考虑孔洞影响的铸造镁合金ZM6疲劳寿命预估方法

Casting magnesium alloy ZM6 is a typical material used in the manufacturing of reduction gearboxes of helicopters. However, the internal defects produced during the casting process have a significant effect on the fatigue properties of the material. In this paper, the fatigue damage model and life prediction method for ZM6 material with internal pore defects are studied. First, the X-ray tomography scan of specimens made of three batches of blank material is conducted, and the distribution characteristics of internal pores are obtained. It can be observed that the pore distributions are much different for three batches of specimens. In addition, the randomly distributed small pores and a few large-dimension pores are both observed for each tested specimen, which will provide the basic data for the following influencing analysis of pores. Afterwards, high-cycle fatigue tests for 48 specimens are conducted under two stress ratios and several stress levels, and the fatigue lives of each batch of specimens are obtained. After that, the correlation between fatigue life and the distribution of internal pores is further studied. It can be concluded that the porosity and the large near-surface pores are the two key factors affecting the fatigue life of a specimen. Accordingly, based on the theory of damage mechanics, a fatigue damage model is then proposed to reflect the influences of porosity and large near-surface pores by introducing the initial elastic modulus of representative volume element (RVE) and the equivalent local stress and strain fields around the key pore. The corresponding calibration method for parameters is presented as well. This model is then combined with the ABAQUS software platform to implement the numerical calculation of fatigue damage evolution for specimens with internal pores. Finally, the proposed theoretical model and calculation method are used to predict the fatigue lives of specimens, and the prediction results agree well with the experimental data, which validates the applicability of the proposed method.