Microscopic Mechanism And Numerical Simulation Of Short Fatigue Cracks At High Temperature

In the modern industries, many metallic materials work under the environment of both high temperature and alternating loads. Thus, fatigue fracture was easy to occur. With the development of smelting and metal detection technology, the emergence of long cracks in the material and structures becomes less and less, however short cracks are mostly observed. And a large number of statistical data prove that most parts the fatigue life are consumed in the micro stage of short crack, which makes the research of high temperature fatigue short crack a particularly important direction. Compared with long cracks, short fatigue cracks are strongly influenced by microstructure, such as grain size, grain orientation and grain boundary distribution and material composition. Thus, it has a great randomness. High temperature lead to surface oxidation and grain boundary sliding, which makes the microscopic mechanism of fatigue short crack behavior more complicated and badly in need of further study. Compared with mathematical model, physics model can response problem intuitively, especially when the object has great randomness. Therefore, by setting up physical models, numerical simulation is an effective way to reveal microscopic damage mechanism of the high temperature fatigue short crack behavior and hence a reasonable life prediction by simulation resulted. There are limitations for previous numerical simulation, and it is necessary for further exploration.Base on large number of low cycle fatigue test dates of20#steel at high temperature. The specimen was partial corrosion by4%nitric acid alcohol solution. And crack on the surface of the specimen was observed through a microscope. We can see the influence of microstructure on the crack’s behavior under different testing condition. Thus, the model of fatigue short crack under high temperature was built using statistical test data. Finally numerical simulation is realized using Matlab program.The main contents are given as follow:1. Large number of high temperature low cycle fatigue tests proved that both GB (grain boundary) and PSB (persistent slip band) are preferential sites for cracks initiation. GB initiation usually become trans-granular cracks and PSB initiation usually become grain inter-granular cracks. Due to the high temperature effect, the early cracks are mostly extend along the grain boundary. And pearlite grain has a strong inhibition to the propagation of the cracks, when plastic zone around the crack tip overlapped, coalescence of the involved cracks occur. The radius of the interaction zone was given by crack length and testing stress state. Furthermore, the interaction between neighboring cracks was considered. The effect of stress shielding next to the cracks (responsible for a reduced local crack initiation tendency and a decreased local crack propagation rate) was modeled assuming a linear stress gradient.2. The microstructure of a material was modeled using Voronoi-polygons. Refer to the actual distribution of pearlite on the surface of20#steel,20%convex cells were chosen as pearlite grains in the Voronoi-polygons using a series of particular random numbers and the others were ferrite grain. Then, give each ferrite grain a3*3random direction cosine matrix to show its orientation, and a dotted line through the center was adopted to represent the slip band on the surface of the specimen. At last we put forward the concept of basic energy to represent different resistance to crack initiation and propagation for different types of grain.3. Base on the established microstructure, visualization of initiation, propagation, coalescence, interference and other group evolution behavior (inter-granular and trans-granular) of the high temperature fatigue short crack under different cyclic was realized by Matlab software programming. Thus, a numerical simulation of short fatigue cracks was built. And the result agrees well with the experiment data. The model can be used to simulate the short fatigue cracks effectively.