DZ125高温合金超高周疲劳裂纹萌生与扩展

裂纹的萌生与扩展是研究合金材料超高周疲劳行为的重要方面。本研究分析与探讨了温度和表面状态对DZ125合金的超高周疲劳裂纹萌生与扩展特征的影响。不同温度下,DZ125合金的超高周疲劳裂纹萌生位置和扩展方式不同。室温下,裂纹均沿表面起源,裂纹扩展以拉伸模式为主;700℃下,裂纹均沿亚表面起源,裂纹扩展以剪切模式为主。室温下,DZ125合金经激光冲击处理前后的超高周疲劳裂纹萌生位置和扩展方式均存在差异。经过激光冲击处理后,裂纹萌生于合金的内部孔洞缺陷,裂纹扩展完全以剪切模式进行。     

金属材料超高周疲劳的试验方法及失效特征

超高周疲劳是指材料经受1×10~7循环周次以上而发生断裂损伤的疲劳行为,彻底打破材料"无限寿命"的传统观点,逐渐成为疲劳研究领域一个新的热点问题。回顾近几十年国内外的研究成果,总结了超声疲劳试验方法的进展和应用,分析了金属材料超高周疲劳S-N曲线的典型特征以及内部断裂的裂纹萌生机制和扩展特征,探讨影响超高周疲劳行为的主要因素,即加载频率、材料组织和氢含量。在此基础上,提出了今后超高周疲劳研究的一些方向:裂纹源表面萌生和内部萌生的竞争性机制及影响因素,超高周疲劳的寿命预测和再制造零部件的超高周疲劳行为。     

热处理工艺对50钢超高周疲劳性能的影响

对50钢进行了正火 高温回火、中温回火、高温回火三种不同热处理状态的对称拉压超声疲劳试验,同时用扫描电子显微分析技术和X射线能谱分析方法对50钢的疲劳断口形貌进行显微分析,研究其疲劳断裂行为和疲劳断裂机理.结果表明,50钢存在表面和内部两种裂纹萌生机制;在本试验范围内,热处理工艺对50钢超高周疲劳性能的影响较低周和高周为小,随着寿命的增加,影响越来越弱;热处理工艺对50钢不同的疲劳阶段有不同的影响起源于不同的裂纹萌生机制,对常规疲劳性能起改善作用的热处理工艺可能对材料超高周疲劳性能的改善没有太大作用.     

微结构和应力比对Ti-6Al-4V高周和超高周疲劳行为的影响

采用旋转弯曲和超声疲劳实验分别测试了全等轴和等轴双态组织Ti-6Al-4V合金的高周和超高周疲劳性能,并用SEM观察了疲劳断口特征.结果表明:2种组织Ti-6Al-4V合金的高周和超高周疲劳行为相似,不同应力比下,其S-N曲线均表现出单线形或双线形的形式;存在滑移机制和解理机制2种疲劳破坏机制.随应力比增加,2种组织Ti-6Al-4V合金的高周和超高周疲劳破坏机制均从滑移机制向解理机制转变.基于疲劳寿命和疲劳强度建立模型分析了应力比对2种机制之间竞争行为的影响,模型预测结果与实验结果趋势吻合.     

铝合金焊接接头超高周疲劳性能研究进展

超高周疲劳是指材料应力循环次数达到1×107以上发生疲劳断裂的行为,不同于传统疲劳"无限寿命"的观点。为了能够合理化设计疲劳构件、有效评估寿命、减少安全隐患和经济损失,有必要对材料超高周疲劳性能进行深入研究。总结了铝合金超高周疲劳S-N曲线的特征、疲劳裂纹的产生、裂纹扩展以及影响疲劳破坏的因素,在此基础上提出利用TIG熔修、机加工打磨、超声冲击等延寿技术提高疲劳寿命;最后在分析现状的基础上,提出一些有关铝合金超高周疲劳寿命的研究方向。     

钢铁材料超高周疲劳的研究进展

随着工程需求的提升和试验手段的进步,对钢铁材料超高循环周次疲劳性能的研究逐渐深入,发现其机理与低周疲劳和高周疲劳有所不同。对钢铁材料超高周疲劳性能进行综述,总结了钢铁材料超高周疲劳的试验方法、S-N曲线、断裂机理、寿命评估模型以及超高周疲劳的影响因素,并提出了超高周疲劳今后的研究方向。     

