The fatigue crack propagation behavior of pure magnesium single crystals has been investigated in laboratory air at room temperature. Two types of CT specimens with different notch orientations were prepared from the magnesium single crystal made by the Bridgeman technique. Fatigue crack propagation behaviors of each specimen were different related to notch orientation. In the case of C specimen with (10˜10)[0001] notch, a crack was inclined to (0001) gradually at low DK. To investigate crack propagation behavior along [0001], a tri crystal specimen in which has (10˜10)[0001] notch was prepared. A crack propagated to [0001] in this specimen and the fatigue surface shows striation like pattern. In the case of E specimen with (0001) [10˜10] notch, a crack propagates parallel to basal plane. {10˜12} twin occurred in front the crack at higher DK level. A fatigue surface of the E specimen was changed by {10˜12} twin formation.

Fatigue test of titanium single crystals with different crystallographic orientations has been carried. To investigate fatigue fracture behavior of small single crystals, plain bending fatigue test method for thin sheet specimen was developed. One end of the sheet specimen is fixed at a voice coil of the loudspeaker and the other end is set free. A bending mode resonance occurs in the specimen due to forced vibration at the fixed end. In A-specimen, a notch plane and the direction are (11 20) and [1100], respectively, the crack propagates parallel to {1100} plane and striation-like markings are observed on the fatigue surfaces. In B-specimen with a notch of (1100) [1120], the crack also propagates parallel to {1100} plane as similar to A-specimen. These cracks are deduced to extend by alternating shear on two intersecting prismatic slip systems at the crack tip. In F-specimen with a notch of (0001) [11 20], the crack propagates parallel to (0001) and some trace of pyramidal slip was observed on the fatigue surface. As a result, S-N curves of each specimen showed strong orientation dependence. Fatigue strength of F-specimen is higher than that of A- and B-specimen.

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