【作者】 郭隽；

【导师】 陈家骅； 郭成璧；

【作者基本信息】 大连理工大学 ， 动力机械及工程， 2000， 博士

【摘要】 据统计，在各类构件的失效中，大约有80％是由疲劳破坏引起的。对疲劳问题的研究具有重要的学术意义和工程意义。实验证实，疲劳断裂过程归根到底取决于那些或明或暗的裂纹。虽然宏观断裂力学研究已经产生了很大的社会/经济效益，然而实验表明，许多疲劳断裂过程取决于短裂纹的萌生、扩展过程，短裂纹阶段有时可占总寿命的80％以上。传统的宏观断裂力学理论不再适用于短裂纹，而且短裂纹扩展速率表现出很大的随机性。因此要求我们从宏观重返微观，深入研究短裂纹行为规律。此方面的深入研究有助于正确地进行寿命预测、适时进行设备检修、及时更换部件，以避免事故的发生，而且也有助于探讨抑制裂纹萌生、扩展的方法，从而延长构件寿命。目前对短裂纹的研究还处于刚起步阶段，还需要更深入的研究与发展。 本文就是以高温疲劳表面短裂纹群体演化行为为研究对象，通过实验分析来获得了短裂纹特征，并建立了描述其演化过程的物理模型，进而实现寿命、余寿命预测。 本文主要是基于Monte Carlo方法，来对短裂纹群体萌生、扩展、合体直至破坏的物理过程进行可视化再现，吸收实验和理论分析结果并考虑短裂纹的随机行为，同时引入分形理论，模拟并预测材料的破坏。本文主要内容可概述如下： 1、论述了高温强度研究的工程背景及意义，系统地评述了当前疲劳学界正在探讨的各种疲劳长、短裂纹模型。 2、对2.25Cr-1Mo进行了多种条件下的高温疲劳表面短裂纹的实验研究，采用中断试验和复膜技术相结合的方法实现了高温疲劳表面短裂纹群体演化行为的观测与分析，获得了其演化特征及损伤破坏的细观机理。 3、把图像处理技术应用到材料金相组织与裂纹的观测实验中。通过将图形图像技术相结合，设计了材料金相组织和短裂纹扫描照片的图形图像处理程序，免去了照片的人工量化任务，从而提高了处理效率和精度。 4、根据金属凝固过程是—Monte Carlo过程，建立了金属材料显微组织的简化Monte Carlo方法，采用“随机投放，等速生长”的方法形成矩齿形晶界图，并通过逐点判断法识别出晶界三重点，剔除奇异三重 大连理工大学博士学位论文点，并识别出晶界及晶界问相邻关系，从而获得晶界结构的直线近似形态图，为细观组织短裂纹的数值模拟作好准备。 5、建立了高温疲劳短裂纹群体演化厅为模型，模型计及了细观组织结构的障碍作用及裂纹间的干涉效应，可以描述裂纹萌生、扩展、合体的全过程，使疲劳短裂纹的发生发展过程可视化，并可对短裂纹进行统计分析。 6、将分形几何学引入到疲劳短裂纹分析中，用数格子法对实验结果和模拟结果分别进行了分形分析，得到了裂纹分形维数随循环过程的演化特征，指出短裂纹的分形维数可以作为材料总体损伤的参数。 7、口顾了当前高温疲劳余寿命预测的各种方法，提出了本文模型在余寿命预测中的应用方案及前景。通过将短裂纹的物理模型与分形分析相结合，并设定临界最大裂纹长度和临界表面裂纹分形维数等多个参敢，来进行结构余寿命预测。结果表明，以分形维数作为失效判据是合理可行的。更多还原

【Abstract】 With the progress of the world industrialization level and the needs of the development of intensive economy, the technology of petroleum, chemistry and energy industry are in pursuit of elevated temperature, high pressure and large scale. Especially, there are more equipments of chemistry and power plant having reached designed life, but from the point of economy and safety, more precise and more practicable life/remaining life prediction models are needed. It is well known that most components? invalidations are caused by fatigue, and the process of fatigue fracture is charged by the collective behavior of short cracks, sometimes it may account for nearly 80% of the whole fatigue process. But classic macro- mechanics can be applied to the research of short cracks. Therefore, study on short fatigue cracks is of great importance. The mechanism of surface short inter-granular fatigue cracks at elevated temperature in type 2.25Cr- IMo stainless steel and its simulation were chosen as the subjects of this paper. Several tests were conducted to elucidate the evolution process of fatigue damage. Through the analysis of the characteristics of short cracks, a model was proposed to evaluate the spatial and temporal distribution of short surface cracks. The followings are the contents of the work. I. The significance of study on high temperature strength was firstly discussed. Some main models of long and short fatigue cracks were also summarized. 2. Through interrupt test and replication method, several tests were conducted to observe the processes of short cracks?initiation and propagation by using cylindrical specimens of type 2.25Cr-lMo stainless steel. 3. Image process techniques were applied to the analysis of the mtcrostructure of material and the characteristic of short cracks. A program was made for the measure of material microstructure picture and crack picture, thus improving the efficiency and precision of the analysis of pictures. 4. A simplified model was proposed to simulate material microstructure according that metal solidification is a Monte Carlo process. First, use the point of utting randomly, growing uniformly?to generate grain boundary, then recognize grain boundary trifurcate points, eliminate bad trifurcate points, and distinguish the connections among these points, thus obtain straight grain boundary picture. 5. Based on the idea of a stochastic process, where a crack is viewed as an object that propagates randomly in time, a model was established to evaluate the damage process of short cracks. The effect of microstructure and the cracks?interaction were also considered in the model. The model can visualize the process of short cracks?initiation, propagation and coalescence. Short cracks?statistical characteristics can also be obtained using the model. 6. Fractal theory was also introduced into the short-crack model, 揃ox counting method?was used to calculate the short cracks?fractal dimension. The fractal analysis of experiment and simulated results shows that fractal dimension increases steadily with the fatigue process, which implies that the fractal dimension can be viewed as a total damage parameter. 7. In the end, the paper reviewed the methods for predicting high a temperature fatigue remaining life. A practicable plan using the model proposed in this paper was also designed to predict residual life, where critical maximal crack length a更多还原