Bending fatigue behavior in an advanced SiC/SiC ceramic matrix composite component at elevated temperature in air

Abstract

The present study aims to understand the high-temperature bending fatigue behavior at the stress-concentrated Y-shape junction of a component made of Tyranno SA3 fiber/BN interphase/SiC matrix composite. The SiC/SiC CMC component was processed via a hybrid technique by combining Chemical Vapor Infiltration (CVI) and Polymer Impregnation and Pyrolysis (PIP) processes. Two different types of tests, namely, point-load-bending-static-test and point-load-bending-fatigue-test, were performed at 1100 °C. The point-load-bending-static test revealed quasi-ductile or shear failure owing to the lower elastic modulus of the PIP-SiC matrix. The first matrix-cracking occurred when the high-stress region reached approximately 180 MPa, which was facilitated by the pores and cracks that remained after the PIP process around the intricate Y-shape junction of the sample. The point-load-bending-fatigue tests were performed under the maximum of the fatigue stresses ranging from 180 to 290 MPa. When the maximum of the fatigue stress was lower than 60% of the static strength, multiple matrix-cracking were pronounced such that the component did not fail and withstand 1000 fatigue cycles. At higher stresses of 250 and 290 MPa, delamination due to shear stress generation at the fiber tows interface and degradation of the BN interphase ultimately led to the fracture of the component at low fatigue cycles of less than 100. Moreover, the fracture morphology of the 290-MPa-fatigue sample showed brittle failure, which was due to matrix-crack-assisted oxygen embrittlement. In addition, a brief comparison between the point-load-bending-fatigue test and the conventional tension-compression fatigue test was discussed.