2007 Fiscal Year Final Research Report Summary
Investigation on Fatigue Strength of Nano/Micromatereials Subjected to Very High Cycle Fatigue
| Project/Area Number |
18560095
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| Research Category |
Grant-in-Aid for Scientific Research (C)
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| Allocation Type | Single-year Grants |
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| Section | 一般 |
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| Research Field |
Materials/Mechanics of materials
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| Research Institution | Kochi National College of Technology |
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Principal Investigator |
CHEN Qiang Kochi National College of Technology, Dept of Mech. Eng., Associate Professor (30264451)
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| Co-Investigator(Kenkyū-buntansha) |
HASHIGUCHI Gen Kagawa University, Faculty of Eng., Professor (70314903)
KAWAGOISHI Norio Kagoshima University, Faculty of Eng., Professor (00117491)
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| Project Period (FY) |
2006 – 2007
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| Keywords | Fatigue / Nano / Micromaterials / Crack Initiation / Very High Cycle / Thin Film |
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| Research Abstract |
The application of silicon-based MEMS/NEMS has already resulted in significant improvements in our lives. Micromechanical components are routinely subjected to cyclic stresses at kilo- and megahertz frequencies, accumulating large numbers of stress cycles. To ensure performance and reliability of these tiny systems, fatigue mechanism must be elucidated to account for the timely and cyclically dependent degradation of the material at the size-scales of interest.
In the present study, fatigue strength and fracture mechanism of LP_CVD polycrystalline silicon thin films were investigated in the domain of 10^7-10^9 cycles by using a novel strain controlled, piezoelectric actuated free standing tensile fatigue system that can installed either within or outside a scanning electron microscope (SEM). Fatigue small crack behavior were observed and taped by CCD video camera. The following results were obtained and the possible mechanism was discussed.
A) The piezoelectric actuated tensile fatigue s
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ystem developed in the present study possesses a position sensor, which was fabricated by MEMS technology and can be used to monitor the movement of specimen center during cyclic loading.
B) The geometry design of the free standing tensile specimen was optimized with the help of commercial FEM software, ANSYS, in terms of the maximum stress available at the center beam of the specimen and the minimum out of plane displacement occurred accordingly.
C) It was fund that LP-CVD polycrystalline silicon thin films fatigued even at room temperature (~20℃) and at atmospheric environment (~26-35% of humidity). However, S-N diagram scattered tremendously, implying that the fatigue strength of the LP-CVD polycrystalline silicon thin films was quite sensitive to the difference in specimen morphology caused by micromachining as well as the deviation of humidity.
D) Specimens failed with little remarkable damage evolution that can be recognized by either an optical or an electron microscope. Furthermore, limited fractographic analysis assumes that fatigue nucleation and early growth are localized, and the other fracture morphologies are most brittle featured. In other words, the most of fatigue life was consumed in nucleating tiny cracks. Less
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