| Budget Amount *help |
¥3,300,000 (Direct Cost: ¥3,300,000)
Fiscal Year 2004: ¥1,500,000 (Direct Cost: ¥1,500,000)
Fiscal Year 2003: ¥1,800,000 (Direct Cost: ¥1,800,000)
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| Research Abstract |
In recent years, microelectromechanical systems (MEMS or micro-machines) have found increasing applications in automobiles, aeronautic, man-made satellites as well as biomedical industries, where the mechanical reliability of machines or components has to be secured during a typical life span of〜10 years. However, information regarding the strength reliability as well as the fracture mechanism of MEMS materials is extremely important. On the other hand, when the dimensions of a material are reduced down to micron/nanometer scale, the material usually demonstrates stronger physical and mechanical properties than the same bulk material due to the decrease of defects or porosities as well as the accelerated emission of dislocations. Meanwhile, MEMS materials are more environmentally affected because of the increasing exposure of surface area to the surrounding environment. It was reported that nearly 90 % out of the total mechanical failures were caused by or related with fatigue. Obvious
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ly, MEMS devices cannot stand longer than a few thousands of cycles if the device is fatigued and the initiated cracks propagate at a speed fester than 〜10^-9 mm/cycle, which is defined as file threshold growth rate for non-propagating cracks for bulk materials. Therefore, it is critically important to investigate into the growth behavior of a fatigue crack in its very slow extension range in order to secure the safety of MEMS devices. This is, however, not realistic by utilizing traditional fatigue machines because fatigue tests at very slow growth region are tremendously time consumed and expensive.
In the present study, a novel ultrasonic fatigue methodology was employed to investigate fatigue behavior of small cracks in the region of nanometer order propagation, especially the exist of a threshold crack growth rate in typical alloys that have been using in MEMS devices. The effects of strain rate as well as the influences of environmental factors such as moisture and elevated temperature were examined in terms of fatigue strength, crack initiation and propagation, and fracture mechanism. Less
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