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Two different sets of fatigue specimens were heat treated at different times or temperatures to investigate the effect of grain size on the low-cycle fatigue behavior of Al-2024 polycrystalline alloy. Straincontrolled low-cycle fatigue testing with a strain rate of 1 ϫ 10 Ϫ4 s Ϫ1 was conducted at room temperature. The fatigue response of the alloy was evaluated macroscopically in terms of cyclic stress strain response and microscopically in terms of appearance of cyclic slip bands. The cyclic stress strain response of Al-2024 polycrystalline alloy exhibited a definite plateau region where saturation stress remained constant with plastic strain. It was found that the smaller the grain size, the lower the saturation stress and the longer the plateau, whereas the larger the grain size, the higher the saturation stress and shorter the plateau (i.e., reverse grain size effect). Microscopic observations using scanning electron microscope revealed that persistent slip bands (PSBs) were observed at 45 deg orientations from the grain boundary. The volume fraction of PSBs was higher in small-grained Al-2024 polycrystalline alloy as compared to large-grained Al-2024 polycrystalline alloy.

The permanent research of reliability and precision for failure criteria is a major concern for proper dimensioning of composite structures. Discrepancies are observed between theoretical results based on laminated theory or finite element analysis [Greif, R. and Chapon, E. (1993). Investigation of Successive Failure Modes in Graphite Epoxy Laminated Composites Beams, Journal of Reinforced Plastics and Composites, 29; Echaabi, J., Trochu, F., Pham, F. X. T. and Ouelette, M. (1996). Theroretical and Experimental Investigation of Failure and Damage Progression of Quasi-Isotropic Graphite Epoxy Composites in Flexural Bending Test, Journal of Reinforced Plastics and Composites, 15(7)] and experiments. In order to investigate these problems, numerical simulations were carried out to study the behavior of graphite-epoxy [[+45/-45/90/0]3] laminates subjected to three point bending tests. Two approaches were followed. The first one is based on laminate theory with transverse shear. The successive failures, failure modes, progression of failure and the effect of the geometrical characteristics of the specimens were analyzed during loading. Various failure criteria have been studied to predict the load when the weakest ply fails under flexural bending. Traditional failure criteria and new ones taking into account physical considerations were used. These criteria are tested with two models of stiffness reduction to account for ply failure. The second approach is based on a finite element analysis of laminate, in which three-dimensional failure criteria were implemented. Finally, the results obtained with these two approaches are compared with theoretical and experimental predictions of the scientific literature [Echaabi, J., Trochu, F., Pham, F. X. T. and Ouelette, M. (1996). Theroretical and Experimental Investigation of Failure and Damage Progression of Quasi-Isotropic Graphite Epoxy Composites in Flexural Bending Test, Journal of Reinforced Plastics and Composites, 15(7)].