航空发动机钛合金材料的高周和超高周疲劳性能研究1）

通过对航空发动机空心风扇叶片用Ti-6Al-4V 随炉试样的高周和超高周疲劳试验研究,揭示了Ti-6Al-4V 材料在10⁷ 循环周次以上同样会发生疲劳破坏. 采用三参数幂函数寿命曲线拟合了高周和超高周的疲劳性能数据,发现可以较好地将两种试验下的数据衔接起来,结果显示在此试验条件下基于超声的超高周疲劳试验的频率效应可以忽略. 通过断口分析表明,超高周试样在试样表面没有缺陷的情况下,裂纹大多数是从材料内部或次表面萌生,而高周疲劳试样的裂纹是从材料表面开始萌生.     

合金材料超高周疲劳的机理与模型综述

在循环载荷作用下, 合金材料发生裂纹萌生、扩展直至断裂的周次在107以上的过程被称为超高周疲劳 (very-high-cycle fatigue, VHCF).本综述将从30年前超高周疲劳的研究起源讲起, 直到近年的最新进展.引言之后的内容包括: 超高周疲劳研究的起源, 超高周疲劳的主要特征, 超高周疲劳裂纹萌生特征区和特征参量, 裂纹萌生特征区的形成机理与模型, 超高周疲劳性能预测模型. 在叙述中, 试图回答下列问题: 什么是超高周疲劳?为什么要研究超高周疲劳?超高周疲劳的关键科学问题是什么?超高周疲劳的S-N曲线趋势为什么发生变化?超高周疲劳裂纹为什么萌生于材料 (试样) 内部?裂纹内部萌生的过程和机理是什么? 上述问题有的可以给出明确的回答, 有的则是现阶段的最新结果, 并有待于对问题的继续探索.      

45CrNiMoVA钢的低周疲劳特性和表面疲劳裂纹的在位观测

利用MTS810材料试验机对带中心圆孔的45CrNiMoVA钢低周疲劳特性进行了研究,并借助长焦显微镜和CCD摄像头对试样表面裂纹的演化规律进行了在位停机观测和在位不停机连续观测尝试,试验发现,裂纹均萌生于与拉伸应力垂直的圆孔边缘;疲劳裂纹的萌生与Lueders形变带密切相关,萌生期超过整个疲劳寿命的70%,主裂纹的发展既可以共面扩展的方式向前连续延伸,也可以裂纹连接的方式向前跳跃传播,应力控制的低周疲劳条件下,疲劳寿命与应力面形貌图像,频闪光源照明可以实现表面疲劳裂纹的在位不停机观测。     

航空发动机钛合金材料的高周和超高周疲劳性能研究

通过对航空发动机空心风扇叶片用Ti-6Al-4V随炉试样的高周和超高周疲劳试验研究,揭示了Ti-6Al-4V材料在10~7循环周次以上同样会发生疲劳破坏.采用三参数幂函数寿命曲线拟合了高周和超高周的疲劳性能数据,发现可以较好地将两种试验下的数据衔接起来,结果显示在此试验条件下基于超声的超高周疲劳试验的频率效应可以忽略.通过断口分析表明,超高周试样在试样表面没有缺陷的情况下,裂纹大多数是从材料内部或次表面萌生,而高周疲劳试样的裂纹是从材料表面开始萌生.     

A review on mechanisms and models for very-high-cycle fatigue of metallic materials

在循环载荷作用下, 合金材料发生裂纹萌生、扩展直至断裂的周次在10⁷以上的过程被称为超高周疲劳 (very-high-cycle fatigue, VHCF).本综述将从30年前超高周疲劳的研究起源讲起, 直到近年的最新进展.引言之后的内容包括: 超高周疲劳研究的起源, 超高周疲劳的主要特征, 超高周疲劳裂纹萌生特征区和特征参量, 裂纹萌生特征区的形成机理与模型, 超高周疲劳性能预测模型. 在叙述中, 试图回答下列问题: 什么是超高周疲劳?为什么要研究超高周疲劳?超高周疲劳的关键科学问题是什么?超高周疲劳的S-N曲线趋势为什么发生变化?超高周疲劳裂纹为什么萌生于材料 (试样) 内部?裂纹内部萌生的过程和机理是什么? 上述问题有的可以给出明确的回答, 有的则是现阶段的最新结果, 并有待于对问题的继续探索.     

