Observation of fatigue damage process in SiC fiber-reinforced Ti-15-3 composite at high temperature
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I.
INTRODUCTION
CONTINUOUS silicon-carbide (SiC) fiber-reinforced titanium alloy matrix composites are attractive for intermediate temperature structural applications because of their high specific modulus and strength as well as good stability at high temperatures. The strongest candidate system is SiC (SCS-6) fiber-reinforced Ti alloy, e.g., Ti-15-3 and Ti-6Al4V composites. Recent research on such materials and reports has shown that the fatigue resistance of composites is an important property for predicting their application field. Understanding of the evolution of fatigue damage and improving fatigue resistance under various loading and environmental conditions therefore are important factors. The fatigue properties of the SiC (SCS-6) fiber-reinforced Ti alloy matrix composites, both unidirectional and 0 deg/90 deg cross-ply,[1–4] from room to high temperature, under various loading conditions[5,6,7] have been reported. Among them, unidirectional composites were extensively used to understand fatigue damage behavior. It was reported that the fatigue damage evolution differs with both specimen and test condition, as well as with test environments.[8,9] The fatigue crack propagation of a notched specimen at room and high temperatures was decelerated by a fiber bridging, which operated behind an advancing matrix crack tip, and the fiber bridging mechanism was believed to be promising for use in improving fatigue crack propagation.[10–13] In contrast, the fatigue damage evolution of an un-notched composite under a tension-tension fatigue test at room temperature showed that early stage fiber fracture, followed by interface wear degradation, was a major source of fatigue crack initiation and propagation, thus predicting Y. TANAKA, Researcher, C. MASUDA, Reader of Laboratory, and Y.-F. LIU, Doctor, are with the National Research Institute for Metals, Ibaraki-ken 305-0047, Japan. Y. KAGAWA, Professor, and S.Q. GUO, Doctor, are with the Institute of Industrial Science, The University of Tokyo, Tokyo 106-8558, Japan. Manuscript submitted March 3, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
total fatigue life of the composites.[14,15] The major reason for this damage evolution was attributed to the weakening of fiber strength by the partial polishing and debonding of the SCS coating layer from the SiC fiber.[14] If the early fiber fracture stage was avoided, the fiber bridging mechanism could operate and the matrix crack growth rate was significantly decelerated.[16] Such a damage evolution process was reported only for room temperature, and whether there is a similar process at high temperature is unknown. In this study, major attention has focused on a detailed observation of the fatigue damage process in un-notched SiC (SCS-6) fiber-reinforced Ti-153 composite at high temperature. Changes of mechanical properties are compared with the microdamage process of the composite. II.
EXPERIMENTAL PROCEDURE
A. Composite Material Unidirectional continuous SiC fiber (SCS-6, Textron, Lowell, MA) reinforced Ti-15-3 allo
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