Abstract
This study tries to evaluate the more realistic number of equivalent load cycles for extremely-low-cycle fatigue evaluation to earthquake inertial loads in the seismic design of mechanical components. A simple “energy equivalent” method, analogizing balance between the accumulated plastic deformation energy due to input earthquake wave and the total hysteretic energy loaded typically during strain-controlled cyclic load test, was proposed and applied. Several single degree of freedom vibration systems with bi-linear load-deflection function were analyzed for 48 strong earthquake motions observed in Japan. The calculated number of equivalent load cycles were found strongly dependent on the characteristics of wave forms and system parameters such as natural frequencies and elastic limit load, however it was realized that the numbers lie mostly below 20 to 30 cycles. These numbers, as the estimated equivalent load cycles, might be concluded enough for the extremely-low-cycle fatigue evaluation of mechanical components during earthquake. As future investigations, more systematic studies especially on the in-structure earthquake response and cyclic loads from relative displacement such as Anchor-Movement would be valuable and necessary.
