Fatigue of LIGA nickel

Abstract

Several applications of metallic MEMS devices require that the component can endure cyclic stresses. Mechanisms for fatigue failure may be altered at length scales where the size of the component becomes comparable with the microstructure. For this reason, it is necessary to characterize the fatigue performance of MEMS-scale structures and understand the role of microstructure on potential failure modes. A new specimen configuration has been designed which allows for simple gripping and actuation using a fixedfree beam in bending. The cross-section of the beam is tapered to create a finite width gage section of constant maximum stress, as can be derived from elastic beam theory. This method has been applied to characterize the fatigue behavior of LIGA Nickel with a nominal cross-section of 26x260 microns, replicating the dimensions of a potential accelerometer device. The common stress-life approach was used to characterize the number of cycles to failure for a range of applied cyclic stresses. We found that the stress-life curve was similar to what has been observed for conventional Nickel. The endurance limit (defined in this study as the stress required to cause failure in ~10M cycles, below which the device has practically infinite life) was found to be 35-40% of the ultimate tensile strength. The surface condition of specimens at various stages in the fatigue life, characterized by scanning electron microscopy, revealed that failure initiated as microcracks within localized persistent slip bands (PSBs).