Methodology for the high-frequency testing of fiber-reinforced plastics

Abstract

Nowadays, composite materials are used in many structural applications. In order to achieve maximum reliability and safety, it is necessary to perform fatigue tests that cover load cases ranging from short and intense lives 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. Conservative testing methods use testing frequencies below 10 Hz to avoid specimen heating or strainrate-related effects in the material. This results in long testing times and, given the number of specimens required, very long test periods. The only way to shorten these testing times is to raise the testing frequency by a significant factor. Positively driven test machines such as standard servohydraulic test rigs, however, have a limited testing frequency. To improve 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 it. Following this approach, a dual-mass oscillator is presented in order to demonstrate the working principle. This principle is then validated against an analytic model of the oscillator. The control circuit, which tracks the resonant frequency of the oscillator and maintains a constant stress amplitude in the specimen, is also discussed, followed by the presentation of the working principle of this nested controller and its ability to measure data over the fatigue life of the specimen. The study is then concluded with the results of the fatigue test.