Fuzhen Xuan

Fatigue Crack Initiation Potential from Defects in terms of Local Stress Analysis

The competition of surface and subsurface crack initiation induced failure is critical to understand very high cycle fatigue(VHCF) behavior, which necessitates the elucidation of the underlying mechanisms for the transition of crack initiation from surface to interior defects. Crack initiation potential in materials containing defects is investigated numerically by focusing on defect types, size, shape, location, and residual stress influences. Results show that the crack initiation potency is higher in case of serious property mismatching between matrix and defects, and higher strength materials are more sensitive to soft inclusions(elastic modulus lower than the matrix). The stress localization around inclusions are correlated to interior crack initiation mechanisms in the VHCF regime such as inclusion-matrix debonding at soft inclusions and inclusion-cracking for hard inclusions(elastic modulus higher than the matrix). It is easier to emanate cracks from the subsurface pores with the depth 0.7 times as large as their diameter. There exists an inclusion size independent region for crack incubation, outside which crack initiation will transfer from the subsurface soft inclusion to the interior larger one. As for elliptical inclusions, reducing the short-axis length can decrease the crack nucleation potential and promote the interior crack formation, whereas the long-axis length controls the site of peak stress concentration. The compressive residual stress at surface is helpful to shift crack initiation from surface to interior inclusions. Some relaxation of residual stress can not change the inherent crack initiation from interior inclusions in the VHCF regime. The work reveals the crack initiation potential and the transition among various defects under the influences of both intrinsic and extrinsic factors in the VHCF regime, and is helpful to understand the failure mechanism of materials containing defects under long-term cyclic loadings.

Residual stresses in the elastoplastic multilayer thin film structures: The cases of Si/Al bilayer and Si/Al/SiO2 trilayer structures

A theoretical model was developed to predict the thermal residual stresses within the elastoplastic multilayer thin film structures. The plastic deformation of one of the films was considered. Special analyses were made on the bilayer structures, i.e., a film overlaid on a substrate, as they are more of practical interest. Closed-form solutions were derived to estimate the residual stresses in the films and curvature of the multilayer film structure, and the relationship between the temperature difference and the thickness of the plastic zone. The cases of Si/Al bilayer and Si/Al/SiO2 multilayer structures were studied to illustrate the implementation of this model. In these structures, Al film was assumed to be plastically deformed. Results showed that, for both structures, there was a linear relationship existing between the thickness of the plastic zone in Al film and the temperature difference. SiO2 layer deposited on the aluminum film had an obvious influence on the critical temperature at which Al f...

Effect of laser power on the microstructure and mechanical properties of TiN/Ti 3 Al composite coatings on Ti6Al4V

Laser nitriding is one of the effective techniques to improve the surface properties of titanium alloys and has potential application in the life extension of last-stage steam turbine blades. However, cracking of surface coating is a common problem due to heat concentration in laser nitriding process. Conventionally, the cracks can be avoided through heat treatment, which may have an important influence on the mechanical properties of coating. Crack-free TiN/Ti3Al IMC coatings on Ti6Al4V are prepared by plasma spraying and laser nitriding. The microstructures, phase constitutes and compositions of the coating are observed and analyzed with scanning electron microscopy(SEM), X-ray diffraction(XRD) and X-ray energy-dispersive spectroscopy(EDS). Microhardness, elastic modulus, fracture toughness of the coating are measured. The results show that the crack- and pore-free IMC coatings can be made through the proposed method; with increasing laser power, the amount and density of TiN phase in the coating first increased and then decreased, leading to the similar trend of microhardness and elastic modulus and the reverse trend of fracture toughness of the coating. Both the average microhardness and elastic modulus of the coating increase three times higher than those of the substrate. The volume fraction of the TiN reinforced phase in composite can be controlled by varying the laser power and the cracking problem in laser nitriding process is successfully solved.

Analysis on multiple cracking in film/substrate systems with residual stresses

The multiple cracks might be initiated in the film on a substrate due to the presence of residual stress. In this paper, the multiple film cracking in the film/substrate systems with residual stress was analyzed. First, a relatively simplified model with the closed solutions considering the edge effect was developed to predict the residual stress within the film segment. Some obvious relationships between the material properties and the dimensions of the film and substrate and the stress distribution in the film could be reflected by using this model. By comparing the analytical results with those from the existing analytical models, finite element analysis, and the existing experimental measurements, it could be concluded that the present analytical model was very rigorous. Then, the energy criterion was adopted to perform the analysis on multiple film cracking on the basis of the knowledge on the residual stress distributions. The closed-form solution for the critical misfit strain for the crack initiat...

Multiple film cracking in film/substrate systems with mismatch strain and applied strain

Multiple cracking of brittle films deposited on the relatively soft substrate was commonly used in different applications. In the present paper, multiple film cracking in the film/substrate systems was analyzed. The system was subjected to the combination of the residual stresses due to the mismatch strain between the film and substrate and the applied stress due to the unidirectional strain on the substrate. First, a simply analytical model was developed to derive the closed-form solutions for the residual stress distribution and redistribution due to the edge effects in the film segment. Second, the energy and strength criteria were adopted to perform the analysis on the multiple film cracking on the basis of the knowledge on the residual stress distributions. Third, the case of SiOx film/polyethylene terephthalate substrate system was used to illustrate the implementation of this model. The relationship between the fracture energy as well as the film strength and the film thickness was obtained. The pr...