Investigating torsional fatigue with a novel resonant testing fixture

In this paper the development of a prototype of a machine for torsional fatigue testing is presented. The prototype was built for testing with constant or variable amplitude loading. The operation principle of this equipment is based on the Frequency Response Function (FRF) for a system subjected to harmonic excitation. The tests are conducted with the driving frequency below the first natural frequency of the system which makes it easier to detect changes in the FRF and possibly reduces the torsional rigidity of the specimens associated with nucleation and growth of crack. The mathematical model for this machine was obtained by using the differential equations for a viscously damped single-degree-of-freedom system and the estimated equivalent physical parameters for this model are shown step by step. The experimental results are obtained with constant and variable amplitude loading. These results show the feasibility of using this type of equipment for torsional fatigue testing and possibly identifying the moment when there is significant change in the torsional rigidity of the specimen.