【作者】 王胜；

【导师】 王东坡；

【作者基本信息】 天津大学 ， 材料加工工程， 2008， 硕士

【摘要】 传统的疲劳极限定义为107周次,使用常规疲劳试验技术可以满足要求。但是随着现代工业技术的发展,许多工业部门(目前对焊接接头和结构所进行的疲劳设计与评定均是建立在单周疲劳载荷试验的数据基础上)。事实上一些重要焊接工程结构,如飞机、核反应堆、水轮机、汽车、燃气轮机以及涡轮发动机等,在承受低频高幅的交变载荷过程中,还不时地发生不同程度的高频低幅振动。这些振动频率可达数千赫兹,循环数常常可达108~109次。现有规范在估算实际工程结构疲劳寿命时经常不考虑高频振动的有害作用,仅将高频分量叠加在主交变载荷上,按单周化处理或干脆将其彻底忽略。如果这种振动仅仅是依附于平均载荷之上,由于其振幅较小,对结构产生的破坏相对较轻。然而如果是叠加在低频率循环载荷之上,它对材料所造成的复合损伤将大大地加速工程结构尤其是焊接结构的损坏和失效,严重地威胁到整个设备或结构的安全可靠性和使用寿命。在双周疲劳试验中,疲劳试件必须满足谐振条件,所以试件通常设计为狗骨形,这样不仅使试件获得很大的应力放大系数,还使其中部产生最大应力。本文中对双周疲劳试件的设计,主要有共振长度的设计及不同几何外形试件的位移、应力分布情况。使用带超声频载荷分量的双周实验装置分别进行了16Mn焊接接头在纯低周(LCF)、纯高周(HCF)以及双周循环(CCF)加载条件下的疲劳试验,研究了焊接接头在双周疲劳载荷作用下的疲劳性能。研究发现,焊接接头的双周疲劳性能低于纯低周而高于纯高周。叠加于大幅低周载荷上的小幅高周载荷及叠加于大幅高周载荷上的小幅低周载荷都显著降低焊接接头的疲劳强度。随着高低周振幅比的增大,疲劳强度也迅速降低。模拟含高值焊接残余应力的焊接接头双周疲劳试验结果表明,高周分量载荷相对高的情况下,焊接残余应力显著降低焊接接头的疲劳性能。更多还原

【Abstract】 The traditional definition of fatigue limit for 107 cycles, and the use of conventional technology to meet the requirements of fatigue test。However, with modern industrial and technological development, many of the industrial sector (the current structure of welded joints and the fatigue design and evaluation are based on low-cycle fatigue load test on the basis of the data) In fact a number of important structural welding works, such as aircraft, nuclear reactors, turbine, automotive, gas turbine engines and turbines, and so on, to bear in the high rate of low-frequency alternating load process, from time to time in varying degrees to the low rate of high-frequency vibration. Existing norms in the project estimate the actual structure of fatigue life, often do not consider the harmful effects of high-frequency vibration, the only high-frequency components in the main stacking alternating load, according to single-cycle treatment, or simply be completely ignored. If the vibration is only dependent on the load on the average, because of its smaller amplitude of the structure of the relatively minor damage. If, however, is superimposed on low-frequency load on the cycle, and its material injury caused by the complex will greatly speed up the project structure in particular, welded structure damage and failure, a serious threat to the entire structure of the equipment or the use of reliability and safety Life.Since performing experiments of the ultra-high-cycle regime in the range of 109~1010 cycles using a conventional fatigue testing method is very time consuming and expensive, so we studied and developed the ultrasonic fatigue testing system to accelerate fatigue test. In this paper, based on the ultrasonic fatigue testing system, the fatigue behavior of 16Mn including smooth and welding joint specimen was studied, and the fracture surface of fatigue specimen was examined by scanning electron microscopy (SEM).Specimen design is critical for optimum ultrasonic fatigue testing, dog-bone-shaped specimen is usually designed in ultrasonic fatigue testing. An analysis was carried out to calculate the resonance length, strain and displacement along the length of fatigue specimen with different geometries and for various resonant frequencies.Fatigue test of 16Mn steel welded joints has been carried out under low cycle fatigue(LCF), high cycle fatigue(HCF) and combined cycle fatigue(CCF) respectively, in order to investigate the fatigue properties of welded joints being subjected to CCF loading. The stress ratio was 0.5 and the ratio of HCF cycles to LCF cycles was about 260:1. The loading frequency of HCF was about 20kHz and performed by an ultrasonic fatigue testing system. The results indicate that fatigue strength of welded joints has been decreased further under CCF. HCF is detrimental to fatigue life of welded joints under CCF loading. With the increasing of the ratio of HCF loading to LCF loading, fatigue life of welded joints becomes shorter.更多还原