## 3 Results and Discussion
### Investigations on Sandwich Beams
However, the occurrence of cracks had a smaller impact on the initial stiffness S\({}_{1}\) compared to cracked TRC coupons. The stiffness in the last branch S\({}_{3}\) was not affected by the cyclic preloading. The average ultimate capacity was 23% lower for the SW CYCL sandwich beams compared to the SW REF beams (Figure 18). \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{3}{*}{**Specimen**} & \multicolumn{2}{c}{**S\({}_{1}\)**} & \multicolumn{2}{c}{**S\({}_{3}\)**} & \multicolumn{2}{c}{**P\({}_{\text{crack}}\)**} & \multicolumn{2}{c}{**P\({}_{\text{ultimate}}\)**} \\ \cline{3-8}  & \multicolumn{2}{c}{**kN/mm**} & \multicolumn{2}{c}{**kN/mm**} & \multicolumn{2}{c}{**kN**} & \multicolumn{2}{c}{**kN**} & \multicolumn{2}{c}{**After**} \\ \cline{3-8}  & \multicolumn{2}{c}{} & \multicolumn{2}{c}{} & \multicolumn{2}{c}{**CyclicPreloading**} & \multicolumn{2}{c}{**At Failure**} \\ \hline \multirow{3}{*}{**REF**} & avg & 0.451 & 0.098 & 1.12 & 4.99 & - & 17 \\  & st dev & 0.009 & 0.002 & 0.27 & 0.45 & - & 4 \\ \hline \multirow{3}{*}{**SW CYCL**} & A & 0.437 & 0.091 & 1.42 & 3.95 & 0 & 12 \\  & B & 0.474 & 0.090 & 1.89 & 5.14 & 0 & 13 \\ \cline{1-1}  & C & 0.418 & 0.091 & 0.98 & 5.35 & 0 & 13 \\ \cline{1-1}  & D & 0.488 & 0.090 & 1.04 & 4.62 & 0 & 13 \\ \cline{1-1} \cline{2-8}  & avg & 0.454 & 0.090 & 1.34 & 4.76 & - & 13 \\ \cline{1-1}  & st dev & 0.028 & 0.000 & 0.36 & 0.54 & - & 0 \\ \hline \hline \end{tabular}
\end{table} 
Table 10: Quantitive comparison of some distinctive parameters of the static load-displacement curve: the average reference versus the beams cyclically preloaded to 1.0 MPa. Figure 17: Residual behavior of SW CYCL. 1.0 MPa. As shown in Figure 5, a part of the tensile face was monitored using DIC. An analysis of the cracks was performed as for the TRC coupons. However, due to the narrow area of interest of the DIC a limited amount of the cracks was captured, and, thus, only the maximum crack width was looked at. The evolution of the maximum crack width of SW REF and SW CYCL 1.0 MPa was very similar (Figure 19). SW CYCL 2.0 MPa showed larger crack widths for all load levels, which could be attributed to the occurrence of cracks during the cyclic preloading and the inherent debonding between fibers and matrix. This observation did not match the results of the TRC coupons, no increase of the maximum crack width was seen for TRC CYCL. 2.0 MPa. For the moment no clear reasoning could be made regarding this particular discrepancy between TRC coupons and SW beams. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Specimen**} & \multicolumn{2}{c}{\(\mathbf{S_{1}}\)} & \multicolumn{2}{c}{\(\mathbf{S_{3}}\)} & \multicolumn{2}{c}{\(\mathbf{P_{crack}}\)} & \multicolumn{2}{c}{\(\mathbf{P_{ultimate}}\)} & \multicolumn{2}{c}{\# **Cracks**} \\ \cline{3-8}  & \multicolumn{1}{c}{**kN/mm**} & \multicolumn{1}{c}{**kN/mm**} & \multicolumn{1}{c}{**kN**} & \multicolumn{1}{c}{**kN**} & \multicolumn{1}{c}{**After**} & \\ \cline{3-8}  & \multicolumn{1}{c}{} & & & & \multicolumn{1}{c}{**CyclicPreloading**} & \multicolumn{1}{c}{**At Failure**} \\ \hline \multirow{2}{*}{**REF**} & avg & 0.451 & 0.098 & 1.12 & 4.99 & - & 17 \\  & st dev & 0.009 & 0.002 & 0.27 & 0.45 & - & 4 \\ \hline \multirow{5}{*}{**SW CYCL**} & A & 0.412 & 0.082 & 1.25 & 3.60 & 2 & 9 \\  & B & 0.455 & 0.097 & 1.36 & 4.15 & 1 & 10 \\ \cline{1-1}  & C & 0.336 & 0.086 & 1.32 & 3.11 & 2 & 15 \\ \cline{1-1}  & D & 0.254 & 0.096 & 1.22 & 4.52 & 6 & 24 \\ \cline{1-1} \cline{2-8}  & avg & - & 0.090 & 1.29 & 3.84 & - & 15 \\ \cline{1-1}  & st dev & - & 0.006 & 0.055 & 0.54 & - & 6 \\ \hline \hline \end{tabular}
\end{table} 
Table 11: Quantitive comparison of some distinctive parameters of the static load-displacement curve: the average reference versus the beams cyclically preloaded to 2.0 MPa. Figure 18: Residual behavior of SW CYCL. 2.0 MPa.