Fatigue crack growth resistance of unidirectional fiber-reinforced titanium metal-matrix composites under transverse loa
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INTRODUCTION
CONTINUOUS fiber-reinforced Ti matrix composites are of particular interest in the development of new generation hypersonic aircraft and advanced gas turbine engines, because of their high strength-to-weight and stiffness-toweight ratios along the longitudinal direction of the fiber Extensive research has been carried out, which has mainly focused on the response of unidirectional fiber-reinforced composites to longitudinal tensile and cyclic stresses. The tolerance of such materials to stressing in the transverse direction is also of some concern, since transverse stresses can be generated in cracked components, even when they are subjected to applied longitudinal loading alone.[1] In addition, a debonded matrix/fiber interface might also serve as a defect and initiate microcracks. Transverse crack growth in unidirectional metal-matrix composites (MMCs) can be essentially of two types: type 1 (denoted D-1 in this article), with the crack plane and crack front parallel to the fiber axis; and type 2 (denoted D-2 in this article), with the crack plane parallel to fibers and with the crack front perpendicular to the fiber axis (Figures 1(a) and (b)). Several investigations have been conducted on the transverse response under tensile loading.[2,3,4] The transverse fatigue crack propagation resistance of unidirectional MMCs of type 2 (D-2) has also been investigated, to a limited extent.[5,6,7] To date, however, transverse fatigue crack growth resistance of type 1 (D-1) has never been reported, partly because plates of 8- or 12ply unidirectional MMCs have a thickness of ,2 mm. Thus, it is extremely difficult experimentally to introduce a notch and propagate fatigue cracks in a stable manner in this D1 orientation. The limited matrix ligament length between two adjacent fibers and interaction between the many debonded fiber matrix interfaces in high volume fraction MMCs also make it very difficult to obtain a stable crack in such composites. In order to overcome these difficulties, the transverse fatigue crack growth resistance in both 8 and X. WU, Research Fellow, IRC in Materials for High Performance Applications, and P. BOWEN, Professor, School of Metallurgy and Materials, are with The University of Birmingham, Edgbaston, Birmingham, B15 2TT, United Kingdom. Manuscript submitted August 23, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
35 pct SM11401/Ti-6Al-4V (wt pct) unidirectional fiberreinforced composites is investigated in the present article by using some individually designed testpieces and experimental techniques.
II. EXPERIMENTAL PROCEDURE The materials studied here are 12-ply 8 pct SM11401/ Ti-6Al-4V (wt pct) and clad 8-ply 35 pct SM11401/Ti-6Al4V (wt pct) composites. Sigma 11401 fibers are a composite 100-mm-diameter silicon carbide fiber carbon coated to a thickness of 4.5 mm. This gives an overall coated fiber diameter of approximately 110 mm. The 8 pct SM11401/ Ti-6-4 plate had a thickness of 4.2 mm, length of 180 mm, and width of 120 mm. It was made by a foil-fiber-foil “hot isothermal
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