Matrix cracking in fiber-reinforced ceramic composites. Ph.D. Thesis
Matrix cracking in fiber-reinforced ceramic composites with unbonded frictional interface was studied using fracture mechanics theory. The critical stress for extension of a fiber-bridged crack was analyzed using the stress-intensity approach. The analysis used a new shear-lag formulation of the crack-closure traction applied by the bridging fibers based on the assumption of a constant sliding friction stress over the sliding length of the fiber-matrix interface. The new formulation satisfied two required limiting conditions: (1) when the stress in the bridging fiber approached the far-field applied stress, the crack-opening displacement approached a steady-state upper limit that was in agreement with the previous formulations; and (2) in the limit of zero crack opening, the stress in the bridging fiber approached the far-field fiber stress. This lower limit of the bridging stress was distinctly different from the previous formulations. Numerical calculations using the stress-intensity approach indicated that the critical stress for crack extension decreased with increasing crack length and approached a constant steady-state value for large cracks. The steady-state matrix-cracking stress agreed with a steady-state energy balance analysis applied to the continuum model, but it was slightly less than the matrix-cracking stress predicted by such theories of steady-state cracking as that of Aveston, Cooper, and Kelly (ACK). The origin of this difference and a method for reconciliation of the two theoretical approaches were discussed. The effects of residual stresses and partially bridged crack was studied and incorporated into the calculation of the critical stress for extension of matrix cracks. Matrix cracking was studied in a model, unidirectional SiC (fiber)-reinforced epoxy-alumina matrix composite. The fiber-matrix interface was tailored with coatings to achieve an unbonded frictional interface.
- Research Organization:
- Utah Univ., Salt Lake City, UT (United States)
- OSTI ID:
- 236770
- Report Number(s):
- N--96-21488; NIPS--96-33278
- Country of Publication:
- United States
- Language:
- English
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