A Rapid Fatigue Test Method on Micro Structures for High-Cycle Fatigue

Abstract

Here, a rapid fatigue test method for micro structure is proposed, where the test sample is excited at its resonant frequency by an external actuator. Due to the scaling effect, a micro cantilever beam has high resonant frequency, so the testing time can be greatly reduced. In addition, no complicated built-in micro actuator is needed; therefore, it is easy to fabricate the test sample. Single-crystal silicon (SCS) beams are fabricated to demonstrate the proposed method here with two types of beams. A notch is designed near the fixed end to have stress concentration effect. The input voltage of the piezoelectric patch is adjusted to excite the cantilever beams at its resonant frequency to have different amplitudes to find the corresponding fatigue life, where a laser Doppler vibrometer is used to monitor displacements of the cantilever beam to control the stress amplitude. Then, the stress–life (S/N) curve of the silicon beam can be obtained. It is found that the fatigue limit of the SCS beam is about 1.33 GPa, and it takes only about 5.2 h to test $\mbox{1.35}\times \mbox{10}^{9}$ cycles at a resonant frequency of 72 kHz. Comparing with previous fatigue test methods with an external probe, testing $\mbox{10}^{9}$ cycles at a frequency of 20 Hz will need 1.58 years, which indicates that the proposed rapid fatigue test method is particularly suitable for high-cycle fatigue test of micro structure.