Investigation of Fiber/Matrix Interfacial Mechanical Behavior in Ceramic Matrix Composites by Cyclic Fiber Push-In Testing

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Cyclic fiber push-in testing is used to examine the stability of interfacial frictional sliding stresses and fiber debond lengths with continued push-in load/unload cycles. The measured response to applying load cycling to a single fiber reveals the susceptibility of the fiber/matrix interface to degrade under cyclic loading conditions, and thus, helps evaluate the contribution of the interface to the cyclic fatigue behavior of the composite after the occurrence of matrix cracks. From cyclic push-in testing in room temperature air, decreasing interfacial sliding stresses and increasing debond lengths are observed with continued load cycling for SCS-6 SiC fiber reinforced reaction-bonded silicon nitride (SCS-6/RBSN), whereas stable interfacial sliding stresses and no increase in debond lengths are observed with continued load cycling for SCS-6 SiC fiber reinforced strontium aluminosilicate (SCS-6/SAS). These results indicate that fiber-bridged matrix cracks should be stable under cyclic fatigue loading conditions in SCS-6/SAS, but should exhibit increasing crack opening displacements and fiber pull-out with continued cycling in SCS-6/RBSN. In addition, changing the test environment from room air to nitrogen significantly affects the cyclic push-in test results for SCS-6/RBSN, but not for SCS-6/SAS. The different responses to this change in test environment are attributed to different locations of interfacial failure.