Fatigue crack growth in Ti-matrix composites with spatially varied interfaces
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I.
INTRODUCTION
SPATIALLY varied interfaces (SVI) is a design concept for composite materials, which potentially enables control of the interface for obtaining a wide range of mechanical properties. Optimization of the mechanical behavior of titanium matrix composites (TMCs) involves trade-offs between longitudinal fatigue life and transverse tensile strength, both of which are strongly influenced by the properties of the interface.[2,3,4] Fiber crack bridging, which enhances fatigue life, is impeded by too strong an interface; too weak an interface causes debonding at low stresses in transverse tension, precluding any further strength and stiffness contribution by the fiber. A brief description of SVIs is given here (a more complete description can be found in Reference 1). In its most abstract sense, spatially varied interfaces are those whose mechanical properties are varied by design along the length and circumference of the reinforcement/matrix interface. A simple example of an SVI is a pattern of circumferential bands whose interface properties alternate between ‘‘weak’’ and ‘‘strong’’ (Figure 1). To increase the overall strength of the interface, the area fraction of strongly bonded interface would be increased; to decrease it, the area fraction of weakly bonded interface would be increased. In theory, an entire range of interface strengths between fully weak and fully strong can be obtained by changing these proportions. A positive correlation of overall interface strength with strong area fraction was demonstrated experimentally using single fiber transverse tension experiments.[1] In addition to controlling the off-axis response, other mechanical properties such as toughness, creep resistance, crack growth re[1]
BENJI MARUYAMA, Materials Research Engineer, is with the Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright Patterson AFB, OH 45387. SUNIL G. WARRIER, formerly Senior Scientist with UES, Inc., Dayton, OH 45432, is Senior Scientist with the United Technologies Research Center, East Hartford, CT. 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
sistance, and ductility, all of which depend on the properties of the interfaces, can be systematically varied. An advantage of SVIs over uniform interfaces is their ability to be systematically modified using just two types of interface (e.g., weak and strong), while a uniform interface essentially has a fixed set of mechanical properties. In this article, the fatigue crack growth (FCG) behavior of composites with SVIs is investigated. The objectives are to probe and understand interface failure mechanisms and to explore the possibility of interface optimization using SVI. Issues of crack tip shielding through both modulus mismatch effects (elastic shielding) and bridging will be di
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