Fatigue Behavior and Damage Evolution of SiC Fiberreinforced Ti-6Al-4V Alloy Matrix Composites

The fatigue behavior and damage evolution of an unnotched SiC fiber reinforced Ti-6Al-4V alloy matrix composite (SiCf/Ti-6Al-4V) at room temperature were investigated. The fatigue tests were conducted under a load-controlled mode at a load ratio of 0.1 and a frequency of 10 Hz. The S-N curve was established from tests conducted to failure under a maximum applied stress ranging from 600 MPa to 1200 MPa. The effects of the applied stress levels on damage evolution were investigated by SEM microstructural analysis of specimens from tests interrupted prior to the end of life. The results show that the fatigue damage initiation way and the evolution process are closely related to the applied stress levels. At high stress level (Smax = 1000 MPa), fiber cracking is the major fatigue damage initiation modes. After two or three fibers fracture, fiber cracks and matrix cracks begin to connect and form a macroscopic propagating crack. At intermediate stress level (Smax = 800 MPa), matrix crack initiation and propagation are the major modes of fatigue damage. Multiple matrix cracks initiate from the ground surface edge and the cracked fiber/matrix interfacial reaction layer near the specimen edge. Matrix cracks propagate almost perpendicularly to the loading axis and most of the fibers remain intact and bridge the cracks. At low stress level (Smax = 600 MPa), only partial interface debonding can be observed both between the C coating layer and the interface reaction layer and within C coating layer