7075航空铝合金原位腐蚀-多轴疲劳行为分析

针对沿海地区飞机服役过程中的疲劳失效问题,利用自行开发的腐蚀装置对7075铝合金进行未腐蚀和3.5%NaCl盐雾原位腐蚀条件下,等效应力分别为200 MPa、250 MPa和300 MPa时的多轴疲劳试验。结合循环曲线和断口形貌对未腐蚀和原位腐蚀条件下的多轴疲劳失效机理进行对比分析,并提出改进的Manson-Coffin-Basquin（MCB）准则进行寿命预测。结果表明,随着等效应力的增大,7075铝合金未腐蚀和原位腐蚀条件下多轴疲劳寿命均下降;等效应力相同时,相对于未腐蚀试样,原位腐蚀试样的多轴疲劳寿命显著缩短,轴向和扭向滞回线的面积均不同程度增大;200 MPa等效应力条件下,试样多轴疲劳寿命在未腐蚀条件下为99 675周次,原位腐蚀条件下下降至55 284周次;未腐蚀和原位腐蚀条件下,裂纹源均出现在外表面,原位腐蚀试样扩展区有盐粒出现;修正的MCB准则引入加载条件和环境因素,得到了良好的预测效果,寿命预测值均位于两倍分散带内。     

复合材料涡轮轴低循环疲劳寿命预估方法研究

计算连续纤维增强金属基复合材料轴结构应力/应变响应,将仿真计算结果与国外试验数据对比分析,验证金属基复合材料轴结构响应计算方法的准确性。在此基础上,根据发动机涡轮轴真实工况载荷条件下,计算发动机在动载荷下的响应情况及危险位置,将动载荷与静载荷进行等效转换,并验证金属基复合材料轴结构在动载荷及静载荷作用下响应状态的一致性,从而基于静载荷条件工况下,进行疲劳寿命预测,并与国外试验数据对比分析,验证模型有效性。最终,以连续纤维增强金属基复合材料航空发动机涡轮轴为例,计算其在扭矩载荷及轴向力载荷作用下,铺层角度对轴结构响应及寿命的影响规律。     

2A12航空铝合金多轴疲劳试验及应力准则寿命预测模型研究

针对多轴疲劳失效问题,选取2A12航空铝合金进行应力幅比变量、相位差变量和平均应力变量的多轴疲劳试验。对常用的3种多轴疲劳应力准则寿命预测模型（即Lee准则、Carpinteri准则和Sines准则）进行讨论,并通过引入应力幅比参量和相位差参量,提出基于Carpinteri准则的修正模型。将不同条件下2A12航空铝合金的试验寿命与不同应力准则下模型的预测寿命进行比较,结果表明：Lee准则对上述多轴疲劳试验的预测结果过于危险;Carpinteri准则和Sines准则由于未考虑拉-扭应力幅比和相位差因素,预测寿命与实际寿命相比均出现了较大偏差;修正后的应力准则寿命预测模型在不同条件下90%的寿命预测数据在两倍误差带内。     

多轴载荷下基于权平均最大剪切应力临界面的疲劳寿命预测方法

提出一种基于权平均最大剪切应力临界面的疲劳寿命预测方法.首先,基于改进的Wang-Brown多轴计数法,提出了一种多轴载荷下临界面的确定方法,该方法通过权平均所有计数反复中具有较大法向拉伸应力的最大剪切应力平面的位相角来确定整个载荷历程的临界面,提出的权函数定义为各个计数反复中的剪切应力范围与所有反复中最大剪切应力范围的比值.提出的临界面确定方法通过7050-T651铝合金薄壁管试验数据来进行验证.然后,结合提出的临界面确定方法,高周疲劳损伤准则和Miner线性损伤法则来建立多轴变幅载荷下疲劳寿命预测方法.最后,采用7050-T651和2024-T4铝合金试验数据来验证提出的疲劳寿命预测方法的准确性,结果表明提出的方法具有较好的预测效果.     

疲劳失效准则在钛合金BT9低周疲劳寿命估算中的应用

在应变控制条件下对钛合金 BT9进行了单轴拉 -压、纯扭及拉 -压与扭转比例加载的低周疲劳试验 ,得到了相应的试验结果。讨论了几种寿命预测准则及其寿命预测精度。在分析 Brown- Miller理论及 Makinde-Neale广义疲劳失效准则的基础上 ,提出了 Makinde-Neale广义疲劳失效准则的变异方程。应用所提出的方程对钛合金 BT9进行了多轴应变条件下的寿命预测 ,并得出了更为符合试验结果的预测结果     

纤维增强复合材料涡轮轴结构静强度计算与分析

基于适用于纤维增强复合材料的细观力学代表体积元(RVE)法和材料强度失效参数计算公式,计算体积分数为40%的SCS-ULTRA纤维增强TC17基复合材料的力学性能参数和强度失效参数,建立以实际某型号发动机低压涡轮轴为背景,并用SCS-ULTRA纤维增强TC17基复合材料代替原有高温合金材料,应用复合材料宏观强度准则中的蔡—吴张量失效准则,对其进行静强度计算与失效判定,总结作为衡量趋近失效程度的蔡—吴强度指数随轴结构复合材料铺层数的变化规律,同时对铺层总数为27层的轴结构进行给定载荷下的强度计算,得出轴结构内部每一铺层的强度指数及铺层强度指数随铺层位置、45°铺角不同的变化规律,并预测在极限载荷下轴结构铺层首先出现失效的情况。     

