Abstract
Fracture and fatigue tests have been performed on micro-sized specimens for MEMS/MST applications. Cantilever beam type specimens (10 x 12 x 50μm^3) with notches were prepared from a Ni-P amorphous thin film by focused ion beam machining. Fatigue crack growth and fracture toughness tests were carried out in air at room temperature using a mechanical testing machine for micro-sized specimens. Fracture tests were performed for the specimens with fatigue pre-cracks ahead of the notches. Fatigue crack growth resistance curves were obtained from the measurement of striation spacing on the fatigue surface and closure effects were observed even for micro-sized specimens. Once fatigue crack growth occurs, the specimens were failed within one thousand cycles. This indicates that the fatigue life of micro-sized specimens is mainly dominated by a crack initiation. This also suggests that even micro-sized surface flaw may be an initiation site of fatigue crack and this will shorten the fatigue life of micro-sized specimens. As the results of fracture toughness tests, plane strain fracture toughness, K_<IC>, values were not obtained since the criteria of plane strain were not satisfied for this specimen size. As the plane strain requirements are determined by stress intensity, K, and yield stress of the material, it is rather difficult for micro-sized specimens to satisfy these requirements. Plane stress and plane strain dominated regions were clearly observed on the fracture surfaces and their sizes were consistent with those estimated by fracture mechanics calculations. This indicates that fracture mechanics is still valid for such micro-sized specimens. It is required to consider the results obtained in this investigation when designing actual MEMS/MST devices.
