【作者】 肖维灵；

【导师】 陈海波；

【作者基本信息】 中国科学技术大学 ， 固体力学， 2014， 博士

【摘要】 大科学装置是为解决重大科技中基础性、前瞻性的问题而投资兴建的基础科学研究和多学科交叉研究的公共平台,对促进经济社会全面、协调、可持续发展和国家安全有着重要作用。大科学装置中的关键零部件经常处于高温、高压和强辐射的极端环境中,承受着疲劳载荷,容易发生疲劳破坏,在设计阶段就需要重视疲劳破坏问题。坐落于我国上海的同步辐射光源是一台高性能的大科学装置,电子存储环设计能量位居世界第四。装置中介于存储环和光束线之间的一段光路称为前端区。前端区内的挡光元件,具有吸收同步辐射大部分高热负载的作用。挡光元件的设计面临第三代同步辐射极高功率密度的问题,目前常用的静强度设计方法虽然暂时解决了这一问题,但不够经济合理,而面对未来更高的要求还需要提出新的设计方法。本文针对上海光源的前端挡光元件设计问题,结合所在课题组承担的国家自然科学基金项目"Glidcop受同步辐射高热负载的疲劳失效机理研究”,对Glidcop材料的低周疲劳性能及挡光元件的热疲劳寿命评估展开研究,探索有限寿命的疲劳设计方法,采用了实验测试、数值模拟和理论建模相结合的研究方法。主要内容包括：1.总结了Glidcop材料的热物性参数随温度变化的特性。实验测试了材料在不同温度下的应力应变关系,获得了其弹塑性本构关系；进而研究了材料的低周疲劳性能,建立了不同温度下表征材料疲劳寿命的总应变-寿命关系。对Glidcop和第一、二代同步辐射装置常用材料无氧铜的力学性能进行全面对比,分析各自的优劣势,为两种材料的选择与使用提供依据。2.针对较多研究Glidcop的宏观低周疲劳性能,而对材料微裂纹起裂和扩展行为却鲜有研究的现象,设计了表面加工微孔的疲劳试样进行应力控制的低周疲劳试验,对试样的疲劳断裂过程进行相机拍摄,后期定量测量孔边裂纹的情况,探讨其特有的微裂纹起裂与扩展特性,给出描述微裂纹扩展速率的Tomkins公式,加深对材料疲劳破坏机理的认识。3.为考察Glidcop材料的低周疲劳寿命受表面粗糙度的影响,加工了不同表面粗糙度等级的疲劳试样,进行了应力控制的低周疲劳试验,通过拟合试验结果得到了材料的疲劳寿命与表面粗糙度的定量关系。鉴于粗糙表面对疲劳寿命的显著削弱作用,建议挡光元件的加工应采取有效方法控制内空腔壁面的粗糙度。4.应用热弹塑性有限元模拟了APS和SPring-8热疲劳实验试样的时变温度场和循环应力应变场,分析了试样的受力特点,以多轴疲劳中的等效应变法来预估挡光试样的热疲劳寿命。列举了五个不同的等效应变模型进行比较,分别依据各自文献的疲劳参数对两个实验进行了疲劳寿命预测,结果表明修正的Von Mises等效应变模型对所有工况预测的结果和实验结果吻合最好。该模型考虑了平均应力和弹塑性泊松比的影响,比较全面合理,工程上简单实用,是预测高功率光束辐照下的结构热疲劳寿命的理想模型。5.以上海光源的挡光元件为研究对象,对目前正服役的挡光元件采用的线弹性有限元法和静强度设计准则进行回顾,指出其中的不合理性。改用弹塑性有限元模拟得到元件上危险点的循环应力应变,用修正的Von Mises等效应变模型预估出束流总是正常或总是漂移下的寿命结果,而针对束流漂移发生较少的事实,结合Miner线性累积损伤理论综合给出挡光元件的最终寿命预测结果,形成了挡光元件的热疲劳寿命评估方法。上海光源的挡光元件目前的设计过于保守,即便束流强度增加到500mA仍有余量,考虑到挡光元件30年约104次循环加卸载的设计寿命,提出有限寿命的疲劳设计方法,为其优化设计提供参考。更多还原

