# The Flexural Fatigue Behavior of Honeycomb Sandwich Composites Following Low Velocity Impacts
## 3 Results and Discussion
### Fatigue Tests  
Impact failure may happen any time during the lifetime of the structures. Impact failure can be observed aviation structures such as airplanes due to bird strikes or collision of ground vehicles. The main aim of this paper is to investigate the effect of impact damage on the fatigue life of honeycomb sandwich structures. The fact that impact damage was not examined for the later stages of fatigue damage constitutes the missing aspect of this study. Sandwich structures suffers not only flexural loads but also compression or membrane stretch loads. Figure 7a shows the fatigue graphs of specimen 6 AL10a with a 0.5 mm-thick aluminum face sheet obtained under three-point bending loads. It was seen that impact testing decreased the fatigue strength of specimens for all loading ratios. Reductions in fatigue strength increased as the applied loading ratio (r = F\({}_{\text{fatigue}}\)/F\({}_{\text{ultimate}}\)) increased. The greatest fatigue life decrease (59%) was determined at r = 0.7. Figure 7b shows the loading ratio vs. cycles to failure graphs for 6 AL10b specimens obtained from fatigue tests. Decreasing the loading ratio decreased the impact effect. For a loading ratio of 0.9, the average life of undamaged specimens was 1767 cycles, while the average life of the specimen damaged by 10 Joules of impact energy decreased by 69%, or 556 cycles. When the loading ratio was 0.5, the reduction was 49%. Figure 7c shows graphs with cycles to failure corresponding to different loading ratios for the 6 AL10c specimen. For all loading ratios, the fatigue lives of damaged specimens were lower. Increasing the fatigue load increased the difference between undamaged and damaged specimens. Decreasing the applied load decreased the strength reduction in damaged specimens. Figure 7: Loading ratio vs. cycles to failure graphs for (**a**) 6 AL10a, (**b**) 6 AL10b and (**c**) 6 AL10c specimens. Figure 8a shows cycles to failure values corresponding to different loading ratios for the 6 AL20b specimen. The specimen affected most by preliminary damage was 6 AL20b. For r = 0.7, the preliminary damage produced by 5 Joules and 10 Joules of impact energy decreased the specimen's life by 85% and 97.5% compared to that of the undamaged specimen. Decreasing the loading ratio increased the life of damaged and undamaged specimens. Figure 8c shows cycles to failure values corresponding to loading ratios for the 9 AL10b specimen. The static damage load of the specimen for 5 Joules of impact energy was 1739 N, while it decreased to 1539 N for 10 Joules of impact energy. When fatigue tests were performed at 90% and 80% of the static damage value for undamaged specimens, the applied static loads were found to be 1750 N and 1550 N, respectively. The static damage loads for specimens damaged by 5 Joules of impact energy were below 90% of the static damage load of the undamaged specimen, while those damaged by 10 Joules of impact energy were below 90% and 80% of the static damage load of the undamaged specimen. Thus, the fatigue tests were not carried out at these loading ratios. Figure 9 shows the fatigue test results for the 6 CFRP10a, 6 CFRP10b and 6 CFRP10c specimens. It was seen that the specimen affected most by the preliminary impact damage was 6 CFRP10c with a face sheet material thickness of 1.5 mm. The reason for this could be that increasing face sheet thickness increased the preliminary damage resistance of the face sheet, which caused more cores to be crushed. Cycles to failure values determined in the 6 CFRP10b specimen damaged by 5 Joules of impact energy diverged from the cycles to failure value of the undamaged specimen due to the  
Figure 8: Loading ratio vs. cycles to failure graphs for the (**a**) 6 AL15b, (**b**) 6 AL20b and (**c**) 9 AL10b specimens. decreasing loading ratio. Loading ratio vs. cycles to failure graph of specimens damaged by 10 Joules of energy corresponded to that of the undamaged specimen. Fatigue crack was not observed during fatigue tests due to loading type.