合金材料超高周疲劳的机理与模型综述

在循环载荷作用下,合金材料发生裂纹萌生、扩展直至断裂的周次在10~7以上的过程被称为超高周疲劳(very-high-cycle fatigue,VHCF).本综述将从30年前超高周疲劳的研究起源讲起,直到近年的最新进展.引言之后的内容包括:超高周疲劳研究的起源,超高周疲劳的主要特征,超高周疲劳裂纹萌生特征区和特征参量,裂纹萌生特征区的形成机理与模型,超高周疲劳性能预测模型.在叙述中,试图回答下列问题:什么是超高周疲劳?为什么要研究超高周疲劳?超高周疲劳的关键科学问题是什么?超高周疲劳的S-N曲线趋势为什么发生变化?超高周疲劳裂纹为什么萌生于材料(试样)内部?裂纹内部萌生的过程和机理是什么?上述问题有的可以给出明确的回答,有的则是现阶段的最新结果,并有待于对问题的继续探索.     

孔径尺寸对干涉配合铆接件超高周疲劳性能的影响

合理的干涉配合铆接工艺可以有效提高构件疲劳性能,本文通过试验研究与数值模拟相结合分析孔径尺寸对干涉配合铆接件超高周疲劳性能的影响。利用20kHz超声疲劳试验系统测试了三种不同孔径尺寸下干涉配合铆接件的超高周疲劳性能;基于ABAQUS模拟分析了铆接工艺过程以及孔径尺寸对干涉量和孔边残余应力的影响,通过分析高频低幅加载下孔边应力分布,结合超高周疲劳试验及断口形貌观察,分析了孔径尺寸对铆接件超高周疲劳性能的影响。结果表明:不同孔径尺寸铆接件的裂纹萌生扩展方式类似,疲劳裂纹萌生均发生在孔边距表面1.5mm最大残余拉应力处;适当减小孔径尺寸所形成的合理干涉量可以提高铆接件疲劳寿命,φ4.10mm的孔径可以形成0.66%的孔边平均干涉量,铆接件疲劳寿命最高达到3.63×108周。     

干态下车轮材料表面疲劳裂纹萌生试验研究

利用WR-1轮轨滚动磨损试验机,结合安定极限理论研究了干态下影响车轮材料表面疲劳裂纹萌生与扩展的因素,探究了表面疲劳损伤形成机理和演变规律.结果表明:随垂向力、横向力和冲角增大,表面疲劳裂纹越容易萌生扩展;冲角对表面疲劳裂纹的萌生与扩展起着重要作用,大冲角下斜线状表面疲劳裂纹萌生扩展明显;只有横向力而不存在冲角时,试样表面不会出现斜线状表面疲劳损伤;车轮试样在周期性循环载荷作用下在表面先形成塑性流动,然后沿轮轨表面切向力方向扩展成斜线状的表面疲劳起皮剥落损伤;垂向力是影响表面裂纹萌生时间的重要因素之一.     