钢的超高周疲劳性能研究现状及发展趋势

综述了国内外钢的超高周疲劳研究进展,包括超高周疲劳的典型特征--"鱼眼",基于断面分析疲劳裂纹萌生和扩展的特征;分析了疲劳机理和相关的模型及探讨影响超高周疲劳的若干因素,如加载频率和环境介质等,并对超高周疲劳未来的研究方向进行了展望。     

金属材料超高周疲劳失效的基本特征

随着工业技术水平和金属材料强度的提高,超高周疲劳已经成为工程部件失效的新问题,超声疲劳试验是目前研究金属材料的超高周疲劳性能的主要方法.对金属材料超高周疲劳失效的基本特征进行了总结分析.超高周疲劳的S-N曲线一般分为3种形态特征:持续下降型、阶梯下降型和传统无限寿命型;裂纹萌生的位置与金属材料的种类、夹杂物的尺寸和残余...     

LC4CS铝合金的超高周疲劳寿命分布

采用超声高频疲劳试验机进行了LC4CS铝合金实验样本容量为66的超高周疲劳寿命实验.结果表明,超高周疲劳寿命具有双峰分布特征,这一特征与疲劳裂纹的萌生点有关:较短疲劳寿命的样品的裂纹萌生于夹杂等缺陷处,而较长疲劳寿命的样品的裂纹萌生于表面.寿命分布的双峰特征使得传统的升降法不能用于确定材料的超高周疲劳的条件疲劳极限,超高周疲劳寿命的分散性远大于低周和高周疲劳寿命的分散性.     

TC4钛合金三点弯曲超高周疲劳性能研究

针对航空发动机压气机叶片复杂载荷环境下的超高周疲劳问题,开展了TC4钛合金三点弯曲超高周疲劳试验,研究其在弯曲加载下的超高周疲劳破坏行为。疲劳试验结果表明：在两种应力比（R=0.3、0.5）下,当循环次数超过107次时,试件仍发生疲劳断裂,S-N曲线均呈现双线性特征;SEM断口分析表明,随着最大应力的降低,裂纹萌生位置由试件表面向次表面转移,疲劳裂纹萌生是表面滑移和内部解理断裂之间相互竞争的结果;基于疲劳寿命建立模型分析了应力比对2种裂纹萌生机制之间竞争行为的影响。采用红外热像仪监测试件表面的温度,高周疲劳试件的温度变化分为四个阶段：稳定升高、稳定降低、快速升高和快速降低阶段,而超高周疲劳试件的温度变化分为三个阶段：稳定升高、快速升高和降低阶段。最后,阐述了疲劳过程中热产生和传热的特点,并分析了温度变化与应力分布的关系。     

Influence of Microstructural Inhomogeneity and Residual Stress on Very High Cycle Fatigue Property of Clean Spring Steel

The present study investigated the very high cycle fatigue (VHCF) properties of a spring steel SUP7-T386 under the conditions of surface grinding and electro-polishing by performing the axial loading test at a stress ratio of ?1. The influence of the microstructural inhomogeneity (MI) generated in the process of heat treatment and the residual stress induced by surface grinding on the VHCF properties was discussed. This steel with surface grinding exhibits the continuously descending S-N characteristics, corresponding to the surface flaw-induced failure at high stress level and the interior flaw-induced failure at low stress level. Otherwise, with surface electro-polishing, it exhibits continuously descending S-N characteristics with lower fatigue strength, but only corresponding to the surface flaw-induced failure even at low stress level. Compared with the evaluated maximum inclusion size of about 11.5 μm, the larger MI size and the compressive residual stress play a key role in determining fatigue failure mechanism of this steel under axial loading in the VHCF regime. From the viewpoint of fracture mechanics, MI-induced crack growth behavior belongs to the category of small crack growth, and threshold stress intensity factors controlling surface and interior crack growth are evaluated to be 2.85 and 2.51 MPa m^1/2, respectively. The predicted maximum MI size of about 27.6 μm can be well used to evaluate surface and interior fatigue limit of this steel under axial loading in the VHCF regime, combined with the correction of residual stress.     

Damage mechanism of a nodular cast iron under the very high cycle fatigue regime

Fatigue behaviour in the very high cycle regime (VHCF) of 10¹⁰ cycles were investigated with a cast iron (GS51) under ultrasonic fatigue test system (20 kHz) in ambient air at room temperature with a stress ratio R = −1.

