The authors describe the design and theoretical performance of a single crystal silicon micromechanical device developed to evaluate the static and dynamic fatigue properties of micromechanical devices. The structure is a cantilever with a large end plate and gold mass. Torquing and sensing electrodes extend over the plate, and with associated electronics, drive the structure at resonance. Fatigue crack propagation is measured by detecting shifts in the natural frequency caused by the extension of a crack from an initiation site near the base. Experimental data are presented demonstrating the operation of the device. >