## 3 Results and Discussion
### Investigations on Sandwich Beams  
In addition to the fatigue behavior of TRC coupons under tensile loading, the fatigue behavior in bending of sandwich (SW) beams with TRC faces was investigated. In total 13 beams were tested. To determine the virgin, reference behavior of the panels (SW REF), five samples were tested with a quasi-static four-point bending test, as described in Section 2.3.2. The other beams were subjected to a cyclic preloading, divided into two different series analogous to the experiments on the TRC coupons. Since no influence was observed for a cyclic preloading of the coupons up to 0.5 MPa this was excluded from further experiments. The stress levels of 1.0 MPa and 2.0 MPa, respectively, in the tensile face were converted to equivalent loads for the sandwich beams using a validated numerical model [33], respectively, 0.5 kN and 1.0 kN. These beams will be referred to as SW CYCL 1.0 MPa and SW CYCL 2.0 MPa. Similar to the coupons, the typical hysteresis curve was analyzed by comparing some fatigue parameters. The cycle stiffness and the dissipated energy were calculated similarly as explained in Section 3.1. The evolution of the stiffness and the dissipated energy show the same trends as for the coupons. The stiffness and the dissipated energy evolved in the opposite direction and directly linked to the formation of cracks. For uncracked specimens, they remained constant (Figure 15a). The occurrence of a crack was accompanied by a drop in stiffness and a jump in dissipated energy, as was seen for sample SW CYCL. 2.0 MPa A at cycle 93.000 in Figure 15b. Overall, fewer cracks were observed for the sandwich bending tests (Table 9) compared to the coupon uniaxial tensile tests. A possible reason for this was the different anchorage length in the different configurations. When applied in the sandwich beam, the textile was embedded over a longer length in the matrix, leading to a better anchorage. In addition, the presence of the insulating core could have had a beneficial effect, the bond between the EPS-core and the TRC faces restricted the crack widths. Figure 14: The cracking patterns of the different series were compared using the: (**a**) the average crack width, (**b**) the maximum crack width and (**c**) the total crack width. Similar to the TRC coupons, once a crack was formed in the tensile face of the sandwich beam, its width increased during the first subsequent thousand cycles but stabilized gradually afterwards (Figure 16). The occurrence of cracks was the only observed damage mechanism, the core and the face-core interface were not affected. The fatigue behavior of the sandwich beams was dependent on the behavior of the TRC faces. Figure 16: Evolution of the crack width of some representative cracks formed during cyclic loading of sandwich beams. Figure 15: The evolution of the stiffness and the dissipated energy of sandwich beams during the loading–unloading cycles: (**a**) an uncracked sample and (**b**) a cracked sample. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Specimen** & \
\begin{tabular}{c} **SW CYCL** \\ **1.0 MPa** \\ \end{tabular} &
\begin{tabular}{c} **SW CYCL** \\ **2.0 MPa** \\ \end{tabular} \\ \hline A & 0 & 2 \\ B & 0 & 1 \\ C & 0 & 2 \\ D & 0 & 6 \\ \hline \hline \end{tabular}
\end{table} 
Table 9: Number of cracks formed during cyclic preloading of the sandwich beams. After the loading-unloading cycles, the residual static behavior was quantified by means of a quasi-static four-point bending test. As for the TRC coupons, the only observed degradation phenomenon was the formation of cracks. For SW CYCL 1.0 MPa no cracks were observed during the cyclic loading and, thus, the residual behavior was very similar to the behavior of SW REF (Figure 17). As can be seen in Table 10 both the stiffness and strength of SW REF and SW CYCL. 1.0 MPa were situated in the same range. All specimens of SW CYCL 2.0 MPa showed cracks after subjection to the loading cycles (Table 11).