Figure 6. Effect of face sheet thickness, type and core height on flexural strength
Increased face sheet thickness reduced the amount of damage formed on the face sheet after impact tests. Similarly, increasing face sheet thickness decreased flexural strength more after impact. The reason for this was that during impact loading on increasing face sheet thicknesses, more of the applied load was transferred to the cores.
4.2. 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. 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 bending loads but also compression or membrane stretch loads.
Sandwich composites, whose strength values were determined by static tests, were subjected to testing both without impact and with impacts of variable energies; after that, fatigue tests were carried out. For fatigue tests, three-point bending loads were applied to specimens as for the static tests. Damaged specimens were subjected to fatigue tests using the load values determined by the static values of undamaged specimens. The reason for this was to determine changes in fatigue behaviors of damaged specimens with respect to undamaged specimens.
All fatigue tests were repeated three times and mean values were used, while fatigue graphs were drawn to avoid confusion. Because of decreases in the strengths of specimens with preliminary impact damage, fatigue tests were not performed for some values.
Figure 7 (a) shows the fatigue graphs of specimen 6AL10a 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 = Ffatigue/Fultimate) increased. The greatest fatigue life decrease (59%) was determined at r = 0.7.
Figure 7 (b) shows the loading ratio vs. cycles to failure graphs for 6AL10b 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 7 (c) shows graphs with cycles to failure corresponding to different loading ratios for the 6AL10c 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.