Computer Simulation Of The Indentation Experiment And The Fatigue Of The Grain Boundary Response

Analytical and finite element (FE) methods were used to simulate the indentation testing on multi film/substrate systems. Special attentions were put on the relationships between the indentation experimental results and the material properties of the matrix and thin films, so that the methods have been presented to obtain the material properties from the indentation experimental results. Elastic, plastic and creep behaviors of the multi film/substrate systems were investigated. Based on homogeneous contact elastic solutions, the energy conservation law was adopted to construct close-form and first-order-accurate contact elastic solutions. Compared with the results obtained from numerical computations (FE-method), such solutions were accurate enough for the film/substrate systems. The effect of Poisson抯 ratio of films and ratios of film thickness to indenter radius were examined. By the definition of 慹ffective?shear modulus, the method to obtain shear module and elastic module was provided. According to the FE simulation, a new method has been presented to derive the material creep properties (in Norton equation) from the indention creep experimental results (indentation stress ?indenter depth ?time) for thin film/substrate systems. The method can also be applied to other cases of the material and structures. In the simulation, two steady states were found. Maybe the indentation creep behavior has two steady states, but only the first steady state can be used to construct the relationship between traditional uniaxial tensile creep testing and indentation testing. In simulation on the plastic case for the indentation testing, FE-results showed that the simulation results would have some error from the ignoring the influence of the substrates even if indentation depth did not exceed 5-10% depth of film. The polynomial equations have been developed to predict the relationship between the material properties of film and substrate. The blunt ratio of the indenter has been considered in the indentation - loading curve. Finally, a new method has been proposed to obtain the yield strength and hardening modulus of film from indentation loading. Two sets of bicrystal specimens were studied to expose the fatigue behavior influenced by the grain boundaries. One is the crack along the grain boundary; the other is the crack perpendicular to the grain boundary with different distance. Using the three-point bend specimens, the fatigue crack growth rate of bicrystals was measured. It was found that the rate of the fatigue crack growth of the cracks along the grain boundaries is the fastest. The grain boundary is the weakest. Different characteristics were found in the set of bicrystal specimens of the crack perpendicular to grain boundary. There was a grain boundary-induced shielding effect. The crack growth rates varied with the distance between the crack tip and grain boundary: the crack growth rate was maximum at critical distant, while rate was minimum when the crack reached grain boundary. The crystallographic finite element method was applied to analyze the stress and strain structure ahead of the crack, in order to reveal the characteristics of the fatigue behavior studied above. It is the grain boundary-induced redistribution of stresses near the crack tip that induces the grain boundary-induced shielding.