Intermediate‐Temperature Stress Rupture of a Woven Hi‐Nicalon, BN‐Interphase, SiC‐Matrix Composite in Air

Abstract

Woven Hi-Nicalon TM -reinforced melt-infiltrated SiC-matrix composites were tested under tensile stress-rupture conditions in air at intermediate temperatures. A comprehensive examination of the damage state and the fiber properties at failure was performed. Modal acoustic emission analysis was used to monitor damage during the experiment. Extensive microscopy of the composite fracture surfaces and the individual fiber fracture surfaces was used to determine the mechanisms leading to ultimate failure. The rupture properties of these composites were significantly worse than expected compared with the fiber properties under similar conditions. This was due to the oxidation of the BN interphase. Oxidation occurred through the matrix cracks that intersected the surface or edge of a tensile bar. These oxidation reactions resulted in strong bonding of the fibers to one another at regions of near fiber-to-fiber contact. It was found that two regimes for rupture exist for this material: a high-stress regime where rupture occurs at a fast rate and a low-stress regime where rupture occurs at a slower rate. For the high-stress regime, the matrix damage state consisted of through-thickness cracks. The average fracture strength of fibers that were pulled out (the final fibers to break before ultimate failure) was controlled by the slow-crack-growth rupture criterion in the literature for individual Hi-Nicalon fibers. For the low-stress regime, the matrix damage state consisted of microcracks which grew during the rupture test. The average fracture strength of fibers that were pulled out in this regime was the same as the average fracture strength of individual fibers pulled out in as-produced composites tested at room temperature.

Stress-Rupture and Stress-Relaxation of Sic\Sic Composites at Intermediate Temperature

“…For HNMI composites this was rather straightforward because the unoxidized regions had large pullout lengths [6] and the demarcation between the oxidized (no pullout strongly bonded fibers) and unoxidized (large pullout) region was 09 o21_e'/__°0 very clear. SYLMI composites have very short pullout lengths to begin width (0 to 10 Hm) and a change in pullout length due to strong bonding is not so obvious.…”

“…The "knee" in the stress-strain curve indicates where through-thickness cracking is prevalent. Since the oxidation embrittlement mechanisms controlling failure require access to the fibers and interphases [6], it is not surprising that rupture stresses correlate with stress-controlled damage in the matrix. The 22 epi SYLMIe composite performed the best, probably due to the highest "knee" stress as well as the highest volume fraction of fibers in the loading direction.…”

Growth of Matrix Cracks During Intermediate Temperature Stress Rupture of a SiC/SiC Composite in Air

“…The It should be noted that for stresses above the "knee" in the stress-strain curve, more AE activity occurs during loading than during the constant load part of the stress-rupture test [7]. The measured matrix crack density and furnace temperature profile versus location along the test specimen are shown in Figure 5 For the lower stresses, the crack density in the hot region was 0.5 to 1.0 mm -_ and less than 0.2 mm _ in the cold region.…”

“…As a result, even if the composite does not fail during a rupture test, the retained strength of a rupture specimen at room temperature is severely degraded. For example, the two specimens that survived 140 hours at 110 and 129 MPa applied rupture stresses both failed at stresses below the applied rupture stress at room temperature [7]. …”

“…[8] and Steyer and Zok [9]. Second, for BN interphase, MI matrix composites [7], the slope of the rupture stress versus time curve is steeper at higher stresses than at lower stresses (see Figure 1) Figures 1 and 2 show the stress and strain versus time stress-rupture data, references 6 and 7, for specimens tested at 815°C in air, respectively. The "transition"…”