Role of cladding in the notched tensile properties of a titanium matrix composite
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INTRODUCTION
THE ultimate tensile strength of fiber-reinforced titanium matrix composites (TMCs) is relatively notch sensitive. Typically, for specimens containing circular holes or sharp notches ;3 to 6 mm in size, the strength is reduced by as much as a factor of 2.[1] Such a high degree of notch sensitivity may preclude the design of TMC components that contain holes or other stress concentrating features. The fatigue resistance of TMCs is also notch sensitive. Under cyclic loading, fatigue cracks initiate and grow within the matrix, leaving the fibers intact in the crack wake. There exists a critical peak stress below which the crack growth rate approaches a steady-state value, independent of crack length. Above the critical stress, fiber failure occurs in the crack wake, leading to an acceleration in the crack growth rate and, ultimately, catastrophic fracture. This critical stress represents a threshold below which the fatigue life is infinite. Both experiments[2] and calculations[3,4] indicate that the threshold stress decreases dramatically with increasing notch size. The fatigue notch sensitivity is an additional concern in the design of components containing stress concentrations. One approach to alleviating the notch sensitivity of TMCs is to reinforce components locally in regions surrounding the stress concentrating features. Through judicious selection of the reinforcing material and the geometry, it is anticipated that the local strength and fatigue resistance could be brought up to the levels corresponding to the TMC alone, rendering the material notch insensitive. A simple approach to manufacturing reinforced panel structures involves hot pressing TMC plates between monolithic sheets of the Ti alloy itself and subsequently machining away the excess Ti remote from the stress concentrating features. Figure 1 shows a schematic illustraU. RAMAMURTY, formerly Post-graduate Researcher with the Materials Department, University of California, is Lecturer, School of Mechanical and Production Engineering, Nanyang Technological University, Singapore, 2263. F.W. ZOK Professor, Materials Department, and F.A. LECKIE, Professors, are with the Department of Mechanical and Environmental Engineering, University of California, Santa Barbara, CA 93106. Manuscript submitted October 10, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
tion of this reinforcement scheme for a panel containing a through-thickness hole. Similar schemes have been used in the past in repairing aluminum aircraft components that contain fatigue cracks, using either boron/epoxy or graphite/epoxy composites.[5] The objective of the present study is to assess the utility of Ti cladding in improving the notched properties of TMCs. For this purpose, experiments have been conducted on composite panels with two different clad thicknesses as well as a panel without cladding. The focus of the work is on the ultimate tensile strength and the resistance to fatigue cracking. The un-notched tensile properties of these materials have been reported else
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