得到的Ｓ－Ｎ曲线，而是根据该构件在两种载荷谱下的疲劳试验寿命，推导出能反映在变幅载荷谱条件下构件疲劳特性的当量Ｓ－Ｎ曲线为依据寿命估算。运用此方法，本文对文献（１）所给六种载荷谱下的试片寿命进行了计算，并以试验寿命为依据对按常规Ｓ－Ｎ曲线法修正的Ｓ－Ｎ曲线法，局部应力     

机械零部件的疲劳寿命估算与疲劳试验——程序载荷谱的工程应用

本文在最常用的迈内尔(Miner)线性累积损伤理论的基础上,给出机械零部件在载荷作用下裂纹形成寿命的估算方法,并以汽车后桥壳为例,利用载荷谱,对其疲劳寿命进行估算和疲劳试验验证。     

使用疲劳裂纹扩展数据的疲劳裂纹扩展的可靠性分析方法

利用现有的疲劳裂纹扩展数据或疲劳裂纹扩展试验的不完全数据 ,对疲劳裂纹随机扩展的可靠性进行了研究。基于疲劳裂纹扩展的确定性模型 ,导出了疲劳裂纹扩展的随机公式。利用现有的疲劳裂纹扩展数据或疲劳裂纹扩展试验的不完全数据 ,对疲劳裂纹扩展随机公式中的随机变量的分布进行了估计 ,得到了其分布的数字特征 ,用Monte Carlo方法得到了疲劳裂纹扩展寿命的失效概率的点估计。实例分析表明了本文方法的实用性和可行性     

On the application of a micromechanical small fatigue crack growth model to predict fretting fatigue life in AA7075-T6 under spherical contact

This work presents a method for assessing the fretting fatigue life by estimating the fatigue crack growth rate from the regime of microcracks to the final failure, which is achieved using a two-threshold small fatigue crack growth model. The propagation thresholds are associated with the interaction of the "monotonic plastic zone" and the "cyclic plastic zone" with the microstructure of the material. The predicted fatigue life and the estimated non-propagating cracks agree very well with the experimental fretting fatigue tests with spherical contact in 7075-T6 aluminium alloy.     

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.     

Discrepancies of fatigue crack growth behaviour of API X65 steel

This paper discusses the fatigue life behaviour of the API X65 steel tested under constant amplitude loading in ambient temperature. The influence of repeated loading at a particular stress ratio would favour the initiation of fatigue cracks that would consequently affect the fatigue crack growth. The main objective of this paper is to evaluate the load ratio effects on fatigue crack growth rate, taking into account their statistical characteristics. The fatigue crack growth test was carried out using compact-tension specimen at different load ratios of 0.1, 0.4 and 0.7. The experimental results showed the crack growth rate was dependent on the applied load. It was observed that the load ratio effect was less significant in stable crack growth regions. The result was consistent with the multiple regression test obtained by the least square method at a significance level of 0.05. The empirical model of Paris and Walker was utilised to evaluate the effects of load ratio on the fatigue crack growth rates. The approximation of fatigue life lies between 10-25 % of error using conservative model and 6 % error using the Walker model. The experimental data was scattered within a factor-of-2 correlation line suggesting that the accuracy of the experimental data towards the estimated values was high.     

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.     

三环减速器环板断裂力学分析

用断裂力学方法分析了三环减速器环板的疲劳寿命。应力强度因子幅作为疲劳裂纹扩展的主要参量，利用断裂准则判断裂纹是否失稳扩展，并由裂纹扩展方程估算了减速器环板的疲劳寿命。     

THEORETICAL ANALYSIS AND EXPERIMENTAL VERIFICATION ON TERNARY-WAVES METHOD TO COMPILE ACCELERATED SPECTRA

The equivalent damage calculation formulae of fatigue crack formation and growth are established. In order tocompile the fatigue crack formation and growth accelerated load spectra, the main wave shapes and load sequence of theactual load spectrum are kept constant, and the carrier waves are cut off. And secondary waves are put together into newsecondary waves to shorten the test time according to the equivalent damage calculation formulae respectively. Then bythe fatigue cumulative damage calculation of the fatigue crack formation and growth accelerated load spectra, the onecorresponding to the bigger damage is determined as the fatigue accelerated test load spectrum. Therefore in the test process, the fatigue accelerated test spectrum may be applied till fatigue failure, the engineering fatigue crack length of full-scale structure need not be inspected, and the fatigue crack formation accelerated load spectrum need not be transferredinto the fatigue crack growth accelerated load spectrum. Finally, it     

Service life estimation of extrusion dies by numerical simulation of fatigue-crack-growth

In this study, fatigue behavior of cold extrusion dies is investigated analytically. Experimental studies and practical experience have shown that fatigue cracks occur at the inlet radius of die shoulders. The highest stress concentration is found also at the same location. For this reason, effective stress intensity factors at different locations of the die-shoulder inlet of a typical axisymmetric extrusion die have been calculated by using the finite element method. The crack growth has been simulated by applying the Paris/Erdoĝan fatigue law to the computed data. Finally, service life of the extrusion die has been estimated from the crack-growth-rate. Agreement of theoretical estimations with available data is found to be satisfactory within the limits of experimental uncertainties. Furthermore the experimental behavior of the crack growth (stable-unstable-stable growth with final fracture) is simulated correctly. It has been shown that the unexplained behavior is caused by decrease of stress concentration with increasing crack-length.     

填充天然橡胶材料裂纹扩展模型的建模方法

填充天然橡胶材料疲劳裂纹扩模型是指裂纹扩展速率与撕裂能峰值之间的关系,它是用于橡胶减振元件疲劳寿命预测的重要模型。在裂纹扩展试验得到的填充天然橡胶材料裂纹扩展长度与循环次数的数据中,由于橡胶的应力软化现象,试验后期的部分数据不可用。为此,建立了数据处理方法,获得有效的裂纹扩展长度与循环次数数据。基于单轴拉伸载荷下填充天然橡胶材料最大应变能密度与最大应变满足幂函数关系的假设,建立了变幅加载工况下裂纹扩展长度与循环次数的数学模型和识别其模型参数的优化方法。在建立的裂纹扩展长度与循环次数模型的基础上,建立了裂纹扩展速率与撕裂能峰值关系的模型和确定模型中参数的数值方法。利用一组哑铃型试片的疲劳寿命实测数据,对建立的裂纹扩展速率模型进行了验证,证明了所建立模型的正确性。     

A probabilistic analysis on variability of fatigue crack growth using the markov chain

Understanding the stochastic properties of variability in fatigue crack growth is important to maintaining the reliability and safety of structures. In this study, a stochastic model is proposed to describe crack growth behavior considering the variability of fatigue crack growth rates due to the heterogeneity of material. Fatigue life distribution is then predicted based on this model To construct this model, fatigue tests are conducted on a high strength aluminum alloy 7075 T6 under constant stress intensity factor range control. The variability of fatigue crack growth rates is expressed by random variablesZ and Γ based on the variability of material constantsC andm of the Paris-Erdogan equation. The distribution of fatigue life under constant stress intensity factor ranges is evaluated by the stochastic Markov chain model based on the Paris-Erdogan equation. The merit of the proposed model is that only a small number of tests are required to determine this function, and fatigue life required to reach certain crack length at a given stress intensity factor range can be easily predicted. Department of Mechanical Design and Production Eng.