Transverse behavior and acoustic-emission analysis of titanium metal-matrix composites under tensile loading
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ING the last decade, extensive research has been carried out on titanium-matrix composites, because of their high strength-to-weight and stiffness-to-weight ratios and good damage tolerance. These properties are particularly important for aerospace applications, where the use of such composites could allow significant weight savings. For unidirectionally reinforced composites, transverse properties are of interest, not in the least because transverse damage can be present in cracked testpieces even when the applied stress is along the longitudinal direction alone.[1] Since fiber reinforcement creates potential sites of weakness in such transverse directions, the introduction of significant transverse stresses can be a problem. Extensive research has been carried out on the transverse response of composites under tensile loading, in order to understand the mechanisms and the effects of transverse properties on the life of the material.[2–16] It has been suggested that a composite undergoes elastic deformation of the composite, interfacial debonding, matrix elastic deformation after interfacial debonding, matrix plastic deformation, and final catastrophic failure under tensile loading. Debonding of fiber/matrix interfaces occurs when the applied stress overcomes the thermal residual stress and any chemical bonding strength. This leads to a deviation from linearity on a stress vs strain curve, i.e., the occurrence of a “knee.” However, detailed in-situ observations have found that the dynamic processes during stressing are complex.[5] Premature debonding may start from very low stresses because of undulations, voids, and debris at the interface. Acoustic emission (AE) techniques have also been used to capture the signal and location of fiber X. WU, Senior 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. H. MORI, Research Fellow, is with the Engineering Materials Division, Railway Technical Research Institute (Japan Railway), Tokyo 185-8540, Japan. Manuscript submitted August 17, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
and matrix failure in composites and monolithic-matrix materials.[2,17–19] Here, a detailed study has been carried out to investigate and to verify the damage mechanisms under transverse loading. Two types of titanium metal-matrix composites (TiMMCs) available commercially have been studied. II. EXPERIMENTAL PROCEDURE The types of MMCs studied here are as follows. (1) A Ti-6Al-4V(wt pct) matrix reinforced with either 8 or 21 pct volume fraction 1140⫹ carbon-coated silicon carbide fibers (diameter of ⬇100 m), produced by DERA (Sigma) (Farnbrough, United Kingdom), which were made by a foil-fiberfoil hot isostatic processing (hipping) route at temperatures in the range between 890 ⬚C and 940 ⬚C for 2 hours, at a pressure of 100 MPa. (2) A Ti-21s (Ti-15Mo-2.7Nb-3Al0.2Si (wt pct)) matrix reinforced with 35 pct SCS-6 silicon carbide fi
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