## 3 Results and Discussion
### Flexural Behaviors of Specimens with Carbon Grids
#### 3.2.3 Post-Peak Behavior  
Table 8 lists the ratios of strain to the tensile strain of the longitudinal strands in the carbon grids under and after the maximum load. Figure 13 illustrates the relationship between the mid-span deflection of the carbon grid-reinforced specimens and strain of the longitudinal strands; here, the strain indicates the value measured using the strain gauges installed on the strands (Figure 3). Specimens G1, J1, J2, J3, J4, J5, and J6 exhibited a stage-wise load reduction tendency after reaching the maximum load (Figure 9) as the mid-span deflection of the specimens increased. The strains of the longitudinal strands under the maximum load ranged from 38 to 65% of the tensile strain (Table 7), indicating the occurrence of tensile stress that corresponded to 38 to 65% of the tensile strength. Figure 12: Comparison of stiffness: (**a**) crack formation; and (**b**) stabilization stages. In the case of specimen G1, the load decreased after reaching the maximum load, and a certain amount of the load was recovered as the carbon grid resisted the load. However, the load rapidly decreased again as a strain of approximately 80% of the tensile strain was measured. Finally, the load sharply decreased and failure occurring as a strain of approximately 83% of the tensile strain was measured. In the case of specimen J5, a certain amount of the load was recovered as the carbon grid resisted the load after reaching the maximum load; nevertheless, the load rapidly decreased again as a strain that corresponded  
Figure 13: Relationship between the mid-span deflection and strain in the specimens: (**a**) G1; (**b**) G2; (**c**) G3; (**d**) J2; (**e**) J3; (**f**) J4; (**g**) J5; and (**h**) J6. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Specimen** & **At Peak** & & & **After Peak** \\ \hline G1 & 0.64 & 0.80 & 0.83 & \\ G2 & 0.64 & 0.70 & & \\ G3 & 0.82 & 1.01 & & \\ J1 & - & & & \\ J2 & 0.54 & 0.55 & & \\ J3 & 0.38 & 0.44 & & \\ J4 & 0.53 & 0.56 & 0.55 & \\ J5 & 0.38 & 0.47 & 0.58 & 0.70 & 0.93 \\ J6 & 0.65 & 0.83 & 1.02 & \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Ratios of strain to tensile strain. to approximately 47% of the tensile strain was measured. The load gradually decreased, and the final failure occurred because of the sharp reduction in load when a strain of approximately 93% of the tensile strain was measured. In the case of specimen J6, the load rapidly decreased after reaching the maximum load as a strain corresponding to approximately 83% of the tensile strain was measured. Subsequently, the load sharply decreased, and final failure was reached as a strain of approximately 102% of the tensile strain was observed. In the case of specimen G1, J5 and J6, a difference was observed between the strain measurement results and the damage status of the longitudinal strands. The strain gauges were practically installed on the resin on the strand surface and recorded the deformation of resin and some fibers only at the position where the strain gauge was attached; thereby, the measured values could be affected by the strand surface condition. The biaxial warp-knitted structured carbon grid used in specimen G1 used epoxy with relatively high physical properties as resin and had the small cross-sectional area of the strands and the cross-sectional voids generated during manufacturing. Therefore, the strands were considered to fracture by the occurrence of strain corresponding to 83% of the tensile strain. Conversely, the cross-laminate structured carbon grid used in specimens J5 and J6 used vinyl ester, with lower physical properties than epoxy [23] and a larger cross-sectional area. Consequently, it was considered that the strands were not fractured despite the occurrence of strain that corresponded to approximately 93 and 102% of the tensile strain. The load in specimens G2 and G3, with the biaxial warp-knitted structured carbon grids, tended to reduce sharply after reaching the maximum load (Figure 9).