Keywords: cyclic loading, fatigue, composite, axial loading, laminated beam, finite element method.

The increased use of composite materials has necessitated further research in this field. One of the important areas is the cyclic behaviour of composite materials. The composite materials used in aerospace structures have been subjected to the dynamic loads during their service life. Therefore, it is important to predict their safe fatigue life or fatigue safety factors for a given life, and there is a need for the mathematical models which are experimentally validated and physically meaningful. Prediction of fatigue life for composite materials is a difficult subject and it was investigated by many researchers. Fatigue of composites depends on numerous properties and conditions such as humidity, uniformity of fibres, homogeneity, load type [1,2]. The fatigue experiments can be classified based on the loading type such as axial, bending, torsional. Each of them gives different information on the fatigue behaviour of materials. Beams are often used to characterize the material properties of the laminated composites. In this study, the fatigue behaviour of laminated composite beams subjected to the axial cyclic loading has been investigated experimentally and numerically.

The axial fatigue tests of the laminated beams are performed to obtain the fatigue properties of the laminated composites. The two different types of laminated beams are manufactured. These beams are made of carbon-epoxy and glass-epoxy. The test specimens are manufactured based on the ASTM D3039 (standard test method for tensile properties of polymer matrix composite materials) in a size of 300 mm x 30 mm using a wet hand lay-up technique in a heated vacuum table. The fatigue tests are achieved using a universal test machine. The load is controlled during the fatigue tests and the displacement and load are recorded at the peak points. The displacement amplitude in each test is found to be almost constant as the number of cycles increases. However, the displacement in both minimum and maximum load is increasing as the number of cycles is increased. The numerical fatigue models found in the literature are studied together with the experimental data obtained in this study and the parameters used in these fatigue models are determined. The experimental data is incorporated into a numerical model and the numerical fatigue model is applied on the prediction of the fatigue life of a blade. The fatigue safety factor is calculated for the helicopter blade. The method presented here shows that the SN data could be used for the fatigue evaluation of the composite structures together with the finite element method.

1

F. Huimin, W. Huang, "A Relational Expression Between the Fatigue Life and Stress Level, Temperature, Humidity for Composite Materials", Journal of Reinforced Plastics and Composites , 23, 1245, 2004.

2

H.T. Hahn, R.Y. Kim, "Fatigue Behavior of Composite Laminates", Journal of Composite Materials, 10: 156-180, 1976. doi:10.1177/002199837601000205

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