Research On The Fatigue Crack Initiation And Propagation Of 16Mn Steel Welded Structure

In the research of material fatigue, the fatigue crack is described as an important physical quantity. Although many researches of mental fatigue cracks initiation and propagation have been done and some of the theories have been created, but the fatigue cracks initiation and propagation of welded structures have been done little, especially for short cracks. Along with the development of the aerospace industry, the metallurgical quality of the materials and precision of parts are gradually improved, short cracks propagation of this parts is an important part of the fatigue failure, therefore the research of the cracks initiation and propagation is of great significance, especially for short crack research.In this paper, an experimental of the fatigue cracks initiation and propagation of 16Mn steel welded structure which is often used in welding has been researched. The main points are following:(1)Cracks initiation research: Under the cyclic loading, the phenomena are observed by TEM. Such as the dislocation tangles and dislocation bands are formed because of the dislocation slipping, and then become sub-grains, the cracking of subgrain boundary forms micro cracks; Cementite in pearlite fracture, which can create micro cracks, but they cannot extended.(2)Short cracks propagation research: The degree of Microscopic structure influence to the fatigue crack propagation depends on crack length. In the stage of MSC, because of the microstructure influence, the cracks propagation data is volatile. With the growth of cracks length, the influence has gradually diminished. In the stage of PSC, the cracks propagation rate is monotonic increase by the cracks growth, until achieve the long crack propagation rate.(3) Micro characteristic of cracks propagation research: The path of crack propagation in the heat-affected zone and weld metal is the mix of transgranular and intergranular.(4)The U-notched specimen fatigue research: Notched specimen fatigue is mainly formed by the propagation of the notch surface cracks. Within a certain length, the surface crack growth data is very scattered and showing strong randomness. The penetrated crack steady expands with a certain growth rate since it is initiation, and has been little impacted by Microstructure.