Fatigue crack growth tests have been performed for micro-sized Ni-P amorphous alloy specimens to investigate the size effects on fatigue crack growth behavior of such micro-sized specimens. Two types of cantilever beam type micro-sized specimens with different breadth (B10 × W12 × L50 μm3 and B30 × W 12 × L 50 μm3) were prepared from an electroless plated Ni-P amorphous alloy thin film by focused ion beam machining. Notches with a depth of 3 μm were introduced in the specimens. Fatigue crack growth tests were performed using a newly developed fatigue testing machine for micro-sized specimens in air at room temperature under constant load range and stress ratios of 0.1, 0.3 and 0.5. Striations were observed on the fatigue fracture surfaces and fatigue crack propagation rates were estimated by a careful measurement of the striation spacings. The fatigue crack growth rates at stress ratios of 0.3 and 0.5 were higher than that at 0.1. This suggests that crack closure may occur even in such micro-sized specimens. The fatigue crack growth resistance is also dependent on the specimen breadth. Shear lips which correspond to plane stress dominated region and a flat fatigue surface which corresponds to plane strain region are clearly observed on the fatigue surfaces, and the size of the plane strain region is different between the specimens with different breadth. This difference in stress state ahead of the crack may affect the crack growth behavior. The results obtained in this investigation are the first measurements of fatigue crack growth properties for micro-sized specimens and provide important information on reliability of actual micro systems and microelectromecahinical devices.
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