Self-Induced and Machine-Induced Specimen Vibrations in Fretting Testing

Vibrations are often observed overlapping unidirectional sliding in wear testing. The vibrations are either an effect of sliding at the contact interface (self-induced), or induced as a secondary effect by unavoidable vibrations in the load transfer system of the testing machine (system-induced). In the present work the effects of overlapping vibrations have been studied in an actuator for tangential vibrations. Longitudinal vibrations are generated by a giant-magnetostrictive rod, and fed, via a mechanical load transfer system, to a vibrating specimen, clamped against a stationary specimen. The actuator was designed to minimize system-induced vibrations. In particular, possible machine resonance conditions were analyzed, with the purpose of choosing unavoidable resonance frequencies as far from the applied vibrational frequencies as possible. A number of mechanical models were set up, analyzed with respect to spring and damping constants and vibrating masses, and adapted to the design of the different parts of the machine. Spring constants were controlled by the size and elastic modulus of relevant machine elements. An evaluation of the vibrator shows that system-induced vibrations can be effectively controlled within large frequency intervals. It will be demonstrated how the device makes it possible to distinguish between self-induced and machine-induced vibrations of the “stationary” specimen. A fiber optic specimen displacement transducer system is used to measure the relative specimen displacements, which are found to be considerably smaller than the displacement of the vibrating specimen, and can be measured with an accuracy of 2 nm. Tangential and normal forces are measured and recorded as functions of time, compared as to phase and amplitude. A “vibrational friction coefficient” is discussed.