The influence of frequency was examined by comparing similar data generated on conventional servo hydraulic test systems. An advanced, high-speed, and high-sensitivity infrared imaging system was used to measure the temperature changes during ultrasonic fatigue test at various load levels caused by internal damping due to a very high frequency cycling. The temperature field on the surface specimen was determined by using a non-destructive measurement technique called infrared pyrometer. An infrared camera made up of a matrix of 320 × 240 detectors was used.

The S-N curves obtained show that fatigue failure occurred beyond 10⁹ cycles, fatigue limit does not exist for the cast iron and there is no evidence of frequency effect on the test results. A detailed study on fatigue specimens subjected to ultrasonic frequency shows that the temperature evolution of the cast iron specimen is very evident, the temperature increased just at the beginning of the test, the temperature increased depending on the maximum stress amplitude. Under the current test conditions, the high cycle fatigue (VHCF) behaviour of the cast iron exhibited a typical fatigue crack growth process, that is, fatigue initiation takes place always at the surface graphite or subsurface void; the distinctive stable fatigue crack growth zone can be found around the fatigue crack initiation site, the change of fatigue initiation site from surface to subsurface is associated with the complex effects of applied maximum stress level, surface condition.

Under lower stress amplitude and high cycle condition, surface graphite fatigue initiation is predominantly depended on cyclic stress amplitude; subsurface void fatigue initiation is determined by maximum cyclic stress.

In the process of small crack propagation, the temperature in local plastic zone increase very sharply. The temperature field of ultrasonic fatigue specimen can be changed with the cooling condition; internal heating can accelerate surface fatigue crack initiation and propagation.

Fatigue properties in VHCF regime were studied for cast iron (GS51) at 20 kHz frequency and for the first time, crack initiation and propagation stages were analyzed using a high-sensitivity infrared camera. The new Paris's model for fish eye formation in the gigacycle fatigue were also confirmed by this study.     

7050铝合金搅拌摩擦焊接头超高周疲劳性能

采用超高周疲劳试验系统研究7050-T7451铝合金搅拌摩擦焊接头的超高周疲劳性能. 试验结果表明,焊接接头在107周次以上仍然会发生疲劳失效,S-N曲线在108周次左右出现转折点,呈折线型下降;通过SEM对超高周疲劳断口形貌进行观察发现,当应力范围较高时,试件的疲劳裂纹往往在表面萌生,随着应力范围的降低,裂纹有亚表面和内部萌生的倾向;裂纹萌生位置取决于表面起裂和内部起裂相互竞争的结果;试件的断裂位置多为焊接接头的热力影响区和热影响区,EBSD和接头硬度的分析结果表明断裂位置与接头组织不均匀密切相关.     

Investigation of fatigue crack initiation in Ti-6Al-4V during tensile-tensile fatigue

Fatigue crack initiation in Ti-6Al-4V has been investigated in high cycle fatigue (HCF) and low cycle fatigue (LCF) regimes at stress ratio R=0.1 using the replication technique. In all four tested α/β microstructures, the crack was initiated by fracture of equiaxed alpha grain. Fractured alpha grains are seen on the fracture surface as flat facets with features characteristics of cleavage fracture. In the regime of low stress amplitudes and in the absence of reverse loading, cleavage fracture contributes to crack initiation and early stages of crack growth in Ti-6Al-4V. This mechanism is discussed in relation to the anomalous mean stress fatigue behavior exhibited by this alloy.     

纳米晶Ni疲劳行为的实验研究

系统研究了纳米晶Ni与粗晶Ni的疲劳行为. 通过疲劳实验获得了这2种材料的疲劳应力--寿命曲线, 并采用AFM对纳米晶Ni样品表面进行观察以研究其裂纹萌生的微观机制, 利用纳米压痕仪对疲劳实验前后样品的力学性能和显微组织变化进行了研究. 结果表明, 纳米晶Ni具有比粗晶Ni更高的疲劳极限. AFM观察表明,纳米晶疲劳后样品表面出现平均尺寸为73 nm的胞状起伏, 疲劳后样品的晶粒尺寸未发生明显改变. 压痕硬度结果表明, 疲劳过程材料的力学性能也未发生明显变化.     

