Finite Element Modeling And Experimentation Study Of Compressive Loading Effects On Fatigue Crack Propagation Of Aluminium Alloy

Today, traditional strength design of static loading condition and non-defects material has been studied deeply with the development of fracture mechanics and damage mechanics. So the fatigue has become one of the main factors which induced failure of metal components. The aluminum alloy is used in aviation field widely and its fatigue life is crack steady propagation life. So, it is a big engineering problem to predict the real propagation life accurately of aluminum alloy under applied fatigue loading. But the knowledge of fatigue crack can not grow under compressive loading for the closure of crack for a long time. The compressive stress will be omitted in the fatigue strength design. From the research result, it has been found that the compressive stress affects fatigue crack propagation of metal materials. For the aluminum alloy, it is very dangerous to ignore it. Compressive stress promotes crack growth. So, the study of the effects of compressive stress on crack growth in the aluminum alloy under tensile-compressive loading spectrum will satisfy engineering application.Neither analytical nor universal models have been found, because of the complexity of fatigue life. Now, finite element method is used widely in modeling fatigue crack growth. It is a good way to cut down the cost of fatigue test and shorten the period of fatigue study. From the mechanism research of aluminium alloy fatigue crack initiation and propagation, we know that crack initiation and propagation is closely connected to the plastic deformation in front of the crack tip. The plastic deformation which caused by the fatigue loading is adjacent to the crack tip. However, the elastic deformation occurred away from crack tip. According to the different mechanical responses of materials around crack tip, combined with the expressions of the elastic-plastic mechanics and the fracture mechanics, a two-dimensional static elastic-plastic finite element model which describe the change of the aluminium alloy crack tip mechanical parameters has been set up. Through the comparison of the difference and similar between static crack model and dynamic crack model, as well as the 2D model and 3D model at the same time, the computation results of two dimensional static elastic-plastic finite element model are conservative. It is a simple modeling method with high calculation efficiency, and suits for engineering application.After analyzing elastic-plastic finite element results, such as crack tip stress analysis, crack opening displacement analysis, equivalent plastic strain analysis and the analysis of plastic zone size in front of the crack tip, it proved that compressive loading promoted crack growth in aluminium alloy, and the effects mainly occurred in the zone beyond crack tip less than 1μm. It is dangerous to neglect the effects of compressive loading when predicting fatigue life of aluminium alloy. Based on the finite element analysis results, a new parameter has been redefined, named effective stress intensive factor range-△K'_eff. Based on this new parameter, a model has been set up which can calculate the fatigue crack propagation rate of aluminium alloy under stress ratio R=0～-1. According to the fatigue crack growth mechanism of aluminium alloy, engineering rationality of△K'_eff has been analyzed. The test datum of aluminium alloys 7049-UA and 2A12-T4 have been verified that the model based on the parameter△K'_eff can accurately described the effects of compressive stress on fatigue crack propagation of aluminium alloy. Compared with the computation results of the model based propagation parameter K_max,△K, and K_eff, fatigue life predicted by the model based parameter△K'_eff is safer.Rigidity of plate specimen fixture used on the high frequency fatigue test machine can deeply affect the test frequency and test precision. So, we analyzed the joining structure of the plate specimen fixture by the finite element method. The analysis results showed that the whole rigidity of fixture with circular groove joining structures is higher than those of other types of joining structure, the radius of circular groove and screw length connecting the fixture and test machine are also affect whole rigidity. The reasonable magnitude of circular groove radius is about (0.17-0.25) times of the board thickness, and screw length is about (1.43-1.55) times of screw nominal diameter, furthermore, the reasonable magnitude of screw pitch determined by self-locking performance and wear resistance of screw pairs.