The micro-mechanics of fracture and creep of metal and ceramic matrix composites

Advanced metal matrix composites such as metals reinforced by ceramic fibers or layers are being developed for use at relatively high temperatures. The performance of such composites is controlled by the behavior of the metal and ceramic phases, and the behavior of the metal/ceramic interface. To aid the microstructural design of the composite and the safe-design of structural components, micromechanics models are developed in this work for the fracture and creep of continuously reinforced fiber composites, as well as metal/ceramic multilayer composites.; First, crack bridging models are developed for fiber composites with slip-dependent interfaces. For a unidirectional fiber reinforced composite containing a matrix fatigue crack bridged by intact fibers, three interface models are proposed based on the observations of interface degradation owing to asperity wear and damage of fiber coatings. The corresponding crack bridging traction laws are derived. Systematic calculations are carried out for the bridging stress distribution and crack tip stress intensity. It is found that slip-dependent interface properties may have a strong effect on fiber fracture, while the effect on matrix cracking is relatively small. The results suggest that the constant-{dollar}tau{dollar} model is a good approximation; however the slip-dependent interface model should be used in composite design when the applied load is high.; When the working temperature is high, composite creep becomes a critical issue in component design. In particular, broken fibers which introduce stress a concentration in the matrix and increase the stress in the intact fibers, thus have a strong effect on the composite creep. In this thesis, the role of broken fibers in the creep of unidirectional continuous fiber reinforced composites is studied theoretically. Generic relationships between the constitutive behaviors of the individual phases and the composite creep response are established. The fiber composite is approximated using an axisymmetric cell model. The long-term creep behavior of the composite is obtained in closed form. The composite in transient creep behavior is calculated using the finite element method. It is shown that a broken fiber can cause a significant increase of the stress in the surrounding intact fibers. It is also demonstrated that applied load perpendicular to the fiber direction may have a strong influence on fiber fracture. Matrix plasticity is found to have little effect on the composite creep. The results also indicate that an interface optimization is necessary in the composite microstructural design.; A similar analytical approach is used for the study of creep in ceramic/metal multilayer composites. A closed form solution is derived to predict the transient creep of damage-free multilayer composites. The solution for the long-term creep behavior of the multilayers containing fractured ceramic layers is also derived in a closed form, which can be used as a conservative design formula to prevent the catastrophic composite failure. The influence of fractured ceramic layers on the time-dependent composite creep is examined using FEM. The effect of interface sliding on composite creep is also studied; the influence is found to be significant.