Paper number 1343

THE MICROMECHANICAL MODELLING OF THE SHORT CRACK FATIGUE BEHAVIOUR OF AL-SIC MMCS

Mark Bruzzi and Dr. P. E. McHugh

Dept. of Mechanical Engineering,
National University of Ireland, Galway, Ireland.

Summary

The objective of this study is to use micromechanical models to investigate the fatigue crack growth behaviour of microscopically small cracks for two materials: (1) a cast 359 Al reinforced with 20% SiC particles and (2) a forged 2124 Al reinforced with 17% SiC particles. In particular, the focus of this study is on the local crack tip driving force conditions. Micromechanical models, incorporating separate representation of the matrix and the reinforcing particles in 2D, are developed each including a short crack with a progressively increasing length, originating from the surface. The SiC particles are treated as elastic and the matrix as elastic plastic. J2 flow theory is used to model the plasticity within the matrix, and a non-linear isotropic / kinematic hardening model is used to model its hardening behaviour. A quench is simulated to predict input residual stresses which exist within the materials due to the mismatch of thermal expansion coefficients. The effects of increasing crack closure, with short crack length, on the crack tip driving force conditions are included into the model by increasing the threshold for crack propagation for each crack length within the model. The effects of the reinforcing particles, acting as microstructural barriers to crack propagation, is then investigated by the micromechanical simulations. The J-Integral, calculated around the crack tip, is correlated with the crack growth rate and the crack growth rate curve for propagating short cracks is presented. Different positions of crack tips with respect to the particles are examined, and their respective crack growth rates are calculated. Finally, the predicted S-N curve from the micromechanical modelling is compared with experimental results for both the cast and the forged MMCs.

Keywords

micromechanics, Al/SiC MMC, fatigue, crack growth.