Mode I Interlaminar Fracture Properties of Oxide and Nonoxide Ceramic Matrix Composite

This work provides a novel method for determining interlaminar fracture properties at both room and elevated temperature, offering the first glimpse of the interlaminar fracture behavior of CMCs at elevated temperatures.;Interlaminar fracture properties play an important role in predicting failure of structural components for CMC materials. Elevated temperatures induce more severe conditions for interlaminar properties resulting in a weaker interlaminar toughness. The main challenges associated with determining interlaminar fracture toughness are the ability to measure crack growth without visual observation and to develop an experimental setup that can be used at both room and high temperature. Hence, a non-visual crack monitoring technique has been successfully introduced to estimate crack length in CMCs using electrical resistance. In a parallel effort, a wedge-loaded double cantilever beam method has been developed to determine the interlaminar fracture properties of CMCs at room and elevated temperatures. It has been found that the wedge method does not depend on the wedge material, as long as the correct coefficient of friction is taken into consideration. Additionally, the wedge method was found to be comparable to the traditional double cantilever beam method.;The interlaminar fracture properties depend immensely on the composite microstructure and the weave architecture; the interlaminar crack propagates along the longitudinal fiber tows, passing through the porosities, which serve as stress concentration points. Moreover, depending on the fiber tows orientation along the crack propagation path, a rising or flat R-curve behavior can be seen for the same composite system.;High temperature testing revealed that the energy required to initiate a crack at room temperature is greater than that at 815 °C. However, more energy is required to propagate the interlaminar crack at high temperature for some CMC systems (such as PIP SiC/SiNC). This behavior was attributed to softening of the matrix, which was evident when comparing crack growth rate at elevated temperature to room temperature. The data presented provides the first glimpse of the interlaminar fracture properties of CMCs at elevated temperatures.;The wedge method was also verified using finite element analysis and micromechanics approaches. However, in order for a model to accurately predict the interlaminar behavior of the material and assist in optimizing specimen's geometry, the mechanical response of the studied composite should be well-known, especially shear properties.;Finally, a method for determining the out-of-plane electrical resistivity for composite materials has been proposed, while introducing the concept of length constant as a composite property. This method was utilized and successfully verified for two ceramic matrix composite systems with significantly different electrical properties. The out-of-plane electrical resistivity was found to be 8-9 times greater than the in-plane electrical resistivity.