尾减输入齿轮疲劳失效分析

对某发动机尾减系统尾减输入齿轮疲劳失效的原因进行了系统的分析。通过宏观检查、断口分析、金相检验以及受力分析等,确认了该齿轮的失效过程为：在大弯曲载荷作用下疲劳裂纹于锥齿中部齿根部位起始,裂纹初期沿径向呈15°左右的小角度疲劳扩展;齿部出现裂纹后,齿轮振动频率发生变化,进而导致齿轮出现节径型振动,使得裂纹逐渐沿径向疲劳扩展;出现径向裂纹后剩余齿由于截面减少以及起始部位已基本无渗碳层其强度较低等原因,导致剩余齿出现弯曲疲劳断裂。     

压力机摇杆轴断裂分析

采用化学成分、金相检查、粗晶试验、力学性能等试验方法对摇杆轴断口的裂纹进行了分析.结果表明,轴的焊缝中存在气孔、夹渣、未熔合、冷裂纹等焊接缺陷,成为应力集中的发源地,产生数个疲劳裂纹源,大大降低了焊缝金属的疲劳强度,这些裂纹在工作应力的作用下快速扩展,当实际工作应力超过材料的疲劳极限时,导致摇杆轴疲劳断裂.     

某压气机钛合金叶片掉块失效分析

通过对某高压压气机工作叶片掉块的宏观、微观分析,认为叶片的掉块原因是叶片长度超差使叶尖与机匣产生摩擦,磨屑堆积在叶背产生裂纹,形成疲劳源,最后造成疲劳裂纹扩展.     

TC6钛合金压气机叶片裂纹产生原因

某型航空发动机在完成总运转时间2 081 h后进行拆解检查,在一件TC6钛合金高压压气机Ⅵ级转子叶片上发现沿叶片纵向分布有两条裂纹。通过宏观观察、断口分析、金相检验、能谱分析及硬度测试等方法对裂纹产生原因进行了分析。结果表明:该裂纹为试车过程中产生的疲劳裂纹。发动机在完成阶段性试车后进行拆解检查,复装后叶尖间隙不满足设计要求,导致在随后的试车过程中叶尖与机匣封严涂层发生严重刮擦,造成局部超温、掉块,形成疲劳裂纹源并最终扩展为裂纹。     

发动机安装座焊缝开裂分析

采用宏观观察、SEM断口检查、材料检查、金相检查方法对某发动机部件安装座焊缝裂纹进行了检查及分析,确认了该裂纹产生是由于在发动机该部件的薄壁上焊接该安装座时,存在焊接应力;装配焊接定位时产生校正应力;加之导管传给安装座的振动应力,叠加产生较大的应力,在此大应力的作用下,由安装座焊缝边缘应力集中的薄弱部位产生疲劳裂纹.提出了预防安装座焊缝疲劳裂纹失效的防范措施.     

高频载荷下高强钢的超高周疲劳及热耗散研究

应用超声疲劳试验技术,对两种高强度钢(42CrMo4,100Cr6)在20kHz频率下的超高周疲劳性能进行测试分析.实验结果表明:两种钢的S -N曲线在106周发生了明显的变化,出现了水平渐近线.尽管23个42CrMo4钢试样用于1010周的疲劳试验,但在8.76×107循环周次以上,没有疲劳破坏发生,42CrMo4钢存在疲劳极限,而100Cr6钢的S -N曲线呈现台阶型.高精度热成像仪检测不同载荷条件下疲劳试样温度的变化结果显示:温度的变化与试验材料和加载水平有关.试样温度的快速升高发生在超声疲劳试验的初期,温度的变化反映了材料内部的热耗散过程.裂纹萌生后,微裂纹处不可逆的局部塑性变形导致裂纹萌生区温度急剧升高,疲劳试样内部温度场的变化反映材料的疲劳损伤过程.SEM观察表明:在长寿命区,疲劳裂纹常萌生于试样内部或次表层组织缺陷处.     

