Materials characterization of silicon carbide reinforced titanium (Ti/SCS-6) metal matrix composites: Part II. Theoretic
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I.
INTRODUCTION
THE titanium metal matrix composites (TMC) are attractive materials for hot structural applications because of their excellent stiffness-to-density and strength-to-density ratios. Basic material behaviors and fracture mechanisms for the TMC need to be understood. The mechanistic investigation of fatigue behavior is presented in Part I[~1 of this series. During fatigue testing, four stages of flexural deflection behavior were observed. The deflection at stage I increased very slowly with fatigue cycling. The deflection at stage II increased significantly with fatigue cycling, while that at stage III slowed down. Finally, the specimen ruptured at stage IV. Hence, both mechanistic investigation and theoretical modeling were performed to understand the four stages of deflection behavior, as described in Part I. In theoretical modeling of the fatigue behavior, finite element techniques were used to investigate the stress distribution for each composite ply and in the local area around the fiber. [2-6] The stress information will be developed to verify the fracture scenario, reported in Part I. The theoretical predictions of the fatigue deflection behavior for both longitudinal and transverse specimens were found to be in good agreement with the experimental results. The longitudinal specimen is defined herein as that which has the fibers of the surface plies aligned with the length direc-
K.T. CHIANG and D.H. LOH, Members of Technical Staff, are with the Rocketdyne Division, Rockwell International Corporation, Canoga Park, CA 91309-7922. P.K. LIAW, Professor and Ivan Rancheff Chair of Excellence, is with the Department of Materials Science and Engineering, The University of Tennessee, Knoxville, TN 37996-2200. E.S. DIAZ, Senior Engineer, is with the Westinghouse Science and Technology Center, Pittsburgh, PA 15235. This article is based on a presentation made in the symposium entitled "Creep and Fatigue in Metal Matrix Composites" at the 1994 TMS/ASM Spring meeting, held February 28-March 3, 1994, in San Francisco, California, under the auspices of the Joint TSM/SMD/ASM-MDS Composite Materials Committee. METALLURGICALAND MATERIALSTRANSACTIONS A
tion of the fatigue specimen, as reported in Part I. As for the transverse coupon, the fibers of the surface plies are perpendicular to the length direction of the specimen. It was found that the composite damage originated in the fiber/matrix interface and the matrix at multiple sizes. Cracks in the matrix propagated and joined together, which resuited in fiber bridging. Finally, the fiber breakage caused the final failure of the composite material coupons.
II.
THEORETICAL MODELING
A. General Approach The objective of the theoretical stress analysis of the composite is to provide the stress information to further understand the mechanistic investigation conducted in Part I. The analysis began with the estimation of the composite material properties. The properties for each ply were predicted using the Laminate Analysis with Matrix Plasticity (LAMPLAS) progr
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