18 TH INTERNATIONAL CONFERENCE ON COMPOSITE MATERIALS 1 Introduction Triaxially-woven fabric (TWF) composite is a distinctive type of composite system made up of three sets of inter-woven tows placed in three equally distributed in-plane directions combined with the matrix resin impregnated in the tows. TWF composite has a number of advantageous features such as low apparent density, quasi-isotropy in stiffness, and high flexibility, all of which come from its unique contexture. In addition, low thermal sensitivity of the TWF composite comprising of carbon fibers is of great advantage especially for space structures. Research works have been intensively conducted to investigate stiffness characteristics of TWF composite analytically and experimentally[1-6]. In these studies, the fundamental mechanical behaviors have been predicted through numerical simulations either with finite element model or beam network model, and experiments have shown that the analytical solutions fit sufficiently well with experimental results. TWF composite has been utilized in several components of space structures such as parabolic antennas[7-10]. Mars-orbiting aeronomy probe “Nozomi[7] (Planet-B)” was the first Japanese satellite that made use of TWF composite for its antenna. Asteroid sample return spacecraft “Hayabusa[8] (Musec-C)” also equipped with an antenna made of TWF composite. “SPINAR (Space Inflation Actuated Rod)” [9] was Japan‟s first space verification experiment of a space-inflatable assisted extendible structure which made a full use of TWF composite as its extendible rod. Over the past decades, many researchers have studied the design and construction methods of large space structures, and among them inflatable membrane structure has been considered to be one of the most advantageous options. The group including part of the authors has been preparing to conduct a series of experiments to verify the feasibility of inflatable structural components which may be essential for part of the future space programs. The project named “Space Inflatable Membrane structure Pioneering Long-term Experiments[11] (SIMPLE)” is currently underway for scheduled launch to the International Space Station in 2012. As one of the advantages of TWF composite is high flexibility, the project team focuses on the favorable features of this composite when used as the major load-carrying material of inflatable structures. The TWF composite is utilized in the inflatable extensible mast equipment to demonstrate its high adaptability to deployable or inflatable structures. The material system will also be exposed to the orbital environment and the possible degradation will be monitored. Taking into account that the inflatable structures may be set up in orbit for extended period of time, fatigue effects of TWF composite under mechanical or thermal loadings may not be dismissed and may even be critical to the structural design. Most of the existing studies focus on its stiffness characteristics and let alone its fracture mechanisms. In this study, fatigue life characteristic, along with damage propagation and accumulation mechanisms of TWF composite are experimentally investigated. To predict fatigue life features, fatigue experiments are conducted to acquire S-N curves. Damage propagation and accumulation mechanisms are also looked into, with X-ray CT scans focusing on the intersection of the tows. The long-term viability of the TWF composite under mechanical and thermal environment is thus verified, and the possibility of the immediate prediction of the final failure under cyclic loading is also discussed. DURABILITY OF TRI-AXIALLY WOVEN FABRIC COMPOSITES FOR SPACE APPLICATIONS T. Aoki 1 *, Y. Kosugi 1 , A. Watanabe 2 , T. Murakami 3 1 Department of Aeronautics and Astronautics, University of Tokyo, Tokyo, Japan, 2 Sakase Adtech Co. Ltd., Maruoka, Japan 3 Industrial Technology Center of Fukui Prefecture, Fukui, Japan * Corresponding author (aoki@aastr.t.u-tokyo.ac.jp) Keywords: Tensile fatigue, Cyclic loading, Failure mechanism, Debonding, Creep, CT scan.