Investigation of the fatigue behavior of honeycomb sandwich composites after low velocity impact damage

Sandwich composites are often used in areas like transportation where high mechanical and high-energy damping properties are desired in combination with low weight. Honeycomb composites are made by placing a light core material between two plates, and they are especially preferred in aerospace applications. As a result of a literature review, it has been found that static and fatigue tests on honeycomb sandwich composites have been carried out by many researchers, but no study has investigated the fatigue behaviors of the damaged composites and the changes in fatigue values after repair. Especially in the aerospace sector, it is crucial to determine whether a damaged composite needs to be repaired. Considering that the main cause of damage in airplanes, where sandwich composites are frequently used, is fatigue, the proposed study will hopefully eliminate this significant lack of data. In the study, honeycomb sandwich composites were produced by varying the type and thickness of the face-sheet material, and the honeycomb cell size and thickness. Their mechanical properties were investigated using three-point bending, compression, low-velocity impact and fatigue tests. The study determined the decreases in the static strengths and changes in the fatigue lives of honeycomb sandwich composites after impact loading, as well as their static and fatigue strength recovery amounts after the damaged region was repaired. To investigate the fatigue behaviors of specimens, which is responsible for the originality of this study, preliminary damage was created using a spherical component of 12.7 mm in diameter. Low-velocity impact loading tests were performed using CEAST-Fractovis Plus impact test equipment from the mechanical engineering department laboratory of Dokuz Eylül University. After low-velocity impact loading tests were carried out using two different impact energies: 5 J and 10 J, the fatigue behaviors of specimens were determined with compression and three-point bending loads. In addition to the experimental determination of fatigue behaviors of the specimens, numerical calculations were done using the same boundary conditions. The numerical work was performed using an ANSYS Workbench finite elements modeling program. The results of the study showed that increasing honeycomb cell thickness decreased the compressive strength but increased the flexural strength, while increasing honeycomb cell size decreased strength values for both loading cases. Increasing impact energy increased both the contact force and the absorbed energy. However, when honeycomb cell thickness was increased, no effect was observed due to the increased absorbed energy amount and contact force. The loading that was least affected by preliminary impact damage was the compression loading. Repairing the damaged specimens increased the fatigue lives for all specimen types, but the value achieved for the undamaged specimen could not be obtained by any of them. The results of finite element analysis showed consistency with the experimental results. The greatest difference between the experimental and numerical results was observed in the specimens with a 20 mm cell thickness.