Fatigue in selectively fiber-reinforced titanium matrix composites
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INTRODUCTION
STRUCTURAL components made of silicon-carbide fiber-reinforced titanium matrix composites (TMCs) have attractive stiffness, strength, and creep-resistance characteristics. Hence, they are being considered for aircraft engine components such as actuator piston rods, fan blades, and turbine connecting rods. In all these applications, the TMC will only be used to selectively reinforce the structure, in order to enhance its stiffness and fatigue resistance. Factors favoring the selective reinforcement concept are the high cost of the TMCs and difficulties associated with manufacturing entire TMC components. The selective reinforcement concept can also be viewed as the cladding methodology. It is generally expected that TMCs will be clad with monolithic Ti alloys for either of the following reasons. A monolithic Ti layer would facilitate machining tolerance and allow for joining of TMC components to other metallic components in the structure. Another potential use for cladding is to reinforce components locally in regions of high stress concentration such as panels containing through-thickness holes, with the purpose of alleviating the high degree of notch sensitivity exhibited by TMCs.[1] The role of cladding on the mechanical behavior (such as unnotched and notched strength, and fatigue crack growth) of TMCs has been reported previously.[2–5] It was shown that these properties can be rationalized using suitably modified models that are developed for describing mechanical properties of TMCs. However, the unnotched fatigue response of selectively reinforced (or clad) TMCs thus far has not been investigated. It is expected that the fatigue characteristics of these materials will be significantly different from that of all-TMC panels. In all-TMCs multiple matrix cracking occurs upon cycling the load, with each crack being bridged U. RAMAMURTY, Assistant Professor, is with the School of Mechanical and Production Engineering, Nanyang Technological University, Singapore 639798. Manuscript submitted August 27, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
by the fibers.[6–13] Fiber bridging is effective in preventing catastrophic failure of the TMC component and enhancing its fatigue life. However, in the case of selectively reinforced composites, it is possible for the multiplicity in matrix cracking to get entirely suppressed, leading to significant reduction in the fatigue life. Since fatigue life is one of the most critical design-limiting factors in components such as fan blades and connecting rods, it is important to understand the fatigue fracture mechanisms in these materials. Lifeprediction methodologies that are capable of incorporating relevant mechanisms as well as the constituent properties that govern such mechanisms should be developed. The present work is focused on developing an understanding of the mechanisms associated with the unnotched fatigue performance of selectively reinforced TMCs and then using such an understanding to develop fatigue failure criteria as a function of the applied st
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