EFFECT OF STRESS RATIO ON DELAMINATION GROWTH BEHAVIOR IN UNIDIRECTIONAL CARBON/EPOXY UNDER MODE I FATIGUE LOADING R. Khan ,* , C. D. Rans, R. Benedictus Faculty of Aerospace Engineering, Delft University of Technology, Kluyverweg 1, 2629 HS, Delft Netherlands * R.khan@tudelft.nl SUMMARY Effect of stress ratio on the Mode I delamination growth behaviour of composite structures under fatigue loading has been investigated. Mode I cyclic delamination growth tests were conducted on unidirectional M30SC/DT120 laminates under stress ratios of 0.15, 0.35 and 0.5. The fracture surfaces have shown different topography under different cyclic stress ratios. Key Words: Delamination growth, Stress Ratio, Strain Energy Release Rate, Double cantilever Beam, Mode I, SEM 1. INTRIDUCTION The presence of delaminations and their effect on structural performance is the most common threat to the use of composite materials in primary structural applications. Composite structures may contain delaminations due to bad layup and defects during manufacturing and assembling processes. Delaminations may also be developed in a composite structure due to in service impacts, static overloads and fatigue. Cyclic loading can cause delaminations to grow in size, which results in redistribution of structural loads and can cause more delaminations in other locations of the structure and lead towards structural failure[1]. Understanding delamination growth behavior under cyclic loading is necessary for damage tolerant design and reliability assessment of composite structures. However, due to the complex nature of the problem, it is not an easy task. Several researchers [2-12] have studied delamination growth behavior under cyclic loading. The cyclic stress ratio (R-ratio), which is the ratio of minimum cyclic stress to maximum cyclic stress, is known to influence delamination growth behavior [13-16]. Hojo et al [13]. developed a relation between delamination rate and equivalent Stress Intensity Factor (SIF), which is an empirical function of SIF range and R-ratio. Tanaka and Tanaka [14] incorporated the effects of R-ratio by empirically deriving a relation for the exponent of the Paris power law in terms of R-ratio. Schon [15] related Paris law exponent to R-ratio, Strain Energy Release Rate (SERR) range at threshold, critical SERR, and the corresponding delamination rates. Andersons et. al [16] extended the empirical model of Hojo et al [13] for R-ratio effect,