Study On Evolution Behavior Of Short Fatigue Crack For Low-cycle At High Temperature Under Complex Stress State Based On Fractal Theory

With the development of science and economy, the high-temperature and high-pressure apparatus have been more widely adopted, which leads to more problems about safety reliability inevitably. According to statistics, about eighty percent of structural failure is caused by fatigue in practical engineering. Currently, reliability and safety evaluation of structures in service, is generally used of long crack growth rate model, which is based on classical fracture mechanics. However, experiments show that the fatigue fracture process depends on the initiation and propagation of short crack, for most materials, the majority of fatigue life is consumed in short crack stage. In addition, there is obvious discrepancy compared short crack with long crack, and this abnormal behavior can not be explained by using traditional theory. Therefore, study on short fatigue crack will not only help reveal the physical mechanism of fatigue damage, and understand the classical theory of fatigue research, but also provide a theoretical basis on service reliability and safety evaluation of the structure, life prediction and other research.From the view of physical, the solid containing cracks is actually a non-linear dissipation system, so it can be described by fractal theory. The propagation of fatigue short crack is a fractal curve, the irregular path can be described by fractal dimension. The fractal dimension of the crack implicit in many of the physical factors, such as the microstructure of material, the load conditions and so on. As the cracks in the actual engineering materials and components are always initiated at the place of uncontinuous structure and stress concentration, which is in a non-uniform distribution of three-dimensional stress state, therefore, the study on behavior of short crack in the complex stress state has important theoretical and practical significance of engineering. In this paper, short cracks for low cycle at high temperature is studied. The 20 steel under various stress state is experimentally studied, the initiation, propagation and coalescence of short cracks are observed. By analyzing experiment results, the evolution features of crack fractal dimension with the number of cycles are as follows, the behavior of short fatigue cracks satisfies fractal feature, with the development of the fatigue process, the fractal dimension is also increased. The fractal dimension is not related to the gap of the specimen, so it can be applied to characterize the short crack under various stress state. With increases of strain amplitudes, the fractal dimension has a stage performance. Finally, the remaining life of the short crack stage can be predicted based on the evolution law of the fractal dimension.