Matrix cracking initiation stress in fiber-reinforced ceramic matrix composites
Abstract
One of the important design parameters in CMC's in the Matrix Cracking Initiation Stress (MCIS) which corresponds to the stress at which first matrix cracks are observed. Above the MCIS, the fibers will be exposed to the oxidizing environment which may degrade the mechanical property of the fibers and thus of the composite. In this thesis a systematic study to explore the effects of matrix toughness and inherent strength, fiber diameter, stiffness and volume fraction, temperature and interfacial bonding on the MCIS was carried out. Composites were fabricated using three different matrices--borosilicate glass, aluminosilicate glass and polycrystalline zirconium silicate (or zircon), and two different reinforcing fibers--an SiC monofilament (140 micron diameter) and an SiC yarn (16 micron diameter). In-situ observations during 3-point bend test inside the SEM indicate that matrix cracking is a local phenomenon and occurs first in the matrix between widest spaced fibers. In all composites the MCIS was found to increase with fiber additions and scaled with the monolithic strength. The relative increase in MCIS over the monolithic strength with fiber volume fraction, however, was found to depend strongly on the a(sub 0)/S ratio, where a(sub 0) is the inherent unreinforced matrix flaw size and S is the inter-fiber spacing. For small ratios, the effect of fiber additions on enhancing MCIS are minimal. As the ratio approaches unity, the role of the fibers in constraining the inherent flaw increases, thereby increasing the MCIS. Thermal residual stresses were also seen to play an important role in determining the MCIS; systems with compressive residual stresses in the matrix show higher MCIS at room temperature than at a higher temperature. In systems such as the 7740/Nicalon, which had negligible thermal stresses, MCIS showed minimal changes on testing at 520 C. Several theoretical models were reviewed and the predictions were compared to the experimental results. It was shown that the local-flaw model predictions seem to correlate well with the observed data for the different systems considered in this study.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- May 1991
- Bibcode:
- 1991PhDT........43K
- Keywords:
-
- Bend Tests;
- Ceramic Matrix Composites;
- Cracking (Fracturing);
- Fiber Volume Fraction;
- Fracture Strength;
- Glass Fiber Reinforced Plastics;
- Reinforcing Fibers;
- Silicon Carbides;
- Spacing;
- Aluminum Silicates;
- Borosilicate Glass;
- Zirconium Oxides;
- Structural Mechanics