Development of Fatigue Pre-Cracking Method into Micro-Sized Specimens for Measuring Fracture Toughness
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Development of Fatigue Pre-Cracking Method into Micro-Sized Specimens for Measuring Fracture Toughness K. Takashima, S. Koyama, K. Nakai and Y. Higo Precision and Intelligence Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama, 226-8503, Japan ABSTRACT In our previous investigations [1, 2], we have demonstrated that the introduction of fatigue pre-crack ahead of a notch is required to measure reliable fracture toughness values even for micro-sized specimens. However, it is rather difficult to introduce a fatigue pre-crack into a micro-sized specimen as once a fatigue crack starts to grow then the fatigue fracture occurs within one thousand cycles and this makes it extremely difficult to control fatigue crack length. Therefore, a new fatigue pre-cracking method is required for measuring fracture toughness. In this investigation, a new fatigue pre-cracking method has been proposed for micro-sized specimens and fracture toughness tests were carried out for the micro-sized specimens with fatigue pre-crack. Micro-cantilever beam type specimens with dimensions of 10 × 10 × 50 µm3 were prepared from an electroless deposited Ni-P amorphous alloy thin film and notches were introduced by focused ion beam machining. Fatigue pre-cracks were introduced ahead of the notches by far-field cyclic compression method using a mechanical testing machine for micro-sized specimens (MFT2000). Fracture tests were also carried out using the testing machine. Fatigue pre-cracks with length of 0.2 µm were confirmed on the fracture surfaces ahead of the notches in the far-field cyclically compressed specimens. This indicates that the fatigue pre-cracking method developed in this investigation is promising for measuring accurate fracture toughness for micro-sized specimens for MEMS applications. INTRODUCTION
MEMS devices are usually prepared from a thin film deposited on a substrate using suitable surface micromachining techniques, and micro-sized elements prepared from a thin film layer are used as mechanical components. Meanwhile, the microstructure of thin films depends on their preparation method and defects may be introduced into the thin films during processing. The microstructure and defects of thin films are considered to affect the mechanical properties of micro-sized elements in MEMS devices. The evaluation of mechanical properties including elastic modulus, tensile strength, fracture toughness and fatigue properties is therefore essential for practical applications of such MEMS devices. In particular, fracture toughness of micro-sized elements in MEMS devices is important to enable reliable design, as even micro-sized flaw may act as a crack initiation in such micro-sized components. For ordinary-sized specimens, fracture toughness has been measured according to the ASTM standard, but there are no such standards for micro-sized specimens, so the measurements of fracture toughness for micro-sized specimens have been carried out by their own method [3-6] and the comparison of fracture toughness has be
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