# Bonding Behaviors of GFRP/Steel Bonded Joints after Wet-Dry Cyclic and Hydrothermal Curing
## 2 Experimental Program
### Test Specimens  
A total of 19 GFRP/steel bonded specimens were prepared; each specimen consisted of two GFRP plates and two steel substrates, as shown in Figure 1. The dimensions of the two GFRP plates were 100 mm \(\times\) 20 mm \(\times\) 5 mm, and the thickness and width of the steel substrates were 5 mm and 20 mm. To ensure a reliable bonding behavior, sandblasting was used as the surface preparation method before the bonding procedure. Before sandblasting, the substrates were cleaned twice with alcohol and then blasted with sand (first 800 grit, then 2000 grit) until a nonglossy surface was formed. Then, the surfaces  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Mechanical Parameter** & **GFRP** & **Adhesive** & **Steel** \\ \hline \multirow{3}{*}{Young’s Modulus, MPa} & 15,400 (longitudinal direction) & \multirow{3}{*}{1034–1207} & \multirow{3}{*}{204,000} \\  & 6850 (transverse direction) & & \\  & 7630 (thickness direction) & & \\ \hline \multirow{3}{*}{Strength, MPa} & 291.1 (longitudinal direction) & \multirow{3}{*}{24.1–31.0} & \multirow{3}{*}{291.3 (Yield stress)} \\  & 125.3 (transverse direction) & & \\  & 144.6 (thickness direction) & & \\ \hline Poisson’s ratio & 0.37 & 0.40 & 0.30 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Mechanical properties of glass fiber-reinforced polymer (GFRP), adhesive, and steel.  
were cleaned again with alcohol. Lastly, the GFRP was bonded on the substrates with methacrylate, and the thickness of the adhesive layers was formed to 1 mm; images of this bonding procedure are shown in Figure 1b. In addition, to measure the strains of the bonded joints during tensile tests, three strain gauges (S-1, S-2, and S-3) were bonded to the surface of each specimen, including two stain gauges (S-1 and S-2) on the surface of GFRP and another stain gauge (S-3) on the steel substrate. The dimensions of the specimens and locations of stain gauges are shown in Figure 1a.  
Except for a reference specimen cured in an air environment (about 25 \({}^{\circ}\)C and 50% relative humidity) for three days, the bonded specimens were divided into two series: those cured under wet-dry cyclic conditions (WD) and those cured under hygthermal conditions (HY). An environmental curing box that provides a variety of controlled environmental conditions, including moisture and thermal conditions, was used, as shown in Figure 2. In each cycle of the wet-dry (WD) curing, the specimens were placed in the environmental box for 12 h, which provided a fixed environment (e.g., 20 \({}^{\circ}\)C and 95% relative humidity), and then, they were put in a dry, room-temperature environment for another 12 h. Thirty-, 60-, and 90-day wet-dry curing cycles and three temperature levels (i.e., 20 \({}^{\circ}\)C, 40 \({}^{\circ}\)C, and 60 \({}^{\circ}\)C) were considered in this study. In the hygrothermal (HY) pretreatment curing, the specimens were placed in the environmental box for specific durations (i.e., 30-, 60-, and 90-day), and three temperature levels (i.e., 20 \({}^{\circ}\)C, 40 \({}^{\circ}\)C, and 60 \({}^{\circ}\)C) were considered. To ensure full solidification of the adhesives, the bonded specimens were cured for two hours at room temperature (about 25 \({}^{\circ}\)C with 50% relative humidity) before wet-dry cyclic curing or hygothermal pretreatment. During the harsh curing period, the relative humidity in the environmental box was set at 95%. After harsh curing, the specimens were placed in a laboratory to air dry for three days.  
Figure 1: Dimensions of bonded specimen and bonding process (unit: mm). (**a**) Specim; (**b**) Bonding Process.  
The detailed description of the two series of specimens are shown in Table 2, where the testing specimens are labeled as [CC]-[D]-[T], where CC represents the categories of the curing condition, D is the duration of curing, and T is the temperature in the environmental curing box. For instance, WD-30-20 denotes the specimen that was cured with wet-dry cyclic condition for 30 days at 20 \({}^{\circ}\)C.