Papers by Author: Li Sha Niu

Abstract: Specimens of a directionally solidified superalloy with different shot peening pressure were annealed at 1220oC in vacuum condition to get recrystallized surface layers with different micro-structures. Low cycle fatigue tests of these specimens were performed at room temperature and 400oC by using an electrohydraulic load frame in the SEM system for real-time observation. The initiation and propagation of cracks were inspected and the influence of the micro-structure of the recrystallized layer on the material fatigue behavior was analyzed. The low cycle fatigue life of the specimens depends mainly on the characteristics of the recrystallized layer. When the shot peening pressure is lower, the recrystallized layer is thin and not integrated, and the fatigue life decreases obviously in comparison with that of the specimen without recrystallized surface layer. When the shot peening pressure increases, the recrystal grains are more integrated, and the fatigue life rises. A comparison of the recrystallized layers between the blade surface and the specimen surface has been done and it points that the incompact surface recrystal layer is very dangerous to gas turbine blades.

Abstract: Flow-induced localized corrosion is regarded as one of the main degradation mechanisms of materials. As an initial step of the simulation of a pipe, a plate is chosen to simplify the problem. In this paper, finite element method is used to simulate the corrosion process in the plate by employing nonlinear geometry and physics equations of the material to describe the quasi-static process. An elastic modulus iterative procedure was performed to obtain the material parameters in consideration of the nonlinear physical properties of corrosion. The effect of corrosion is then considered by introducing a criterion between depth and time, calculating corrosion depth at progressive given time. Dead and live finite elements are employed to consider the invalidation of the material. Thus the movable boundary conditions can be taken into account and the dynamic status of corrosion can be simulated. Stress corrosion process under flowing fluid condition is analyzed and then the results of representative examples are compared with published results.

Abstract: Sputter deposited SiC films with and without annealing were characterized using X-ray photoelectron spectroscopy (XPS). A complex transition layer, containing silicon oxycarbide (SiCxOy), between the SiO2 layer grown during extended exposure to ambient air or annealing and SiC substrate was investigated. Furthermore, the presence of excessive amorphous carbon was detected in the near-surface region for annealed sample. We justified the differences of composition and chemical bonding in these two oxide layers in terms of different oxidation kinetics involved.

Abstract: A 2-D Finite element simulation method was developed based on the kinetic law and the energy evolution during the whole process of deformation, which is used to investigate the creep size effects in polycrystalline thin metal film on substrates. Three diffusion paths (e.g. surface, grain boundary and lattice diffusion) are considered in the present model. The diffusion rate for these three processes was compared under different loading conditions with corresponding microstructure. It’s found that grain boundary diffusion is coupled with another diffusion channel. Creep size effects result from mass transferring in thin film. The model gave the quantitative results of the influences of the film thickness, grain size, and the constraints of the substrate on polycrystalline metal film diffusion. The simulated results present the evolution of the point defects in grain interior, the strain and stress field. The distribution of the crack-like stress in the grain boundary could explain the stress concentration mechanisms clearly and this also agrees with the literature results.

Abstract: This work has investigated the effect of spherical dent on rolling contact fatigue (RCF). A 3-D finite element simulation model of bearing rolling contact incorporating critical plane approach has been developed to study the fatigue failure location. It was found that the fatigue failure locations were significantly influenced by the dent. The calculation results are in good agreement with the experimental results and comparable with the results from the published literatures in which 2-D models were generally used.