【Abstract】 Large scale scientific facilities are public platforms for basic and multidisciplinary researches, which aim toto solve the fundamental and forward-looking issues in the major technological fields. It plays an important role in the comprehensive, coordinated and sustainable economic and social development and national security. Usually, the key components of large scientific facilities work in the extreme environment with high temperature, high pressure, intense radiation and fatigue loads, and prone to fatigue failure. Therefore, the fatigue failure should be considered in the design stage.Shanghai Synchrotron Radiation Facility (SSRF) is a high performance large scale scientific facility with an electron storage ring ranked fourth in the world. The optical path between the storage ring and the beam line is called front end in the facility. In the front aera, there are a series of key components for absorbing the most of high heat load by synchrotron radiation. The design of the key components faces the challenge of the high power density of the third generation synchrotron radiation. Presently, although the static strength design method provides a temporary solution, it’s not enough economic and reasonable. New design methods are essential for higher requirements in the future.This dissertation relays on the design of the high heat load components at the front end of SSRF and combines with a project of National Natural Science Foundation of China named "study on the fatigue failure of Glidcop under high heat load of synchrotron radiation". It aims at exploring a finite life design method and focuses on low cycle fatigue properties of Glidcop and thermal fatigue life prediction of high heat load components. The experimental test, numerical simulation and theoretical modeling are investigated and studied. The main contents are as follows.1. The temperature dependent thermal properties of Glidcop are reviewed. Stress-strain curves under various temperatures are tested and elastoplastic constitutive laws are obtained. Low cycle fatigue properties of this material at different temperatures were studied. Detailed comparisons of mechanical parameters are made between Glidcop and Oxygen-free copper, a common material in the first and second generation of synchrotron radiation facilities, for the purpose of analyzing their advantages and disadvantages and providing a reference for the selection and use of these two materials.2. Because of extensive studies of macroscopic low cycle fatigue properties of Glidcop and lack of microscopic researches on the behavior of crack initiation and propagation, a stress-controlled low cycle fatigue test is designed using specimens with artificial holes on the surface. The fatigue fracture process is recorded by a camera. The material-dependent micro-crack initiation and propagation characteristics are investigated through measuring the crack length at the hole edge. Furthermore, the Tomkins expression that describes the micro-crack growth rate is derived. This study is useful to well understand the fatigue failure mechanism of this material.3. In order to investigate the effect of surface roughness on the fatigue life of Glidcop, specimens with different levels of roughness are processed and stress-controlled low cycle fatigue tests are carried out. A quantitative relationship between the fatigue life and the surface roughness is obtained through fitting the test data. As the significant weakening effect on the fatigue life for a rough surface, it is suggested that the processing method of high heat load components should be effective to control the surface roughness of the inner cavity.4. Based on the APS and SPring-8thermal fatigue tests, thermal elastoplastic finite element method is used to obtian the transient temperature field and cyclic stress-strain response of the samples. After analyzing the mechanical characteristics, equivalent strain approach of multiaxial fatigue theory is suggested to predict the thermal fatigue lives of the experimental samples. For comparison, five different equivalent strain models are introduced for fatigue life prediction. According to the literature’s fatigue parameters, the fatigue lives are estimated for the samples. The results show that the lives predicted by the modified Von Mises equivalent strain model best agrees with the experimental data in all load cases. The model not only involves the effects of mean stress, but also considers the difference of Poisson’s ratios in the elastic and plastic stages. It is comprehensive, simple and practical in engineering. Therefore, it’s an ideal model to predict the thermal fatigue life of components under high power beam radiation.5. The high heat load components of SSRF are designed presently under the instruction of linear elastic finite element method and static strength design criteria. To avoid its irrationality, an alternative method called elastoplastic finite element is used to simulate the cyclic stress and strain of the components. The fatigue lives are predicted by the modified Von Mises equivalent strain model for beam always normal or beam always drift. Since the beam drift is few, the final estimated fatigue lives should be calculated using Miner linear cumulative damage theory. Therefore, the thermal fatigue life assessment method for the high heat load components is formed. The design of the components in SSRF is too conservative, even if the beam current increases to500mA. Taking into account a design life of30years (equivalent to10,000thermal cycles) for the high heat load components, the present finite life design method is finally proposed to provide a significant direction for the optimal design.更多还原