超高周疲劳寿命预测方法探讨

分析了金属材料超高周疲劳断口形貌特征,介绍了基于Paris公式的裂纹扩展寿命预测模型和基于位错理论的疲劳裂纹萌生寿命预测模型,并结合前期有关金属材料超高周疲劳行为的试验数据,对2种预测模型的误差进行分析。结果表明,基于位错理论的寿命预测模型较为准确;而基于Paris公式的裂纹扩展寿命预测模型,其预测精度随着疲劳寿命的增加而降低,即材料组织缺陷萌生成为疲劳裂纹阶段占据疲劳寿命的绝大部分。在此基础上,提出了超高周疲劳寿命预测的研究方向:疲劳裂纹的萌生机制,特别是裂纹源表面萌生和内部萌生的竞争性机制;建立大样本数据,结合统计学方法,以工程构件的服役安全性和可靠性为基础,精确评价超高周疲劳寿命。     

Effect of corrosive environment on the fatigue crack initiation and propagation behavior of Al 5454-H32

Fatigue behavior of aluminum alloy 5454- H32 was studied under laboratory air and 3 % NaCl solution environment using smooth cylindrical and notched plate specimens. Presence of 3 % NaCl environment during fatigue loading drastically reduced alloy fatigue life. The deleterious effect was pronounced in both types of specimens in the long- life regions, where the fatigue lives were lowered by as much as a factor of 10. However, the sharply notched specimens showed only a modest reduction in fatigue life in corrosive environment. The severe influence of the corrosive environment in the long- life (low- stress) regime cannot be explained merely by the early initiation of the fatigue crack from surface pits; the environmental contribution in the early crack growth regime must also be considered an important factor. Fracture surface studies revealed extensive pitting and some secondary cracking in the crack initiation region. It was shown that lowered fatigue life in Al 5454- H32 occurs by early initiation of fatigue cracks from surface pits. In addition, a corrosion pitting and secondary cracking process may be operative in the small crack growth region. This could have enhanced the early crack growth rate and thus contributed to the lower fatigue life in the long- cycle region.     

Effect of stress ratio on long life fatigue behavior of Ti-Al alloy under flexural loading

A new ultrasonic three-point bending fatigue test device was introduced to investigate fatigue life ranging up to 10^10 cycles and associated fr＇dcture behavior of Ti-Al alloy. Tests were performed at a frequency of 20 kHz with stress ratio R=0.5 and R=0.7 at ambient temperature in air. Three groups of specimens with different surface roughness were applied to investigate the effect of surface roughness on fatigue life. Furthermore, optical microscopy（OM） and scanning electron microscopy（SEM） were used for microstructure characteristic and fracture surface analysis. The S-N curves obtained show that fatigue failure occurs in the range of 10^5-10^10 cycles, and the asymptote of S-N curve inclines slightly in very high cycle regime, but is not horizontal for R=0.5. Fatigue limit appears after 10s cycles for R=0.7. Surface roughness （the maximum roughness is no more than 3 μm） has no influence on the fatigue properties in the high cycle regime. A detailed investigation on fatigue fracture surface shows that the Ti-Al alloy studied here is a binary alloy in the microstructure composed of α2-Ti3Al and γ-Ti-Al with fully lamellar microstructure. Fractography shows that fatigue failures are mostly initiated on the surface of specimens, also, in very high cycle regime, subsurface fatigue crack initiation can be found. Interlamellar fatigue crack initiation is predominant in the Ti-Al alloy with fully lamellar structure. Fatigue crack growth is mainly in transgranular mode.     

挤压态镁合金ZK60的超高周疲劳行为

利用超声疲劳实验研究了挤压态镁合金ZK60的超高周疲劳行为.结果表明,合金的疲劳S-N曲线在5×106-108cyc范围内存在一平台,而在108-109cyc范围内,疲劳强度逐渐降低,对应于109cyc的疲劳强度为(90±5)MPa.SEM断口观察表明,在5×106-108cyc范围内,疲劳裂纹基本上萌生于试样表面或亚表面,而在108-109cyc范围内,疲劳裂纹主要萌生于试样内部的非金属夹杂物.通过测定疲劳源区的尺寸,估算的合金疲劳强度与实验结果基本一致.疲劳源的形成是由微裂纹在多个夹杂物处起裂和合并所引起的.因此,合金的疲劳强度不是由最大夹杂物尺寸决定,而是取决于由多个夹杂物组成的"缺陷区"尺寸.通过测定多个部位的"缺陷区"尺寸,可以有效的估算合金的疲劳强度.