Degradation of residual strength in SCS-6/Ti-15-3 Due to fully reversed fatigue
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INTRODUCTION
There is a continuing demand for advanced materials with properties superior to those currently in use. Titanium matrix composites (TMCs) fall into this category because they have excellent strength- and stiffness-to-weight ratios that are maintained at elevated temperatures. The high-temperature capabilities of TMCs do not approach those of other composite systems such as ceramics, but they are much more ductile, which leads to improved toughness. Because of their unique combination of properties, TMCs are currently considered for programs like the Integrated High-Performance Turbine Engine Technology (IHPTET) initiative. To facilitate the incorporation of these materials into real-world applications, such as turbine rings, combustion chamber casings, actuator rods, etc., numerous studies have been undertaken to characterize their behavior under different types of fatigue loading. These studies researched various TMCs under load and temperature variations that ranged from simple constant-amplitude fatigue[1] to very complex hypersonic flight profiles.[2] There have also been studies dealing with crack growth,[3,4] micromechanical properties,[5,6] and the effect of vacuum exposure,[7] to name a few. Although a large amount of work has been performed on several TMC systems, there has been very little attention devoted to residual strength degradation[8,9] (defined as the remaining tensile strength after exposure to cyclic loading). To address this concern, the present study was undertaken to characterize the residual strength behavior of silicon carbide fiber–reinforced TMCs. The main goal of this study was to determine the effects of strain-controlled, fully reversed, elevated-temperature faJ.R. CALCATERRA, Aerospace Engineer, AFRL/VAUS, S. MALL, Professor and Head, Department of Aeronautics and Astronautics, AFIT/ENY, and S.C. COGHLAN, Mechanical Engineer, Selectech Services, are at Wright-Patterson AFB, OH 45433-7402. This article is based on a presentation made in the symposium ‘‘Fatigue and Creep of Composite Materials’’ presented at the TMS Fall Meeting in Indianapolis, Indiana, September 14–18, 1997, under the auspices of the TMS/ASM Composite Materials Committee. METALLURGICAL AND MATERIALS TRANSACTIONS A
tigue on the degradation of residual strength for TMCs with different fiber volume fractions (Vf). Residual strength degradation is a very important for two main reasons. First, if a component is developed using a fatigue-based methodology, the designer must know what type of overload the material can withstand at any point in its service life. Second, residual strength degradation serves as an indicator of damage accumulation in the composite. Close examination of the data from these tests allows the researcher to determine the sequential progression of damage and may highlight the importance of damage, deformation, or precipitation effects not seen in full-term fatigue. The completion of this study hinged on two secondary objectives. These were the development of a fatigue life (S-N or ε-
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