飞机结构损伤光固化复合材料疲劳寿命研究

疲劳裂纹的萌生和扩展是材料在变载荷作用下的主要失效形式.试验针对飞机光固化复合材料修理补片的疲劳寿命进行了测试,并考核了不同修理工艺(半宽度胶接、全宽度胶接及铆接修理)试样的疲劳寿命,为确定最佳修理工艺提供了试验依据.     

含孔复合材料层合板的疲劳寿命研究

建立了三维有限元模型,分析了复合材料层合板的应力场。使用修正Hashin失效准则判定复合材料的失效模式,并突降失效单元的材料性能。疲劳载荷引起复合材料刚度降和强度降依靠缓降模型实现。笔者将突降模型和缓降模型植入有限元模型中,模拟了复合材料层合板在拉伸和压缩疲劳载荷下的渐进损伤过程,并计算了层合板的纵向刚度损伤和疲劳寿命。层合板的纵向刚度损伤具有三阶段特点,与试验观察是一致的。层合板疲劳寿命预测值与试验值吻合地很好。     

复杂加载条件下压力容器典型用钢疲劳蠕变寿命预测方法

针对多轴应力状态,探讨压力容器典型用钢16MnR缺口试样的高温疲劳与循环蠕变交互作用行为,在延性耗竭理论和损伤力学基础上,建立一种半寿命平均位移速率寿命预测模型,采用该方法对不同缺口半径试样的高温疲劳寿命进行了较好的预测.针对多级加载条件,研究316L钢的循环变形行为,探讨疲劳蠕变与动态应变时效之间的耦合作用,在延性耗竭理论基础上,建立非线性损伤演化模型,考虑多级加载时的载荷历程效应,提出一种新的损伤累积准则,采用该方法对二级加载条件下的疲劳蠕变寿命进行了较好的预测.     

基于最弱环理论的缺口件概率疲劳寿命预测方法

基于最弱环理论和光滑试样疲劳寿命的Weibull分布,建立了一种缺口件概率疲劳寿命预测方法。该方法首先基于最弱环理论和光滑试样的疲劳强度分布,通过定义缺口件的Weibull有效应力,建立了缺口件在给定循环载荷下的疲劳失效概率计算公式。基于Weibull有效应力和光滑试样的疲劳应力-特征寿命方程,可计算得到给定循环载荷时缺口件的特征疲劳寿命,进一步根据光滑试样的Weibull疲劳寿命分布可最终获得缺口件在给定循环载荷下的疲劳寿命分布。采用上述方法对TC4缺口试样进行了概率疲劳寿命预测,并与局部应力应变法预测结果进行了对比。结果表明:局部应力应变法预测结果过于保守,本文方法预测精度较高,50%失效概率时的疲劳寿命预测结果与缺口试样试验均值寿命吻合很好,10%和90%失效概率时的疲劳寿命预测结果基本分布在试验均值寿命的两倍分散带之内。     

The effects of radial stresses on the strength of thin-walled filament wound GRP composite pressure cylinders

Filament wound glass fibre reinforced epoxy cylinders with thickness : internal diameter ratios of approximately 0.01, and winding angle ± 85° have been tested to failure under different combinations of axial load and internal pressure. The effect of the interaction coefficients in the Tsai—Wu failure criterion equation are considered. Results for the 17 specimens tested and published results for other winding angles have been used to assess some commonly used forms of failure criteria. It was shown that inclusion of the radial stress terms had no significant effect on the experimental results considered. The geometries and hoop to axial stress ratios for which radial stresses are likely to be important are stated.     

A new high-cycle fatigue criterion applied to out-of-phase biaxial stress state

A new multiaxial high-cycle fatigue criterion based on the so-called critical plane approach is presented. According to such a criterion, the critical plane orientation is proposed to be correlated with the averaged principal stress directions deduced through the weight function method. Then the fatigue failure assessment is performed by considering a nonlinear combination of the maximum normal stress and the shear stress amplitude acting on the critical plane. The proposed criterion is applied to a general sinusoidal biaxial stress state, for which analytical formulae can be derived. The theoretical results calculated according to the present criterion, together with those of other common critical plane criteria, are compared with experimental data related to different brittle (hard) metals under in-phase or out-of-phase sinusoidal biaxial normal and shear stress states.     

