Collective Damage Evolution Of Short Fatigue Cracks Under Elevated Temperature And Its Simulation

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...