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.