Effects of Sample Size on the Fracture Behavior in Fe-3%Si Alloy Single Crystals
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0976-EE03-04
Effects of Sample Size on the Fracture Behavior in Fe-3%Si Alloy Single Crystals Kazuki Takashima1, Eiji Taki1, Yuji Kawakami2, and Masaaki Otsu1 1 Materials Science & Engineering, Kumamoto University, 2-39-1, Kurokami, Kumamoto, 8608555, Japan 2 Industrial Technology Center of Saga, 114, Yaemizo, Nabeshimamachi, Saga, 849-0932, Japan
ABSTRACT Fracture tests have been performed for both macro- and micro-sized specimens prepared from an Fe-3%Si alloy single crystal, and the effects of specimen size on fracture behavior have been investigated. Micro-sized cantilever beam specimens with dimensions of 10 x 10 x 50 µm3 were prepared by focused ion beam machining. Notches with a depth of 5 µm were introduced into the micro-sized specimens and fatigue pre-crack was also introduced ahead of the notch. Macro-sized three-point bending specimens with dimensions of 2 x 2 x 10 mm3 were also cut from the same Fe-3%Si alloy single crystal. Notches with a depth of 1mm were introduced into the macro-sized specimens, and fatigue pre-crack was also introduced. The notch plane was set to be (100), which is a cleavage plane of this material, and the notch direction was set to be [010] for both size of specimens. For macro-sized specimens, cleavage fracture occurred during introducing fatigue pre-crack. In contrast, the micro-sized specimens were fractured by ductile manner. A plastic zone was clearly observed on the specimen surface near the crack tip and dimples were found on the fracture surface. The plastic zone size of this material is calculated to be 90 µm. This size is small enough to satisfy small scale yielding for macro-sized specimens, although this size corresponds to large scale yielding in micro-sized specimens. This may cause the size effect on the fracture behavior of this material. INTRODUCTION MEMS technology in the fields of information technology, automotive systems and biomedical applications has been developed rapidly, evolving to the stage of industrial manufacturing and commercialization. In MEMS structures, free standing micromechanical components are involved to construct moving parts including micro-beams and membranes. These micro components are prepared from thin films deposited on a substrate using suitable micromachining processes. In order to design reliable MEMS devices, it is required to know the mechanical properties of free standing thin films on the micrometer scale [1-2]. In particular, it is essential to know the fracture properties of thin films, as even micro/nano sized defects in the components provide stress concentration [3-5]. There are several length scale effects on the mechanical properties of such micro-sized elements [2]. These effects, however, are not yet fully understood. In quasi-brittle materials with crack or notch, the transition from brittle to ductile fracture has been observed when the specimen size decreases. This transition has been considered to occur when the ligament size of crack is shorter than a characteristic length (lch) [6-8]. However,
this phenomeno
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