# The Flexural Fatigue Behavior of Honeycomb Sandwich Composites Following Low Velocity Impacts
## 2 Materials and Methods  
The core material of the honeycomb used was an Al-3003 alloy. Honeycomb cores were placed in between two different face sheet materials, one Al-5754 and the other a carbon fiber-reinforced composite plate. As reinforcement material, 0\({}^{\circ}\)/90\({}^{\circ}\) twill weave carbon fiber with a density of 300 gr/m\({}^{2}\) was used. Epoxy resin was chosen as the matrix material. Adhesion between the face sheet materials and the honeycomb cores was provided by 3M 2216 epoxy-based adhesive.  
The honeycomb cores had cell sizes of 6.35 mm and 9.525 mm in diameter (D) and 10 mm, 15 mm and 20 mm in height (T). Face sheet materials had various thicknesses: 0.5 mm, 1 mm and 1.5 mm. These materials produced a composite sample of 80 mm in width and 135 mm in length. The honeycomb sandwich composite layers are shown in Figure 1, while a specimen labeling list and specimen properties for each label are shown in Table 1. There were 45 samples produced for each configuration (3 samples for undamaged static test, 3 samples for 5 J impacted static test, 3 samples for 10 J impacted static test, 12 samples for undamaged fatigue test, 12 samples for 5 J impacted fatigue test, 12 samples for 10 J impacted fatigue test).  
Face sheet materials were first ground with 180 grade sandpaper in accordance with the 3M 2216 adhesive user instructions, and then the aluminum plates were cleaned with acetone, while the composite plates were cleaned with isopropyl alcohol.  
[PERSON] et al. indicated that an 0.7 kg/m\({}^{2}\) amount of adhesive achieved a maximum flexural strength [34]; therefore, 7.5 g of adhesive, corresponding to that amount, was applied to the cleaned side of the  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Specimen Codes** & **Cell Size (D) mm** & \begin{tabular}{c} **Face Sheet** \\ **Material** \\ \end{tabular} & \
\begin{tabular}{c} **Core Height (T)** \\ **mm** \\ \end{tabular} &
\begin{tabular}{c} **Face Sheet** \\ **Thickness mm (h)** \\ \end{tabular} \\ \hline
6 AL10a & 6.35 & Aluminum & 10 & 0.5 \\ \hline
6 AL10b & 6.35 & Aluminum & 10 & 1 \\ \hline
6 AL10c & 6.35 & Aluminum & 10 & 1.5 \\ \hline
6 AL15b & 6.35 & Aluminum & 15 & 1 \\ \hline
6 AL20b & 6.35 & Aluminum & 20 & 1 \\ \hline
9 AL10b & 9.525 & Aluminum & 10 & 1 \\ \hline
6 CFRP10a & 6.35 & CFRP & 10 & 0.5 \\ \hline
6 CFRP10b & 6.35 & CFRP & 10 & 1 \\ \hline
6 CFRP10c & 6.35 & CFRP & 10 & 1.5 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Specimen properties.  
Figure 1: Honeycomb sandwich specimen layers.  
face sheet materials with a spatula. After applying adhesive to a single face, the samples were kept under a 2.5 kg weight for 24 h, and then adhesive was applied to the other face. Before performing tests, all specimens were left at room temperature for 1 week to ensure completion of the adhesive curing process.  
The mechanical properties of the materials used are determined according to the ASTM E8, ASTM D3039, ASTM D7078 standards. The mechanical properties of the 3M 2216 adhesive, and the Al-5754 and Al-3003 alloys, are shown in Table 2, while the mechanical properties of the CFRP composite are shown in Table 3. E\({}_{1}\) and E\({}_{2}\) are the longitudinal and transverse elastic modulus, v\({}_{12}\) is Poisson's ratio, G\({}_{12}\) is the shear modulus, and X\({}^{\text{T}}\) and X\({}^{\text{C}}\) are longitudinal tensile and compressive strengths, respectively. X\({}^{\text{C}}\), Y\({}^{\text{C}}\) and S are longitudinal compressive strength, transverse compressive strength and shear strength, respectively.