The fracture behavior of 2-D SiC/SiC woven composite was investigated at both ambient and elevated temperatures in this research. At ambient temperature, the fracture initiation toughness and R-curve behavior for composite were characterized and related to in-situ microscopic obseniations of damage accumulation and crack advance. Matrix cracking, crack deflection and branching were observed and dominated fracture behavior in the early loading stage. The key to toughening appeared to be associated with the mechanics of crack arrest at fiber bundles in the woven architecture. After the primary crack extension, the composite behaved non-linearly and a J-integral technique was applied to investigate the R-curve behavior. Substantial fibrous pull-out was observed in this regime of crack advance. An insight into the origin of the $J\sb{R}$-curve of a SiC/SiC woven composite was obtained by experimental characterization of the closure stress-crack opening displacement, $\sigma(u)$, relationship in the process zone of the crack. Application of a previously derived theoretical function, $\sigma\sb{b}(u)$, solely based on fiber bridging, showed consistent results with experimental data. The slow crack growth (SCG) behaviors of three different 2-D SiC/SiC composites were investigated at elevated temperatures from 700$\sp\circ$C to 1200$\sp\circ$C. In Dupont composite, crack length was measured in-situ at temperatures by an optical telescope allowing crack growth rate, V, as a function of applied stress intensity, K, to be obtained directly. Catastrophic failure followed after a limited crack extension suggesting limited SCG behavior at the intermediate temperatures. In contrast, an extensive SCG behavior was observed at 1200$\sp\circ$C. Microstructure observation indicated that crack bridging was absent at the intermediate temperatures but was present at 1200$\sp\circ$C. These results were consistent with the role of temperature on the oxidation and mechanical properties of the fiber bundles. In Goodrich composite, an introduction of boron as well as an increase of carbon interface thickness were found to enhance the time dependent deformation at the crack frontal region. Consequently, instead of the stress intensity factor, K, the energy rate C parameter exhibited a better correlation with the crack velocity.
| Identifer | oai:union.ndltd.org:UMASS/oai:scholarworks.umass.edu:dissertations-5453 |
| Date | 01 January 1996 |
| Creators | Wang, Yu-Lin |
| Publisher | ScholarWorks@UMass Amherst |
| Source Sets | University of Massachusetts, Amherst |
| Language | English |
| Detected Language | English |
| Type | text |
| Source | Doctoral Dissertations Available from Proquest |
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