| The objective of the present research is to develop a comprehensive model to study the high-temperature slow crack propagation in fiber-reinforced ceramic matrix composites (CMCs). In these materials, fiber debonding and slip processes which lead to matrix crack bridging were found to control matrix cracking at low temperatures. At high temperatures, however, fiber creep in the bridging zone, which is characterized by lower temperature threshold and higher rates in comparison with the matrix, appears to be the dominant mechanism which affects matrix crack propagation. Nicalon-CVD SiC composites exhibit this type of creep behavior.; In the model developed here, rigorous derivations of defining equations of bridging tractions and fracture criteria in CMCs are developed based on energy considerations. Relationships which describe the dependence of crack propagation speed on the critical stress intensity, the instantaneous crack length and the relaxation rates of the bridging tractions are also obtained. The effects of fiber debonding, interfacial frictional slip and the composite residual stresses on the time-dependent behavior of matrix cracks are included via detailed analyses of these micromechanical phenomena under fiber creep conditions. A new iterative numerical technique is developed to solve the resulting time-dependent non-linear singular integral equation governing crack opening.; It is shown that the high-temperature behavior of bridged matrix cracks is characterized by an incubation phase where bridging tractions are relaxed, a transient growth phase and steady state growth phase, during which a steady state bridging zone is established and matrix cracks propagate at constant speeds, which are independent of the instantaneous crack lengths and exhibit an Arrhenius dependence on temperature. The incubation time for subcritical crack propagation is determined as a function of the initial crack length, applied stress and temperature. Stability domains for matrix cracks are defined, which provide guidelines for conducting high-temperature crack propagation experiments. Evolution of fiber debonding around the matrix crack is found to significantly influence the relaxation and failure of bridging stresses. |
