Transverse Fracture Toughness of Unidirectional Continuous Fiber and Hybrid Ceramic Matrix Composites

Abstract

Abstract Reinforcing ceramic materials with unidirectionally aligned continuous fibers improves the mechanical characteristics in the fiber direction but often results in reduced transverse strength and fracture toughness. As an effort to improve the transverse properties of unidirectional fiber reinforecd ceramic matrix composites (FCMC), randomly dispersed fine whiskers can be added to the matrix. The result is a hybrid ceramic matrix composite (HCMC). The objective of this investigation was to ascertain what improvement, if any, can be made through the process of hybridization. The constituent materials evaluated in this study include cordierite (magnesia-alumino-silicate) for the matrix, silicon carbide continuous fiber tow and silicon carbide whiskers. Specimens were fabricated using filament/slurry winding followed by hot press sintering. The specimen bar lengths ranged from 30 to 35 millimeters and had nominally square cross-sections of 4 millimeters. Three-point chevron-notched flexural testing was performed using an MTS-810 servo-hydraulic system. The method presented is based on that which was provided by S-X Wu in ASTM STP 855. The difference here is that a numerical solution is used instead of calibrating fitted stress intensity coefficient curves to the geometry of a material with a well established fracture toughness. The primary advantage to using the cheveron notch profile is that the high stress concentration at the tip of the cheveron ligament induces crack initiation at low loads. As the crack progresses into the increasingly wider portion of the chevron it experiences a decreasing stress intensity field and grows in a stabile fashion as the load is increased. A second advantage is that the stress intensity as a function of crack length is well defined and reaches a minimum at the critical crack length. The need for pre-cracking and the knowledge of the critical crack length are thus avoided. The plane strain opening mode fracture toughness can be determined from specimen geometry, maximum load and flexural span alone. The results from testing confirmed the chevron-notched three-point-bend test produced limited but stable crack growth prior to failure without the introduction of a starting precrack. Instant failure due to crack initiation overload and the resulting potential over-estimate of the transverse plane strain opening mode fracture toughness was avoided. The method presented successfully reproduced the results obtain using other means. The fracture toughness of monolithic cordierite was found to be within 1% of the value published in the Ceramic Source, v8. Substantial improvement to the transverse fracture toughness of a continuous fiber reinforced ceramic matrix composite through the introduction of whiskers to the matrix was demonstrated. When compared with FCMCs with the same base matrix and approximately the same volume fraction of fibers, the HCMCs evaluated in this study displayed a 99% increase in fracture toughness for a crack propagating along the fiber direction and an 82% increase for a crack propagating across the fibers. A significantly greater amount of damage was observed on the fracture surface of the hybrid composite. This indicates more crack deflection and branching due to the presence of the whiskers occurred in the hybrid during fracture. The transverse fracture toughness that results from a crack progressing along the fiber direction was considerably higher than that resulting from a crack propagating across the fiber. More broken fibers were seen on the fracture surfaces of the former. The energy required to break fibers and peal fibers away from the matrix may be responsible for the greater fracture toughness found for a crack propagating in the plane along the fiber direction. Improved load transfer provided by the whiskers in the matrix may aid in the resistance to fiber pealing.