高强度钢的超高周疲劳裂纹扩展模型研究

当疲劳寿命在106或107周时,Wöhler S-N 曲线被看作渐近于水平轴,107的疲劳强度被看成是疲劳极限。现代应用要求延长零件的工作寿命,实际齿轮部件应用超过107循环的疲劳失效。本文应用压电超声疲劳试验机对经过热处理和渗碳处理后的低铬合金钢材料进行研究,采用红外摄像仪观测试件表面的温度场随疲劳裂纹萌生和扩展的过程。试验条件是室温,应力比为0.1（R=0.1）,频率为20KHz。通过对表面渗碳处理后试件的断口分析,探讨表面渗碳处理、微观结构和与杂质有关的断裂机理,根据Paris公式建立超高周疲劳裂纹扩展模型。对裂纹扩展过程中裂纹尖端的塑性区的分析结果,结合传热学原理,建立热耗散模型,有限元方法的数值解结果较好地符合红外摄像仪的观测的试验结果。     

焊接接头超高周疲劳实验研究

本文基于超声疲劳振动技术,设计了三种焊接接头试样(圆形对接焊接试样及其喷丸处理试样和板状十字焊接试样),并利用超声疲劳试验系统测定了其超高周疲劳性能,实验应力比为-1,频率20kHz,实验在室温条件下进行。实验结果表明,圆形对接焊接接头的疲劳性能高于板状十字焊接接头,喷丸处理能提高焊接接头的疲劳强度。将焊接接头的疲劳性能与对应形状的母材进行对比分析,发现焊接接头的疲劳性能远低于母材。在相同疲劳寿命的条件下,圆形焊接接头试件的疲劳强度仅为母材的45%,十字焊接接头试件仅为母材的29%;圆形对接接头在5×106周次以后,试件仍然发生疲劳断裂,而板状十字焊接接头在超高周区域(107～109周次)存在疲劳极限。超声疲劳断口的扫描电子显微镜分析结果显示,圆形焊接接头试件断口位置主要位于熔合区的焊趾处或焊接接头表面几何非连续处,十字接头试件断口位于焊趾处;焊接接头试件裂纹萌生于焊接缺陷、试样表面夹杂或熔合区的不连续处;喷丸处理对焊接接头的裂纹萌生机制没有显著影响。     

Current understanding of ultra-high cycle fatigue

The fatigue life of numerous aerospace, locomotive, automotive and biomedical structures may go beyond 10⁸ cycles. Determination of long life fatigue behavior becomes extremely important for better understanding and design of the components and structures. Initially, before the invention of ultrasonic fatigue testing, most of the engineering materials were supposed to exhibit fatigue life up to 10⁷ cycles or less. This paper reviews current understanding of some fundamental aspects on the development of accelerated fatigue testing method and its application in ultra-high cycle fatigue, crack initiation and growth mechanisms of internal fracture, S-N diagram, fatigue limit and life prediction, etc.     

Fatigue life assessment of a high strength steel 300 M in the gigacycle regime

The fatigue behavior of a high strength steel 300 M in the gigacycle regime was investigated. Fully reversed tension — compression fatigue tests at ambient temperature were performed using an ultrasonic fatigue system operating at 20 kHz. The staircase test method was employed to obtain accurate values of the mean fatigue strength corresponding to fixed numbers of cycles up to 10⁹. These results were compared to the curve which is estimated by the data tested in the mid-long life regime on conventional servo hydraulic test machine at 20 Hz. Results indicate that the fatigue strength determined from ultrasonic fatigue testing is lightly higher than conventional testing in the range of 10⁶–10⁷ cycles. It is obvious that nucleations of fractures tend to occur below the surface, if fractures happen after more than 10⁷ cycles. All the fractured specimens fails from internal SiO₂ inclusions or smaller carbides and carbide clusters.     

Effect of low cycle fatigue pre-damage on very high cycle fatigue

Carbon-manganese steel is often applied in components of pipes in nuclear plant. Ultrasonic fatigue tests following low cycle fatigue (LCF) cycles damaged are used to study the strength of very high cycle fatigure (VHCF). The comparison of test results of simple VHCF and cumulative fatigue (LCF plus VHCF) shows that LCF load influences the following VHCF strength. Continuum damage mechanics model is extended to VHCF region. The effect of LCF load on VHCF is studied by an improved cumulative damage model.     

