Hee Mann Yun

Matrix Cracking in 3D Orthogonal Melt-Infiltrated SiC/SiC Composites with Various Z-Fiber Types

The occurrence of matrix cracks in melt-infiltrated SiC/SiC composites with a three-dimensional (3D) orthogonal architecture was determined at room temperature for specimens tested in tension parallel to the Y-direction (perpendicular to Z-bundle weave direction). The fiber types were Sylramic and Sylramic-iBN in the X- and Y-directions and lower modulus ZMI, T300, and rayon in the Z-direction. Acoustic emission (AE) was used to monitor the matrix-cracking activity. For Y-direction composites, the AE data were used to determine the location (±0.25 mm) where matrix cracks occurred in the 3D orthogonal architecture. This enabled the determination of the stress-dependent matrix crack distributions for small but repeatable matrix-rich “unidirectional” and the matrix-poor “cross-ply” regions within the architecture. Matrix cracking initiated at very low stresses (∼40 MPa) in the “unidirectional” regions for the largest Z-direction fiber tow composites. Decreasing the size of the Z-fiber bundle increased the stress for matrix cracking in the “unidirectional” regions. Matrix cracking was analyzed on the basis that the source for through-thickness matrix cracks (TTMC) originated in the 90° or Z-fiber tows. It was found that matrix cracking in the “cross-ply” regions was very similar to two-dimensional cross-woven composites. However, in the “unidirectional” regions, matrix cracking followed a Griffith-type relationship, where the stress-distribution for TTMC was inversely proportional to the square root of the height of the Z-fiber tows.

Annealing effects on creep of polycrystalline alumina-based fibers

Abstract Continuous-length polycrystalline aluminum oxide-based fibers are being considered as reinforcements for advanced high-temperature composite materials. For these fine-grained fibers, basic issues arise concerning grain growth and microstructural instability during composite fabrication and the resulting effects on the fibers thermo-mechanical properties. To examine these issues, commercially available Nextel 610 (alumina) and Altex (alumina–silica) fibers were annealed at 1100 and 1300°C for up to 100 h in air. Changes in fiber microstructure, fiber tensile creep, and bend stress relaxation (BSR) that occurred with annealing were then determined. BSR tests were also used to compare as-received and annealed fibers to other polycrystalline oxide fibers. Annealing was shown to have a significant effect, particularly on the Altex fiber, and caused it to have increased creep resistance.

Modeling the Thermostructural Capability of Continuous Fiber-Reinforced Ceramic Composites

There exists today considerable interest in developing continuous fiber-reinforced ceramic matrix composites (CMC) that can operate as hot-section components in advanced gas turbine engines. The objective of this paper is to present simple analytical and empirical models for predicting the effects of time and temperature on CMC tensile rupture under various composite and engine conditions. These models are based on the average rupture behavior measured in air for oxide and SiC-based fibers of current technical interest. For example, assuming a cracked matrix and Larson-Miller rupture curves for single fibers, it is shown that model predictions agree quite well with high-temperature stress-rupture data for SiC/SiC CMC. Rupture models, yet to be validated, are also presented for three other relevant conditions: (a) SiC fibers become oxidatively bonded to each other in a cracked CMC, (b) applied CMC stresses are low enough to avoid matrix cracking, and (c) Si-based CMC are subjected to surface recession in high-temperature combustion gases. The practical implications of the modeling results are discussed, particularly in regard to the optimum fibers and matrices for CMC engine applications and the thermostructural capability of SiC/SiC CMC in comparison to nickel-based superalloys, monolithic ceramics, and oxide/oxide CMC.

Issues for Creep and Rupture Evaluation of Ceramic Fibers

Today there exists a strong technological need to develop ceramic and metal matrix composites that are reinforced by continuous-length ceramic fibers and that display thermostructural behavior better than conventional high-temperature materials, such as nickel-based superalloys. To achieve these high-performance, high-temperature composites (HTC), numerous research and development programs are in progress throughout the world to address three primary technical areas: (1) measurement and understanding of the various factors controlling the thermostructural behavior of the HTC and its constituents, i. e., fiber, matrix, and interphase; (2) mechanism-based models to describe and predict the behavior of the individual constituents and their performance as an integrated composite system; and (3) innovative process and design approaches for optimizing high temperature behavior. As with any high-performance composite, the fiber is the principal load-bearing constituent so that major emphasis is being placed on research efforts aimed at the measurement, modeling, and improvement of the thermostructural properties of advanced ceramic fibers.