【作者】 朱明亮；

【导师】 轩福贞；

【作者基本信息】 华东理工大学 ， 动力工程及工程热物理， 2011， 博士

【摘要】 疲劳失效作为威胁设备可靠性的主要因素,一直是工程技术领域的研究热点和难点。近年来,过程工业领域高参数(高温、高压)、大型化、长周期和高风险的发展新趋势,对机械装备的寿命可靠性提出了挑战,使早期破坏预测和长寿命设计成为本领域研究的新热点。本文以先进火电汽轮机转子和核电焊接转子为对象,围绕微结构影响下的疲劳破坏行为问题,从理论分析、数值模拟和实验研究三个层面,系统研究了近门槛值区的疲劳裂纹扩展和超高周疲劳行为。主要研究内容和结论如下：(1)近门槛值区疲劳裂纹的扩展与材料微结构密切相关,以高低压一体化汽轮机转子钢25Cr2NiMo1V为对象,试验研究了微组织不同的高压端(HP)与低压端(LP)材料近门槛值区疲劳裂纹扩展行为,基于微观分析手段阐明了微结构差异对近门槛值区裂纹扩展和断裂模式的影响。首次发现：裂纹尖端循环塑性区和原始奥氏体晶粒尺寸相等,可作为辉纹型和晶体学型两种疲劳裂纹扩展模式的判定条件,进而基于循环塑性区理论较好解释了面型断裂的形成机制。率先阐明了不同微组织的疲劳抗力及其对裂纹闭合的作用,为进一步建立近门槛值区疲劳裂纹扩展模型奠定了基础。(2)近门槛值区疲劳裂纹扩展影响因素复杂,尚缺乏统一的裂纹扩展描述模型,以汽轮机转子钢25Cr2Ni2MoV为对象,试验研究了不同应力比下近门槛值区疲劳裂纹扩展行为,基于等效驱动力模型从理论上阐明了疲劳裂纹扩展的控制机制。首次发现：近门槛值区疲劳裂纹扩展受恒载等效驱动机制和裂纹闭合共同控制,而裂纹闭合同应力比和疲劳裂纹扩展速率有关。在此基础上阐明了应力比影响裂纹闭合的机理,很好地诠释了裂纹闭合同裂纹扩展控制参量转变的关系,实现了裂纹闭合理论与裂纹扩展驱动机制的统一。(3)以汽轮机转子钢焊接接头为对象,系统开展了温度影响下的超高周疲劳等强度试验研究,主要包括：◆进行了焊接接头不同部位的纳米压痕和显微硬度试验,建立了纳米硬度和显微硬度的关系；进一步以板条马氏体宽度为特征组织尺寸,建立了特征组织尺寸与显微硬度的关系,为焊接接头微区的强度估算提供了依据。◆开展了服役温度范围(200-370°C)内的拉伸试验,发现了焊接接头最薄弱区域同温度的相关性；基于焊缝金属冲击试验结果,阐明了多层焊接工艺改善冲击性能的微观机制,为评价焊接接头强度性能和优化焊接工艺提供了依据。◆进行了服役温度下焊接接头的超高周疲劳试验,首次获得了370°C下低强度焊缝金属的双线性应力-寿命关系,系统分析了温度和试样位置影响超高周疲劳行为的机理,为了解焊接结构超长寿命服役性能提供了依据。(4)超高周疲劳破坏同材料微缺陷密切相关,基于中温焊接接头的超高周疲劳试验结果,系统研究了裂纹萌生的转变机制和疲劳破坏机理,主要包括：◆基于微观分析手段,发现了370°C下大量裂纹萌生于内部夹杂物、气孔和不连续组织,分析了内部裂纹的萌生与扩展特性,为了解低强钢疲劳的内部破坏机制提供了支持。◆开展了微缺陷处应力集中的有限元分析,发现了内部破坏受缺陷尺寸和位置共同影响,阐明了缺陷尺寸与分布的关系,为确定内部裂纹萌生位置和判定临界夹杂尺寸奠定了基础。◆基于数值模拟方法,阐明了高温下材料基体软化和表面氧化提高内部裂纹萌生概率的机理,为了解温度促进疲劳内部破坏提供了依据。◆基于残余应力测试结果和三维有限元模拟,阐明了残余应力及其演化在裂纹萌生模式转变中的作用,为评定制造工艺对疲劳寿命的影响奠定了基础。◆建立了基于应力水平、夹杂物尺寸和位置的超高周疲劳寿命预测模型,提出了新的寿命控制参量,为结构超长寿命预测奠定了基础。更多还原

