Fatigue and static behavior of curved composite laminates

Taşdemir, Burcu
By virtue of the fact that curved composite laminates which are utilized as load carrying subcomponents in aircraft and wind turbine structures are subjected to cyclic loading during their operating time, it is crucial to understand fatigue failure mechanisms at least as much as static failure mechanisms. With the intention of understanding fatigue failure mechanisms and thus improving fatigue life of structures, these curved laminates are investigated experimentally under static and fatigue loadings. The failure mechanisms observed in the tests are supported by analytical and numerical methods. CFRP laminates with four different ply architectures (UD, cross-ply with two different thicknesses and fabric) are examined. A new test fixture is designed to apply moment/axial combined loading to curved specimens properly. Static and fatigue experiments are conducted using servo-hydraulic testing machine and in-situ photographs are taken. In addition, DIC method is used to obtain strain distribution in the curved region just before the failure. The stress state at the curved region for each specimen configuration is analytically calculated using the multilayer curved beam solution. In the experimental results of UD and fabric laminates, it is observed that both static and fatigue failures initiate at roughly the maximum radial stress location (approximately 35% of the thickness from inner radius). For UD laminates, there is no visible difference between the failure mechanisms under static and fatigue loadings. For fabric laminates, fatigue failure is observed to occur as a single major crack at the maximum radial stress location just as in UD laminates, whereas static failure is observed to occur as multiple diffusive cracks at the maximum radial stress location. In contrast to UD and fabric laminates, different failure locations and mechanisms are observed in the fatigue and static experiments of cross-ply specimens. Fatigue failure is observed to form at the maximum radial stress location whereas the static failure is observed to form in the region where the combined radial, tangential and shear stresses attain a maximum value, in the form of Tsai-Wu failure criterion. For fatigue failure, micro-cracks existing in the maximum radial stress location grow more rapidly under cyclic loading compared to cracks in other regions and coalesce into one major matrix crack which reaches the 0/90 interface gradually and continues to propagate as a delamination. As for the static failure, failure initiates as a matrix crack in the group of 90° layers and jumps to the upper 0/90 interface by an abrupt 40-50° angle and propagates as a delamination. Against the common belief in the literature, according to our observations, we conclude that static and fatigue failure mechanisms are not always the same.
Citation Formats
B. Taşdemir, “Fatigue and static behavior of curved composite laminates,” M.S. - Master of Science, Middle East Technical University, 2018.