横管蒸发纵管冷凝式热管充液量、倾角对传热的影响

通过搭建试验台对横管蒸发纵管冷凝式热管进行试验研究,分析该型热管在不同充液量和不同的倾角的情况下传热性能的变化。经分析发现,充液量和倾角两个因素都对该型热管的传热性能有很大影响。当倾角变化时,其热阻随倾角增加而不断增加且存在临界角。倾角在到达临界角之前热阻上升缓慢,倾角大于临界角后其热阻急剧增大。当充液量变化时,随充液量增大热管热阻先减小后增大;且在60%充液量时热阻最小,传热性能最佳。     

复合材料结构材料参数转换及在有限元分析中的应用

多层的纤维缠绕复合材料结构中,各个单层间具有不同的纤维铺设角,呈现各向异性的材料特性,在进行有限元分析时,必须对材料的材料特性进行处理变换。根据基本力学方程,对不同缠绕方向的纤维层材料,把不同材料坐标系下的特性参数等效变换到统一坐标系。并利用ANSYS软件对层合结构进行有限元分析,结果表明此转换是可行的。     

常幅疲劳载荷下复合材料层合板刚度退化试验研究

复合材料层合板在飞机结构中的应用研究一直备受关注。本文主要采用试验方法研究常幅拉伸疲劳载荷作用下3种不同铺层复合材料层合板的刚度随疲劳寿命的变化趋势,取得了大量试验数据,给出了在工程上有价值的结论。     

Effect of stress ratio on fatigue life of GFRP composites for WT blade

Fatigue life of GFRP (glass-fiber reinforced plastic) composites used in wind turbine rotor blades has been evaluated considering the glass fiber orientations. Three different laminate composites with the respective laminating orientation of 0°, ±45°, and 0°±45° were prepared using vacuum infusion method. Tensile properties and S-N curves for these composites were experimentally determined at room temperature. From the tensile tests, it was found that tensile properties were greatly dependent upon the fiber orientation and the tensile strength of unidirectional composite was the largest and bidirectional (45°) composite was the weakest among three composites. The fatigue properties were determined under constant amplitude load control at different stress ratios, R, of 0.5, 0.1 and ?0.2. The properties also show the dependency of stress ratios and fiber orientation. The fatigue life diagrams of these three composite were relatively well presented with the double logarithmic S-N curve. The linear slopes of the respective S-N curves for three composite were not greatly different. The fatigue limits for the composites were evaluated and predicted with linear Goodman and Gerber diagrams.     

Numerical analysis for prediction of fatigue crack opening level

Finite element analysis (FEA) is the most popular numerical method to simulate plasticity-induced fatigue crack closure and can predict fatigue crack closure behavior. Finite element analysis under plane stress state using 4-node isoparametric elements is performed to investigate the detailed closure behavior of fatigue cracks and the numerical results are compared with experimental results. The mesh of constant size elements on the crack surface can not correctly predict the opening level for fatigue crack as shown in the previous works. The crack opening behavior for the size mesh with a linear change shows almost flat stress level after a crack tip has passed by the monotonic plastic zone. The prediction of crack opening level presents a good agreement with published experimental data regardless of stress ratios, which are using the mesh of the elements that are in proportion to the reversed plastic zone size considering the opening stress intensity factors. Numerical interpolation results of finite element analysis can precisely predict the crack opening level. This method shows a good agreement with the experimental data regardless of the stress ratios and kinds of materials.     

A study of cumulative fatigue damage in aluminum alloy 2024-T4

Experimental data were generated using 2024-T4 aluminum alloy specimens under stress ratios of ?1 and ?0.5 for low-high, low-high-mixed, high-low and high-low-mixed stress sequences.Analysis of the data using Kramer's equation has indicated that the predicted cumulative fatigue damage and fatigue life are in close agreement with experimental results for low-high and low-high-mixed stress sequences under all stress ratios, whereas the theoretical values for high-low and high-low-mixed stress sequences under all stress ratios are more conservative than those obtained experimentally.It is suggested that development of Kramer's equation be studied thoroughly for possible modification.     

Modifying the Lambda Ratio to Functional Line Contacts

Lambda ratio, the lubricant film thickness divided by the composite roughness within an operating rolling element bearing, has been recognized for more than 25 years as an important contact parameter related to bearing performance, primarily fatigue life. This paper considers a simplified method of functional filtering within a bearing contact that provides a more appropriate composite roughness for a wider range of bearing sizes and loadings. The concept was tested with a series of regression models of standard and modified lambda ratios, including the uncoupled lambda parameters. The models were rated using three different statistical indices to assess how each model accounted for the variation present in a large collection of bearing test data.     

基于神经网络的金属材料疲劳裂纹扩展规律的预测

人工神经网络是新型的复杂系统预测方法。本文针对金属疲劳裂纹扩展速率建立了 BP 神经网络,并以部分应力比 LD2锻造铝合金疲劳裂纹试验数据作为训练样本,训练建立好的 BP 神经网络;以另一部分应力比条件下的试验数据作为预测样本,验证训练好的 BP 神经网络的预测能力。仿真结果表明,BP 神经网络能够方便地获得不同应力比下的疲劳裂纹扩展速率,对训练样本和测试样本都具有良好的泛化能力。该方法充分利用了已有数据,减少了疲劳试验次数,具有工程应用价值。