Design, testing, and analysis of a novel fatigue testing apparatus

A novel fatigue testing apparatus, its design, testing, and dynamical characterization are described. The apparatus, capable of high frequencies, loads the specimen with inertial forces of arbitrary load history. A sensing system is incorporated to allow for the study of the coupling between fatigue crack evolution and system dynamics. A numerical model of the system is described and its parameters are estimated using nonlinear system identification tools. Testing of the system reveals its capabilities, advantages and disadvantages. Descriptions of modifications to the system are given, and the apparatus is evaluated as a successful platform for the study of the interplay between fatigue and system dynamics. Fatigue experiments at different amplitudes are conducted, and the methods of phase space warping (PSW) and smooth orthogonal decomposition (SOD) are used in order to track the slowly drifting parameters within the fast-time system dynamics. The capabilities of the algorithms are shown for randomly and chaotically excited systems. The rainflow counting method is applied in conjunction with the Palmgren-Miner linear damage law to investigate the effects of time history on fatigue accumulation. Testing results show that linear damage laws are not good models for prediction of fatigue failure and time to failure is largely based on time history. These conclusions are motivation for the further study of a predictive fatigue model which does not rely on microscopic a priori information such as crack length or micro-crack distribution