Experimental investigation of the very high cycle fatigue behaviour of fibre reinforced composites

Kurzfassung

Fibre reinforced composites became more and more important within the last decades. Thereby many different fields of application are covered, such as aircraft components, renewable energy products and automotive lightweight constructions. Parts, built out of composite materials, are designed to last for a long period of time nowadays, like e.g. blades of wind energy plants. Within this time span they suffer from a very high number of load cycles, often in the very high cycle regime past 10^8 cycles. This paper presents a new concept for testing composites until and beyond 10^8 cycles under alternating loads. Therefore a test-stand is being developed that uses an eigenmode of the specimen to apply high loads at the eigenfrequency. The amplitude of the load is controlled by varying the frequency and the level of the excitation. A high testing frequency is used to obtain 10^8 cycles within a reasonable time span. The temperature of the specimen is monitored to keep the autogenous heating of the specimen below a certain threshold to avoid thermal damages. Also the changes of the damped eigenfrequency and the amplification of the excitation are monitored. These states of the specimen are used to show connections between them and the growing defects in the specimen. The test-stand uses tubular specimen to avoid boundary effects and enable inner and outer cooling. The lay-up consists out of unidirectional fibre strands (rovings) that are aligned along the longitudinal direction of the tube. A special tool is being developed to produce these specimens in the process of pultrusion. Together with other partners of the priority programme 1466 started by the German Science Foundation DFG, the results of the tests will be used to build a sustainable basis for further modelling of the observed effects.