3333 Fatigue Crack Growth Behavior in CT Specimens of Nano-crystal Powder Al Alloys
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The present study has investigated crack growth behavior in the nano-crystal powder alloys of GIGAS 20, 30 and 50. Crack propagation tests were made on CT specimens of these alloys at 297K in air under tension-to-tension fatigue loads at three levels of stress ratios, R=0.05 to 0.2. Curved crack propagation was observed in CT specimens of GIGAS 20 and 50, whereas cracks always propagated straight in those of GIGAS 30. Crack closure effects were measured by strain gages mounted near crack tips and on the back surfaces. Crack propagation paths were tracked by advanced CCD camera mounted on a tri-axial X-Y-Z positioning table. The values of the mode I and II stress intensity factors K_I and K_II for curved cracks so observed were calculated by FEM. Crack propagation rates da/dN were plotted against the effective stress intensity factor range ΔK_<eff> and compared for three types of the present nano-crystal alloys tested. The relations between instantaneous crack propagation angle θ and K_II/K_I ratio were also calculated and found to take a sigmoidal curve.
