Numerical simulation of oxidation and its effect on the crack growth resistance of titanium alloys

Abstract

The use of titanium alloys Metal Matrix Composites (MMC) has become more extensive in the aerospace field, especially for elevated temperature applications such as leading edges on hypersonic aircraft and the jet engine turbine blades. Such applications expose the MMC to highly variable mechanical and thermal loading and corrosive environments. The mechanical and thermal loading may introduce macrocracks, which may promote oxidation from the crack surfaces. Previous research has noted that even though the oxidation generally degrades the MMC, the oxidation from the crack surfaces may improve the crack growth resistance of the metal matrix. Titanium alloys are potential candidates for use in MMC. Therefore, a more thorough understanding of titanium alloys used as metal matrix in MMC is imperative. This research effort develops a numerical modeling procedure to simulate the oxidation propagating from the crack surfaces and evaluate the effect on the mechanical behavior. The numerical modeling is divided into two parts: the oxidation modeling and the fracture mechanics modeling. The oxidation of a pre-cracked specimen is modeled by modifying the Fickian diffusion problem in order to simulate the chemical reaction in the metal. Two different variants of a fixed grid finite element method for numerical simulation of oxidation are used. The first approach tracks down the oxidation front and splits the domain into metal and oxide subdomains. The second approach is based on reformulating the governing equation in both the oxide and matrix, resulting in a single, non-linear equation for the whole domain. Two different methods are also used in the fracture mechanics modeling to simulate the effect of stiffness change and the oxide volumetric expansion on the energy release rate. The first approach models the stress measure the energy release rate. The second approach measures the energy required to open the crack as the energy release rate. Establishing both the oxidation and mechanical models, the energy release rate of a pre-oxidized Ti-15-3 compact specimen under monotonic loading is evaluated.