Development of Fatigue Testing Machine for Thin Sheet Specimen and Fatigue Test for Magnesium Single Crystal

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A fatigue testing machine for thin sheet specimens was developed to clarify the crystallographic orientation dependence on fatigue fracture behavior in magnesium single crystals. The size of the thin sheet specimen was 3mm in width and 0.3mm in thickness. One end of the thin sheet specimen was fixed at a voice coil of a loudspeaker and the other end was set free. A bending mode resonance occurs in the specimen due to forced vibration at the fixed end. To estimate stress amplitude, the displacement of the free end of the specimen was measured with a laser displacement meter. The stress amplitude was evaluated as the bending stress in the single cantilever. Fatigue tests were carried out with stress ratio R = –1 at room temperature in laboratory air and the frequency of the cyclic stress was 981Hz. In A-specimen whose surface plane and loading direction were (0001) and [1120], a fatigue crack propagated along [1100] at higher stress level while a crack propagated along [1210] at lower stress level. In B-specimen whose surface and loading direction were (1120) and [1100], a crack propagated partially along {1012} twin interface. As a result, fatigue life of the B-specimen was longer than that of the A-specimen. The fatigue limits of the A- and B-specimens were nearly the same, and it was estimated as 45MPa at 106∼108 cycles. The fatigue testing machine developed in this study showed the crystallographic orientation dependence on fatigue fracture behavior in magnesium single crystals.