Mater. Res. Soc. Symp. Proc. Vol. 1276 © 2010 Materials Research Society Stress Ratio Effect on Fatigue Behavior of Aircraft Aluminum Alloy 2024 T351 M. Benachour 1 , A. Hadjoui 1 , M. Benguediab 2 , N. Benachour 3 1 Automatic Laboratory of Tlemcen, Mechanical Engineering Dpt, University of Tlemcen, BP 230, Tlemcen, 13000, Algeria. 2 Physical Mechanics and Materials Laboratory, Mechanical Engineering Dpt, University of Sidi Bel Abbes, 22000, Algeria 3 Department of Physics, University of Tlemcen, 13000, Algeria. ABSTRACT Aluminum alloy series 2xxx, 6xxx, 7xxxx and 8xxx enjoy the widest use in aircraft structural applications. Among these materials, aluminum alloy 2024 remains the most commonly used and especially in T351 temper situation. The fatigue crack propagation behaviour of aluminum alloy 2024 T351 has been investigated using V-notch specimen in four bending test. A series of stress ratios from 0.10 to 0.50 was investigated in order to observe the influence of stress ratio on the fatigue life and fatigue crack growth rate (FCGR). The increase in FCGR, which occurs as the stress ratio is increased from 0.10 to 0.50, is generally attributed to an extrinsic crack opening effect. In T-S orientation and at low stress intensity factor, the increasing of stress ratio increase the FCG. Experimental results are presented by Paris law when coefficients C and m are affected by stress ratio. Contrary, at high stress intensity factor, the effect of stress ratio is reversed. We notice a decreasing of fatigue crack growth rate with an increasing of stress ratio. This effect may be explained by microstructure effect in (T-S) crack growth. The analysis of stress ratio effect by Elber model, shown that this model gives bad interpolation in this situation and the parameter characterized the crack closure factor will be adjusted. INTRODUCTION The problem of fatigue behavior of materials in mechanical structures, machine parts, etc. is a crucial point in predicting the fatigue life. In general, the fatigue process is depicted by three distinct regions. Region I is associated with the growth of cracks with low K th , and is commonly believed to account for a significant proportion of the fatigue life of a structure. Region II has received the greatest attention as it is in this region where the „„Paris‟‟ crack growth law “Paris” [1] can be applied. Several different variants of the Paris crack growth law have evolved by many researchers [2-4]. Finally, region III is associated with rapid crack growth. Predicting the fatigue crack growth rate at constant, variable or random loading is of practical interest for many aeronautical applications, aerospace, automobile, etc. A major concern of fracture mechanics is the influence of the stress ratio on the behaviour of cracks, which is classically defined as: the ratio of minimum to maximum applied stresses. Many empirical models for fatigue crack growth have been proposed in the literature to account for the stress ratio dependence of FCG curves (Forman model [5]; Walker model [6]). It was argued that the reason for this influence is the crack closure effect, which introduced first by Elber [7].