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

In this study, the bending fatigue behaviour of laminated composite beams has been investigated numerically and experimentally. There are already some studies on the bending fatigue behavior of laminated beams [1,2,3]. The long-term usage of composite structures is simulated by performing experimental fatigue tests. All of the specimens are manufactured in the laboratory by using the wet hand lay-up technique combined with vacuum and heat treatment. Thousands of load cycles are applied to the composite beam specimens on a universal testing machine.

The beams are made of carbon/epoxy (specimen 2 and 3), glass/epoxy (specimen 4) and carbon/aramid/epoxy (specimen 1) composite laminates. A heated vacuum table is used for the production of composite beams. The four different specimens are produced by using different types of fibers, fiber orientation angles, and curing procedures to investigate the effect of these parameters on the cyclic behaviour of composite materials. The bending fatigue tests are performed by using the universal testing machine. The constant amplitude cyclic displacement is applied during the fatigue tests.

The bending test of the laminated beam is modeled using the finite element method. One, two and three dimensional models of the laminated beam are constructed using beam, shell and solid elements respectively. A Fortran code is written for the one dimensional model. The ANSYS finite element software is used for the two and three dimensional models. The displacements, strains and stresses obtained using three different models are compared with the theoretical results obtained using the laminated beam theory. The strains obtained theoretically and numerically are also compared to the measured strains. The static analysis results are compared to the experimental results for the validation of the model. The fatigue safety factors are calculated using the finite element method.

The results can be summarized as follows. The strains are obtained when the free end is displaced 10 mm by using analytical, experimental and numerical methods. Analytical and numerical results are in an agreement. The measured strains are almost 20% lower than the analytically or numerically obtained strains. One reason for that is the clamped boundary conditions are not exactly satisfied in the experiments. The other reason is that the material properties used in the analysis are slightly different from the real material properties. Therefore, it can be said that the results are in an agreement.

1

W. Van Paepegem, J. Degrieck, "Fatigue damage modelling of fibre-reinforced composite materials: review", Applied Mechanics Reviews, 54(4), 279-300, 2001. doi:10.1115/1.1381395

2

Y. Tomita, K. Morioka, M. Iwasa, "Bending fatigue of long carbon fiber-reinforced epoxy composites", Materials Science and Engineering, A319-321, 679-682, 2001. doi:10.1016/S0921-5093(01)01017-6

3

R. Sakin, I. Ay, R. Yaman, "An investigation of bending fatigue behaviour for glass-fiber reinforced polyester composite materials", Materials and Design, 29, 212-217, 2008. doi:10.1016/j.matdes.2006.11.006

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