Multiple matrix cracking in a fiber-reinforced titanium matrix composite under high-cycle fatigue
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INTRODUCTION
MANY fiber-reinforced ceramic matrix composites (CMCs) undergo multiple matrix cracking under monotonic tensile loading, f~,2~Cracking is accommodated by debonding and frictional sliding along the fiber/matrix interfaces, allowing the fibers to stay intact. This process is manifested macroscopically in the form of inelastic strain upon loading and the development of permanent strain upon unloading. Such materials generally exhibit relatively high tensile strengths and failure strains. In addition, their tensile strength is insensitive to the presence of holes or notches or other strain-concentrating features.[3,41In contrast, CMCs that fail by the propagation of a single dominant crack exhibit stronger notch sensitivity and lower failure strains. Analogous behavior has been observed in fiber-reinforced metal matrix composites (MMCs) under cyclic loading55m Notably, MMCs that undergo multiple matrix cracking are highly notch insensitive and exhibit high fatigue strength. In contrast, when fatigue is dominated by a single dominant crack, the fatigue life is reduced substantially and is sensitive to the size of the initiating hole or notch.t71 In this context, multiple cracking is a desirable feature. There are additional ramifications of matrix cracking in MMCs. First, cracking leads to a reduction in the elastic modulus of the composite, m] Second, the inelastic strains arising from interface sliding result in both nonlinearity in the stress-strain response, with a reduced secant modulus, and permanent strain following unloading. Third, matrix cracking and interface sliding alter the vibrational and damping characteristics of the composite. These phenomena are expected to be important in stiffness-critical components. One of the objectives of the present article is to examine the effects of multiple matrix cracking on the tensile response of a fiber-reinforced MMC (Ti-15V-3Cr-3Sn3AI/SCS-6), with particular emphasis on changes in the D.P. WALLS, Graduate Student, formerly with the Materials Department, University of California, Santa Barbara, is Technical Specialist, Advanced Life Systems and Methods Group, United Technologies, Pratt and Whitney, West Palm Beach, FL 33410-9600. J.C. McNULTY, Graduate Student, and F.W. ZOK, Associate Professor, are with the Materials Department, University of California, Santa Barbara, CA 93106. Manuscript submitted January 27, 1995. METALLURGICAL AND MATERIALSTRANSACTIONS A
modulus and the development of inelastic strain. The experimental results are compared with the predictions of micromechanical models, based on analyses of unit cell models. The effects of cycling on the retained strength are also examined. In most cases, fatigue failure in Ti alloys is a macroscopically brittle phenomenon. Fatigue cracks can initiate and propagate at stresses below the yield point, with no macroscopic plasticity. This phenomenon is exemplified by the fatigue-life curve [9] for the Ti-15V-3Cr-3Sn-3A1 alloy (Figure 1), wherein failure occurs at strain levels below yield. Matrix f
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