A resonant testing method for high frequency testing of composites in the VHCF regime

Composite materials are being used in many structural applications nowadays. To achieve maximum reliability and safety, it is necessary to perform fatigue tests that cover load cases from short intense lives up to long lasting lives with low stresses. The latter case leads to a high number of load cycles up to the very-high-cycle-fatigue regime beyond 10^7 loading cycles. Conservative testing methods use testing frequencies below 10 Hz to avoid specimen heating or strain-rate related effects in the material. This results in long testing times, and together with the needed number of specimens, very long testing campaigns. The only option to shorten this testing time is to raise the testing frequency by a significant factor. However, positively driven test machines, like standard servo-hydraulic test rigs, have a limited testing frequency. To avoid this, the testing approach must be shifted from the positively driven concept towards a resonantly driven concept. The specimen then becomes a part of the load-generating system and the entire test rig uses its own mass inertia rather than suppressing them. Following this approach, a two-mass-oscillator is presented to show the working principle. This principle is then validated against an analytic model of the oscillator. The control circuit is presented, that tracks the resonant frequency of the oscillator and maintains a constant stress amplitude in the specimen. The working principle of this nested controller is presented along with the ability to measure data over the fatigue-life of the specimen. Finally results of the fatigue test are shown