Transverse creep of SiC/Ti-6Al-4V fiber-reinforced metal matrix composites
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INTRODUCTION
Fiber-reinforced metal matrix composites (MMCs), such as SiC/Ti-alloy and B/Al-alloy systems, offer significant advantages over conventional alloys when loaded in the fiber direction. However, the transverse (90 deg) performance is generally poor as a result of either fiber-matrix separation or fiber splitting at relatively low stresses.[1–6] Debonding is the primary mechanism of interface failure in titanium matrix composites (TMCs), that contain fibers coated with C or graded C/SiC.[5,7,8] The coatings produce damage tolerance under longitudinal loads in these systems by providing a preferential path for deflection and subsequent fiber bridging of cracks that may form. However, the interface is the weakest constituent (relative to the matrix and the fiber) and controls the response of the composite under transverse loading. The principal application that has motivated research and development of TMCs is a hoop-reinforced compressor ring rotor for gas turbine engines. The transverse stresses in this component, while small, are of sufficient magnitude to require careful understanding and characterization of the transverse tensile and creep response of TMCs. Early efforts to study the transverse response of TMCs have used straight-sided samples, where the fibers intersect the cut edge of the sample.[2,5–11] Results from these studies showed that the minimum creep rate of the TMC was nearly always higher than that of the unreinforced matrix, or that D.B. MIRACLE, Senior Scientist, is with the AF Research Laboratory, Materials and Manufacturing Directorate, Dayton, OH 45433. B.S. MAJUMDAR, Senior Scientist, is with UES, Inc., Dayton, OH 45432. 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
the time to failure was shorter for the TMC. The models used to represent transverse creep have generally assumed that the interface has no intrinsic strength, and separation occurs as soon as the local mechanical stress is sufficient to overcome the residual thermal stresses. These models typically provide a reasonable representation of the data, and so it has generally been accepted that the poor transverse creep response of TMCs is a result of the absence of a chemical bond at the interfacial region. It has recently been established that a tensile stress singularity results at the fiber/matrix interface from thermal loading (i.e., residual stresses) in samples where the fiber intersects a free surface (Figure 1(a)).[12,13] The local interface stresses near the edge are sufficiently high to cause premature fiber/matrix separation, but since the radial residual stress is compressive along the fiber/matrix interface away from the edge, any damage that is formed is not likely to extend much beyond about one fiber radius. However, a tensile singularity is also produced at e
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