Figure 9. Loading ratio vs. cycles to failure graphs for the (a) 6CFRP10a, (b) 6CFRP10b and (c) 6CFRP10c specimens
Fatigue crack didn’t observed during fatigue tests due to loading type. Core crushing failure is the main failure type for all fatigue specimens. No failures observed for adhesive between core and face material. Adhesion tests can be investigated in future studies.
4.3. Damping Ratio and Stiffness Reduction
Damping indicates the amount of energy required to rearrange molecules of the material due to internal friction. The energy consumed by the material due to internal friction and movements is called the loss modulus (E’), while the response energy consumed by the material against impact is the damping modulus (E”). The damping ratio is calculated by the formula tan (δ) = E”/E’, and it is independent of geometric effects.
To determine the amount of energy absorbed by specimens after the fatigue tests, tan (δ) vs. cycles to failure (log N) graphs are shown below.
Figure 10 shows the effect of different loading ratios on the damping ratio for the undamaged 6AL10b specimen under three-point bending loads. The damping ratio decreased with increasing loading ratio, while for all loading ratios, minimum damping ratios were determined at cycles to failure values where fatigue damage occurred.