## 4 Experimental Results
### Specimens Subjected to Tension-Tension Fatigue  
The results of the fatigue tests are presented in this section in terms of number of cycles to failure and residual life of GFRP. In Table 5 the fatigue test results are illustrated, in addition to the tensile strength that was measured under monotonic load for those specimens that did not fail after the final loading history (10\({}^{6}\) cycles). From Table 5 it can be noted that only for a small quantity of GFRP specimens the number of scheduled loading cycles was completed. The most part of specimens, both aged or unconditioned, failed in tension during the fatigue cycles. However, some interesting observations can be raised. The failure modes in the broken cross sections were similar to the failure modes of the static experiments carried out on unconditioned specimens: all failures occurred in the area around the joint, demonstrating the small deviations of glass fibers from the direction of the tensile load may represent a critical defect. Glass fibers in both grid directions 0\({}^{\circ}\)/90\({}^{\circ}\) have to deviate in the joint area and this may produce significant reductions in tensile strength. Furthermore, the tensile failures were brittle without announcement by large deformations or visible cracks on the specimen surfaces. In Table 6 representative failed specimens and failure details are illustrated for GFRP specimens failed under the application of the fatigue cycles. Of particular interest was the number of loading cycles sustained before failure. In fact, the number of loading cycles before failure has been used to indicate the mechanical degradation. Results were obtained for a range of loading cycles between 23,539 and 638,157. It is evident that the GFRP specimens are sensitive to the tension-tension fatigue treatment. The most part of aged specimens sustained a lower number of loading cycles before tensile failure, compared to unconditioned specimens, indicating a degradation effect. The stress-strain response was almost linear-elastic in the load range used for fatigue testing. Furthermore, when tension-tension fatigue tests had a lower maximum load, GFRP specimens could sustain a larger number of loading cycles, indicating the significant effects of the fatigue test. In detail, only unconditioned specimens with a maximum fatigue load of 1 kN (corresponding to a \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**ID**} & **Treatment in** & **Maximum Fatigue** & **Number of** & **Ultimate** & **Residual Tensile** \\  & **Alkaline Bath** & **Load (kN)** & **Cycles at Failure** & **Load (kN)** & **Strength (MPa)** \\ \hline
1 NT-F & & 2.5 & 50,553 & - & - \\
2 NT-F & Unconditioned & 2.5 & 38,546 & - & - \\ \hline
3 AG-F & & 2.5 & 26,942 & - & - \\
4 AG-F & Alkaline bath & 2.5 & 23,539 & - & - \\ \hline
5 NT-F & & 2 & no rupture & 1.17 & 127.17 \\
6 NT-F & Unconditioned & 2 & 163,088 & - & - \\ \hline
7 AG-F & Alkaline bath & 2 & 28,045 & - & - \\
8 AG-F & Alkaline bath & 2 & 232,661 & - & - \\ \hline
9 NT-F & Unconditioned & 1.5 & 638,157 & - & - \\
10 NT-F & & 1.5 & 111,779 & - & - \\ \hline
11 AG-F & Alkaline bath & 1.5 & 98,026 & - & - \\
12 AG-F & Alkaline bath & 1.5 & 62,466 & - & - \\ \hline
13 NT-F & & 1 & no rupture & 3.12 & 339.13 \\
14 NT-F & Unconditioned & 1 & no rupture & 3.86 & 416.56 \\ \hline
15 AG-F & Alkaline bath & 1 & 410,988 & - & - \\
16 AG-F & Alkaline bath & 1 & no rupture & 5.54 & 602.17 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Fatigue tests results of reference and aged GFRP. normalized maximum tensile stress (\(\sigma_{\max}/\sigma_{\text{ult}}\) of approximate 0.25) completed the scheduled number of cycles. Because GFRP specimens were made of a organic polymeric matrix with elastic glass fibers, the typical degradation, commonly designated as plasticity for metals, cannot occur. However other irreversible mechanisms may be activated during a fatigue test, such as matrix cracking and fiber/matrix debonding.