【Abstract】 Fatigue failure is a major factor threatening reliability of equipments, and its investigation has always become a hot but difficult topic in both engineering and technical fields. Recently, many process indurstries work at extreme conditions such as high temperature, high pressure, and thus undergo scale enlargerment, service life extension and higher risk. This new developing trend challenges the life and reliability of structures and components, as a result, failure prediction at early stage and long life design become new hot spots for study. In this work, by focusing on effect of microstructure on fatigue behavior, near-threshold fatigue crack growth (FCG) mechanisms of advanced steam turbine rotor steels in fossil power plant and very high cycle fatigue (VHCF) behavior of welded rotors in nuclear power plant were systematically investigated in terms of theoretical analyses, numerical simulations and experiments.The main research contents and conclusions are listed as follows:(1) It is the fact that the near-threshold FCG is strongly influenced by microstructure. For the high-low-pressure integral steam turbine rotor steel 25Cr2NiMolV, the microstructures in high pressure (HP) and low pressure (LP) parts are different. The microstructure dependence of the near-threshold FCG and fracture mode was illustrated by fatigue tests and microscopic analyses. It was found for the first time that the striation and crystallographic mode of crack propagation could be determined from the cyclic plastic zone size at crack tip being equivalent to prior austenitic grain size. Furthermore, the formation mechanism of surface faceted fracture was also correlated to the theory of the cyclic plastic zone. In addition, fatigue resistance of one specific microstructure and its contribution to crack closure were elucidated. These findings lay great foundations for the establishment of FCG models in the near-threshold regime.(2) It is still short of generalized FCG models in the near-threshold regime due to its complex influencing factors. The near-threshold FCG behavior of rotor steel 25Cr2Ni2MoV at different load ratios was investigated experimentally, and the FCG driving mechanism was theoretically analyzed based on equivalent driving force model. It was found for the first time that the crack growth process was determined by combined effects of equvalent driving force at constant amplitude loading and crack closure, whereas the crack closure was a function of both load ratio and FCG rate. Then the mechanism of the effect of load ratio on the crack closure was clarified, and the crack closure phenomenon was further related with the transitioin of driving forces in crack advance. These findings are beneficial to unify crack closure theory and crack growth driving parameters in the near-threshold regime.(3) A series of strength performance tests especially VHCF experiments were conducted by using welding joint in steam turbine rotors, which include:◆Based on the nano-and micro-hardness tests in the welding joint, the relationship between nano-hardness and micro-hardness was established. Lath width of the tempered martensites was selected as the characteristic microstructure size for correlating the micro-hardness, and therefore gave us confidence for the estimation of strength distribution in local zones.◆It was found that the location for the lowest strength in the welding joint was temperature dependent by tensile tests at service temperature range (200-370℃), and the mechanism of improving impact properties by multilayer welding technique was illuminated by impact tests at weld metal. These work can provide important information for evaluating the performance of welding joint and optimization of welding process.◆It was reported for the first time the duplex shape of stress-life (S-N) curves in the lower strength welding joint at service temperature in the VHCF regime. And factors affecting VHCF behavior were comprehensively analyzed by paying special attention to the temperature and specimen location. These findings are valuable for understanding the long life service behavior of welded structures in power plant.(4) It is known that fatigue failure in the very high cycle regime is strongly related to micro-defects. Based on the VHCF results at moderate temperature, the crack initiation transition from surface to interior and fatigue failure mechanisms were systematically investigated, which include:◆It was found that cracks were always initiated from interior inclusions, porosities and discontinuous microstructures by microscopic observations. And then internal crack initiation and propagation behavior was analyzed. These can help understand the mechanism of fatigue failure from subsurface in lower strength steels.◆Stress concentration behavior at micro-defects were performed by finite element analyses, and it was found that interior failure was affected by both defect size and location. Then the relationship between defect size and distribution was illustrated. These work lay great foundations for defining fracture location from subsurface and estimating critical inclusion size in materials. ◆The mechanisms of higher crack initiation potential in internal material at higher temperature were clarified by numerical simulations of matrix softening and surface oxidation, which can provide important information for understanding the role of temperature in promoting interior failure.◆The role of residual stress and its evolution in the transition behavior of crack initiation modes were illuminated by both residual stress test and 3-D finite element modelling, which can provide a basis for evaluating the effect of manufacturing process on fatigue life.◆The VHCF life prediction model involving fatigue load, inclusion size and location was revised, and a new life controlling parameter was proposed. These can lay the foundwork for life prediction of engineering structrures in ultralong life regime.更多还原