The Effect of Hold Times on the Fatigue Behavior of an Oxide/Oxide Ceramic Matrix Composite

Abstract

Ceramic matrix composites (CMCs) consisting of an oxide matrix, no fiber-matrix interphase, and an oxide fiber are attractive for high-temperature structural applications because of their inherent resistance to oxidation. Such a system has recently been evaluated using tension, tensile fatigue, and tensile creep rupture. The CMC system consists of an aluminosilicate matrix reinforced with Nextel 610 fibers. The Nextel 610 fibers were in the form of an 8HSW. Results from the mechanical behavior studies showed this CMC to perform extremely well in fatigue at both room temperature and 1000°C. However, the system experienced significant creep strain under sustained loading because of creep deformation that occurs in the oxide fiber. To study the interaction between fatigue and creep, fatigue tests with hold times were conducted at a temperature of 1000°C. For all tests, the maximum fatigue stress was 75 MPa and the load ratio was 0.01. Hold times of 1, 10, and 100 s were applied at maximum load to develop the creep deformation fully. Hold times of 10 seconds were also applied at both maximum and minimum load to study creep recovery. The effect of frequency was also characterized using frequencies of 1, 0.5, and 0.1 Hz. In all tests, the measured strain accumulation was found to be linear with time with no evidence of tertiary creep behavior. The CMC exhibited increased rates of strain accumulation with decrease in frequency and increase in hold times.

Notched Fracture Behavior of an Oxide/Oxide Ceramic‐Matrix Composite

“…Table I summarizes the results for the notched fracture tests and unnotched tensile tests. 1,2 The elastic moduli of 73 and 77 GPa at 23°and 1000°C, respectively, were close to the average value of 70 GPa for the notched specimens. The unnotched tensile stress-strain response is shown in Fig.…”

“…The CMC produced by General Electric Aircraft Engines (GEAE) consisting of alumina fibers and an 87% alumina-13% silica matrix (Nextel610/AS) exhibits excellent room-temperature fatigue and tensile strength (170 and 205 MPa, respectively). 1,2 When tested at 1000°C, fatigue and tensile strength decreased by only 15%. 1,2 Current component designs, however, include stress concentration sites such as bolted attachment points and cooling holes.…”

“…Similar to other fiber-dominated systems, the oxide/ oxide CMC Nextel610/AS exhibits nearly linear unnotched stress-strain behavior. 1,2 However, the notched behavior of Nextel610/AS has not been examined.…”

“…Oxide-based CMCs are inherently oxidation resistant at elevated temperature and are expected to show damage tolerance when produced using a strong interface and weak matrix. 1,2,10 The limited experimental data available in the open literature on the notched fracture behavior of an oxide/oxide CMC are restricted to room temperature. 11 This paper discusses the results of a study of the notched fracture behavior and damage progression of an oxide/ oxide composite (Nextel610/AS) at room temperature and 950°C.…”