Two Kinds Of Surface Failure Of Metallic Materials Engineering Mechanics

The failure of engineering surface of metallic material was mainly analyzed by a lot of experiments. Reasonable theoretical model should be developed in order to comprehend the failure mechanism accurately. For mechanical analysis on two the boride surface and high-temperature cyclic oxidation coating on metallic material, finite element method (FEM) method is used to calculate the stress state, to explore failure mechanism and to predict the usage life time.The thermal stress in the borided 45 steel at a process temperature of 840-1050 癈 was calculated using a planar axisymmetric model by FEM based on the characterization of outersurface and FeB/Fe2B/steel substrate interfaces. The high tensile stress was observed in the peripheral zone on the surface and on the interfaces between the FeB/Fe2B and Fe2B/steel substrate for the borided samples. The frequent conversion of the tensile and compressive stresses was detected around the interfaces. The calculated stress profile is similar to that of experimental reported from the references. The boride surface failure due to brittle fracture is dependent on the high tensile stress on the surface. The formation of plastic strain in the steel substrate beside the interfaces between Fe2B/steel substrate leads to the delamination of borides layer on the borided steel.The spallation model of cyclic oxidation has been developed using a mechanical analysis, in which high-temperature cyclic oxidation is described by a parabolic growth law, while spallation is treated in terms of stability analysis. The buckling strip is considered to be equivalent of a strut resting on an elastic foundation to analyze the effect of delamination interface. The isothermal oxidation constant, kp, spallation parameters, q and m, can be determined by experimental data. The value of q at different oxidation temperature is predicted using the FEM Eigenvalue Buckling analysis. Applications of the proposed model for calculating the cyclic oxidation behavior of Fe-Ni-Cr-Al steel at different temperature are demonstrated by comparing the calculations against the experimental data as well as that of other models in the literature. The calculation by the model is shown to be in good agreement with experimental data.