Short Crack Evolution and Fatigue Life Prediction of 42CrMo Steel
|Course||Mechanical Design and Theory|
|Keywords||42CrMo Alloy Steel Fatigue life prediction Short Fatigue Cracks Monte Carlo|
With the development of modern technology and science, more and more mechanical product and engineering equipment are in pursuit of elevated temperature, high pressure and large scale. From the point of economy and safety, more precise and more practicable fatigue life prediction models are needed to avoid great loss. 42CrMo steel is the most useful material in high speed locomotives and gears. Reliability for alloy structure steels has got more and more emphasis.The researches in this dissertation are supported financially by Natural Science Foundation of Shandong Province (Y2002F19). Most of the research work utilize 42CrMo steel as main experiment material. Three fatigue life prediction models are proposed. The influence of short fatigue crack initiation micro mechanism on randomness of fatigue was discussed. Microscopic experiments of short fatigue cracks of 42CrMo alloy steel were conducted, and the experimental results were analyzed respectively.Firstly, fatigue life prediction models concerning mean strain are analyzed and summarized. According to the fatigue test date curve, power function is used to revise the Basquin model. As the relation between variables when the regression equation is nonlinear, the Box-Cox transaction method can be applied to make the variables in a linear relationship. Thus, a new Basquin model with exponential function is put forward to simulate fatigue life. It is proved that this new Basquin mode is more accurate than any other fatigue life prediction models. The energy models are widely used in the analyses of fatigue of materials. They are based on the analysis of the hysteretic loops formed as a consequence of the plastic strains induced in the material. Especially, it made a universal model concerning fatigue crack propagation and initiation. The predicted result of the new energy model made a good agreement between the calculated and experimental results.Secondly, microscopic fatigue tension experiment of 42CrMo steel was conducted, and fatigue crack initiation micro mechanism was analyzed. Surface short fatigue crack initiation and propagation evolution course on the meson-mechanical level was observed dynamically. Microscopic fatigue experimental results revealed that short fatigue crack initiation is stochastic and its evolution exhibits local field features, the Dominant Local Field Short Cracks propagate by self-propagation and interacting propagation. Crack propagation is characterized by decreasing velocity at first and increasing velocity afterwards.Thirdly, the statistical rule of the evolution of the short fatigue cracks for 42CrMo is studied and analyzed. Estimating results revealed that the 2-PWD or 3-PWD was the best probability function to describe cyclic stresses of strain fatigue for 42CrMo steel, and the 3-PWD was the best probability function to describe short crack propagation life. Fatigue life and cyclic stress range under the median and high probability were estimated by means of stochastic fatigue life and performance probability models.Finally, simulation model for short fatigue crack evolution was established. Short fatigue crack evolution process of 42CrMo steel was simulated, and simulating results agreed well with those of microscopic fatigue experiments. The research strategy of short fatigue crack evolution was proposed, the key of which is to simulate material micro structures using Monte Carlo and Cellular Automata and utilize Monte Carlo model to simulate fatigue crack propagation. Availability of simulation method was verified by three sets of crack propagation velocity data from microscopic fatigue experiments of 42CrMo steel. The simulating results agreed well with those of experiments.