# Bonding Behaviors of GFRP/Steel Bonded Joints after Wet-Dry Cyclic and Hydrothermal Curing
## 3 Results and Discussion
### Strain Development During Quasi-Static Tensile Tests  
To investigate the distribution and development of the stress/strain relationship of the testing specimens, the strain data were obtained from the data recorded during the data acquisition system testing. The locations of strain gauges are shown in Figure 1. Figure 9 shows the strain development during some quasi-static tensile tests (Reference, WD-90-60 and HY-90-60 specimens), and the other specimens had similar results. It can be observed that the absolute strains of the specimens increased when the tensile forces increased until the ultimate tensile capacities were achieved. When the ultimate tensile displacements (i.e., elongation of the joints) were reached, the strains were reduced significantly, which is similar to the load-displacement curves in Figure 6.  
Figure 8: Displacements of each specimen under 10 kN. Figure 9: \(Cont.\)A two-stage strain development was found for S-1 and S-2. In stage I (beginning of testing), strains increase relatively moderately, until reaching a certain threshold (for instance, approximate threshold as 3700 \(\upmu\epsilon\) for S-1 of reference specimen), where strains become unstable and quickly increase, leading to failure of the specimen. The following observations were made when each strain history was compared. 1. For strains measured by the gauge S-1, the strain increase accelerated after the strain exceeded a certain threshold, indicating a progressive degradation of GFRP materials when the tensile load was increased and the load was transferred to the outer surfaces of GFRP. While the reference test had a larger tensile capacity, the accelerating strain growth of the S-1 gauge indicated that the GFRP was weakened when the load exceeded a certain threshold. However, for WD-90-60 and HY-90-60 testing, compared with the reference specimen, a premature accelerating increase of S-1 was observed, showing the effect of the harsh environment on the GFRP materials. According to Figure 9b,c, thresholds of accelerating growth of about 2100 and 2500 \(\upmu\epsilon\) could be determined for WD-90-60 and HY-90-60, respectively, which are much smaller than the reference specimen (about 3700 \(\upmu\epsilon\)), indicating that the moisture diffusion combined with heat effect may negatively affect the GFRP materials, and the threshold value of degradation for GFRP has been reduced. 2. For strains measured by gauge S-2, the similar two-stage accelerating growth was found as the displacement/loading increased, which could be attributed to the debonding failure, including the cohesive failure of adhesive, delamination in the GFRP, debonding at the adhesive-steel interface, and shifting of the load-transferring area to the bonding ends. 3. The development of strains measured by gauge S-3 showed the same shape as the load-displacement curves seen in Figures 6 and 7, indicating the linear elastic behavior of the steel substrates during tensile testing (with no larger than 225 MPa for all testing specimens, far less than the yield strength of the steel substrates). The strain data of gauge S-3 were adopted to calculate the strain energy during tensile testing in the discussion of interfacial fracture energy. In order to investigate the degradation of the GFRP materials for the bonded specimens cured in the other conditions, the variation of threshold values of the S-1 curves was adopted and is shown in Figure 10. By comparing the threshold value of S-1 curves of the specimens cured in harsh environments, it was observed that all the threshold values of specimens cured in harsh environment were smaller than the reference specimen (3700 \(\upmu\epsilon\)), indicating the negative effect of the investigated harsh curing conditions on the GFRP composites [21].