Improving fatigue strength of bainite/martensite dual-phase steels in very high cycle fatigue regime by refining microstructures

Very high cycle fatigue behaviors of two bainite/martensite dual-phase steels were investigated. One of the steels was cyclic rapid heat treated and its microstructures were refined. Fatigue strength of the steel is 225 MPa higher than that without refining. Observation of fracture surfaces show that the fatigue cracks initiate at bainites for non-refined steel and at non-metallic inclusions for the refined steel. The size of inclusions is much smaller than that of bainites which results in the improvement of fatigue strength.     

高温疲劳表面短裂纹的分形研究

对2.25Cr-1Mo合金钢进行了高温疲劳实验，采用中断试验与复膜技术相结合的方法观测了试样表面疲劳短裂纹萌生，扩展及合体的演化过程，并用数据格子方法研究了含裂纹表面的分形特征。结果表明：疲劳短裂纹的萌生扩展行为具有分形特征，可以用分形理论加以描述；分形维数随着疲劳过程的继续而稳定地成比例增加；分形维数可以作为把握材料总体损伤状态的参数，可作为寿命评价的依据。     

Correlation of crack growth rate and stress ratio for fatigue damage containing very high cycle fatigue regime

A model is proposed to correlate the crack growth rate and stress ratio containing very high cycle fatigue regime. The model is verified by the experimental data in literature. Then a formula is derived for the effect of mean stress on fatigue strength, and it is used to estimate the fatigue strength of a bearing steel in very high cycle fatigue regime at different stress ratios. The estimated results are also compared with those by Goodman formula.     

Revised damage evolution equation for high cycle fatigue life prediction of aluminum alloy LC4 under uniaxial loading

The fatigue life prediction for components is a difficult task since many factors can affect the final fatigue life. Based on the damage evolution equation of Lemaitre and Desmorat, a revised two-scale damage evolution equation for high cycle fatigue is presented according to the experimental data, in which factors such as the stress amplitude and mean stress are taken into account. Then, a method is proposed to obtain the material parameters of the revised equation from the present fatigue experimental data. Finally, with the utilization of the ANSYS parametric design language (APDL) on the ANSYS platform, the coupling effect between the fatigue damage of materials and the stress distribution in structures is taken into account, and the fatigue life of specimens is predicted. The outcome shows that the numerical prediction is in accord with the experimental results, indicating that the revised two-scale damage evolution model can be well applied for the high cycle fatigue life prediction under uniaxial loading.     

高温超声拉压疲劳试件设计与温度自动控制

本文较深入地开展了高温环境条件下的超声拉压疲劳实验系统研究,深入分析了试件温度高精测量与自动控制和高温试件设计的关键问题。实验系统可以实现高温环境条件下大于1010周次超高周次的疲劳加载。本实验系统利用传统超声疲劳试验机,使用非接触式红外测温仪对试件温度进行实时动态监测,采用间歇振动的方法保证温度波动小于±3℃。实验结果显示,采用本系统得到的实验结果与传统疲劳加载方式的结果一致性很好。本文工作为进一步开展高温环境条件下金属材料高温超高周(1010周次)疲劳损伤和疲劳破坏机理研究提供了新的有效的平台。     

Very high cycle fatigue behavior of bridge steel welded joint

Very high cycle fatigue (VHCF) behaviors of bridge steel (Q345) welded joints were investigated using an ultrasonic fatigue test system at room temperature with a stress ratio R = ?1. The results show that the fatigue strength of welded joints is dropped by an average of 60% comparing to the base metal and the fatigue failure still occurred beyond 10⁷ cycles. The fatigue fracture of welded joints in the low cycle regime generally occurred at the solder while at the heat-affected zone (HAZ) in the very high cycle regime. The fatigue fracture surface was analyzed with scanning electron microscopy (SEM), showing welding defects such as pore, micro-crack and inclusion were the main factors on decreasing the fatigue properties of welded joints. The effect of welding defects on the fatigue behaviors of welded joints was discussed in terms of experimental results and finite element simulations.