直升机尾减速器机匣疲劳试验方法研究

根据减速器机匣疲劳损伤应力循环的动、静应力包罗线方法确定了疲劳试验加载方案;根据设计载荷谱或飞行实测,确定各载荷作用点的状态载荷并编制了疲劳试验谱;根据减速器机匣的受力形式,完成减速器机匣疲劳试验装置的设计。结果表明,该试验装置既可以考虑高频动载荷的影响,又能简化疲劳试验的加载程序,明显缩短了疲劳试验的周期。     

镍基合金超声疲劳裂纹扩展寿命预测研究

针对发动机结构材料承受高频循环载荷的特点,应用超声疲劳试验技术开展了镍基合金材料的疲劳裂纹扩展试验研究。考虑高频载荷下疲劳裂纹扩展过程中的温升效应,测试了超声疲劳裂纹扩展过程中的温度变化,基于温度变化对材料弹性模量的影响和热膨胀效应,数值计算了疲劳裂纹扩展应力强度因子。研究了温度变化对超声疲劳裂纹扩展的影响机制,并在现有模型基础上,建立了考虑温度影响的超声疲劳裂纹扩展模型,完善疲劳裂纹扩展寿命预测方法。     

压气机转子叶片掉块断裂分析

某型发动机外场服役中多次出现压气机转子叶片叶尖掉块断裂故障,通过掉块叶片表面形貌观察、断口分析、硬度检测、金相检验、多起失效件的类比分析以及叶片振动模态分析,对叶片掉块断裂原因进行了分析。结果表明:所有压气机转子叶片掉块断裂模式相同,均属振动疲劳断裂掉块;叶片在发动机工作转速范围内存在高阶复合振动点,引起叶片共振是导致其振动疲劳断裂掉块的主要原因;叶片与机匣摩擦导致叶尖局部过烧或形成摩擦热应力裂纹促进了疲劳裂纹的萌生。最后针对叶片掉块断裂原因提出了相应的解决措施。     

直流电位法检测高温合金的疲劳裂纹扩展性能

介绍了在高温下使用直流电位法测定疲劳裂纹长度的基本原理、试验方法及所需配置的仪器,并对有关影响因素进行了分析;并利用Johnson的分析型关系式,研究了一种适用于自动测定材料高温疲劳裂纹长度的直流电位法,该方法已成功应用于高温合金疲劳裂纹扩展试验,其测得的疲劳裂纹扩展速率da/dN-ΔK数据与长焦聚显微镜法测得的数据完全一致。     

挖掘机销轴断裂分析

利用宏观检验、断口分析、化学成分分析、金相检验以及硬度检测等方法,对42CrMo钢挖掘机销轴的断裂原因进行了分析。结果表明:销轴断裂为双向弯曲疲劳断裂。由于销轴表面存在较脆的白亮层ε相,且白亮层分布有较严重的疏松,增加了销轴表面的脆性,使销轴表面形成了较多的微裂纹,导致了疲劳裂纹的萌生;销轴的渗氮层深度和硬度偏低也降低了销轴的疲劳强度,加速了疲劳裂纹的扩展,最终使销轴发生早期疲劳断裂。     

室温大气环境下过时效态3J21合金疲劳行为

在MTS万能实验机上对室温大气环境下过时效态3J21合金的疲劳行为进行研究,并采用扫描电镜(SEM)对宏观断口及微观断口进行分析.结果表明,过时效态3J21合金的疲劳裂纹呈穿晶扩展和沿晶扩展,且沿晶扩展的比例较大,疲劳裂纹萌生寿命短,扩展途径相对平直,扩展速率大,裂纹扩展抗力小,疲劳寿命低;过时效态3J21合金疲劳断口...     

离心叶轮叶片裂纹产生原因分析

某发动机离心叶轮叶片的排气边多处出现裂纹,通过理化检验和定量分析对裂纹产生原因进行了分析,并估算了叶片的裂纹扩展寿命及其占总寿命的百分比。结果表明:该裂纹为高低周复合疲劳裂纹,叶片在异常振动等大应力作用下产生了疲劳开裂。     

不同时效态3J21合金疲劳行为

在MTS万能实验机上对室温大气环境下欠时效态、峰时效态和过时效态3J21合金的疲劳行为进行研究,并采用扫描电镜(SEM)对宏观断口及微观断口进行分析.结果表明:不同时效态疲劳裂纹均呈穿晶扩展,欠时效态3J21合金疲劳裂纹萌生寿命最长,扩展途径比较曲折,扩展速率最小,表现出最大的疲劳裂纹扩展抗力,疲劳寿命最长.而过时效态3J21合金的疲劳裂纹萌生寿命最短,沿晶扩展的比例增加,扩展途径相对平直,扩展速率最大,裂纹扩展抗力最小,疲劳寿命最低;峰时效态合金介于两者之间.不同时效态的疲劳断口均由疲劳源、疲劳裂纹扩展区和瞬断区组成.在低速扩展区,欠时效态断口呈现小平面,峰时效态和过时效态断口呈现冰糖状花样,在中速扩展区均未看到长的疲劳条纹,仅发现个别细小的疲劳条纹.     

Statistical modeling of fatigue-crack growth in a nickel-base superalloy

Two fracture mechanics-based statistical models for the fatigue crack propagation of engine materials are developed. The models are applied to fatigue crack growth rate data for IN 100, a nickel-based superalloy used in F100 engine disks, at various elevated temperatures, loading frequencies and stress ratios. The lognormal and the randomized parameter models allow the incorporation of the statistical variability associated with crack growth data into the life prediction process. The statistical distributions of (i) the crack growth rate, (ii) the propagation life to reach a given crack size (iii) the crack size at any given service life have been derived. A correlation analysis is performed to compare the results of the statistical models with test data. The correlation is demonstrated to be very good.     

UNSTABLE FATIGUE CRACK PROPAGATION AND FATIGUE FRACTURE TOUGHNESS OF STEELS

This paper presents the results of fatigue crack growth and fatigue fracture toughness studies of a high-pressure vessel steel with particular emphasis on the influence of heat treatment, low temperatures, plastic prestraining, the stress ratio and specimen dimensions. It has been shown that steels in an embrittled state, caused primarily by thermal treatment and low-temperatures, exhibit unstable fatigue crack growth which is characterized by alternate crack jumps (cleavage zones) and zones of fatigue crack growth. The fatigue fracture toughness, which corresponds to the first crack jump, and final fracture can be appreciably lower (i.e. up to 50%) than the static fracture toughness under plane strain conditions at the corresponding temperature. An analysis has been performed of unstable and stable fatigue crack growth and a model of unstable crack propagation is proposed which accounts for the observed experimental behaviour.     

Indentation fatigure testing of soda-lime silicate glass

Dynamic, cyclic and static fatigue testing was performed on soda lime silicate glass using indentation strength measurements. Using the conventional analysis, the cyclic and static fatigue data were inconsistent with the remaining data for the case when the indentations were annealed prior to testing. Using an analysis that included the measured variations in the fracture mechanics geometric parameters, all three data sets were consistent. Using a numerical analysis, the lifetime for materials in the static and cyclic fatigue tests was shown to be sensitive to variations in the stress intensity factor at short crack lengths. It is therefore very important to understand any crack size dependent of the fracture mechanics parameters in this crack size region for accurate lifetime predictions.     

A mathematical equation relating low cycle fatigue data to fatigue crack propagation rates

A mathematical equation is derived to predict fatigue crack growth rates on the basis of a J integral analysis from the fatigue fracture behaviour of low cycle fatigue samples. According to this equation, the fatigue crack propagation curves can be predicted if low cycle fatigue data and an initial microcrack size are available. The results obtained from this study show that the predicted fatigue crack propagation rates for Ti-24V, Ti-6Al-4V and Al-6Zn-2Mg alloys are very close to experimental values.     

Investigations on fretting fatigue in aircraft engine compressor blade

An investigation of several cracked blade tangs in the military aircraft engine compressor was conducted to identify the root cause of the failure. These cracks were found during the scheduled maintenance with fluorescent penetration inspection. The engine compressor blade made of Ti–6Al–4V is attached to compressor rotor by means of inserting retaining pin through rotor and blade tang. By analyzing the fracture surface of the failed blade tang, it is found that the crack in the blade tang was initiated by fretting fatigue and propagated under low cycle fatigue. Stress analysis of the blade using a non-linear finite element method is coincident with the results of fractography. The clearance between retaining pin and tang hole caused small amplitude of sliding motion leading to fretting wear during engine operation. Consequently, the damaged area due to fretting wear acts as a stress raiser inside tang hole and contributes to accelerate fretting fatigue.