## 3 Results and Discussion
### Tensile Test
#### 3.2.4 Area below the SS Curve  
Figure 8 shows the area below the SS curve as a function of treatment time at various NaOH concentrations and temperatures. At 40 \({}^{\circ}\)C, the maximum value was recorded at 24 h for 0.01 M, whereas the maximum value was recorded at 3 h a for 0.1 and 1 M. At 0.1 M for 3 h, the area was almost double that of the pristine membrane. When immersed for 168 h, the value became slightly lower than the initial value. At 60 \({}^{\circ}\)C and 0.01 M, the maximum value was recorded at 3 h. In the case of 1 M, it decreased below the initial value after a few hours. At 80 \({}^{\circ}\)C, the area below the SS curve increased from the initial value for 0.01 M at 3 h, but the other samples indicated values below the initial value. At the same treatment time, the area decreased as the alkali concentration increased. The overall behavior exhibited was similar to that of the burst pressure.  
Figure 6: Normalized strain at break as a function of treatment time at various NaOH concentrations and temperatures. Temperatures of the treatment solutions: (**a**) 40 \({}^{\circ}\)C; (**b**) 60 \({}^{\circ}\)C; (**c**) 80 \({}^{\circ}\)C.  
Figure 7: Normalized Young’s modulus as a function of treatment time at various NaOH concentrations and temperatures. Temperatures of the treatment solutions: (**a**) 40 \({}^{\circ}\)C; (**b**) 60 \({}^{\circ}\)C; (**c**) 80 \({}^{\circ}\)C.  
The area below the SS curve in Figure 8 was almost the same as that of the burst pressure in Figure 2, despite some numerical differences. Therefore, as in Figure 9, the correlation between the normalized area below the SS curve on the horizontal axis and the normalized burst pressure on the vertical axis was examined. The correlation coefficient (R\({}^{2}\)) was 0.6709, which suggested a correlation, although weak. Meanwhile, when the normalized strain at break was represented in the vertical axis, the correlation coefficient (R\({}^{2}\)) decreased to 0.4121. The area below the SS curve exhibited a higher correlation coefficient with the burst pressure.  
Because the horizontal axis of the SS curve denoted strain and the vertical axis denoted stress, the area below the SS curve was assumed to represent the amount of work or energy until burst of a membrane.  
A systematic alkali attack test on the AEM helped to confirm the following. The standard CIP condition, as recommended by the supplier, is that an AEM and 0.1 M NaOH solution should be in contact for 1 h at 40 \({}^{\circ}\)C. If this process is realized 52 times per year for 3 years, the cumulative immersion time will be 156 h. The equivalent load observed at the 168 th hour of the immersion test  
Figure 8: Normalized area below the SS curve as a function of treatment time at various NaOH concentrations and temperatures. Temperatures of the treatment solutions: (**a**) 40 \({}^{\circ}\)C; (**b**) 60 \({}^{\circ}\)C; (**c**) 80 \({}^{\circ}\)C.  
Figure 9: Correlation between the burst pressure and the area below the SS curve.  
conducted in this study almost corresponded to the standard CIP. At 168 h, the burst strength and area below the SS curve are higher than those observed for the pristine membrane. Hence, no problem is expected if CIP is performed solely by passing the above conditioned solution through the ED stack.  
However, the AEM swells during the alkaline treatment and shrinks during the neutralization treatment. This dimensional change facilitates the deterioration of the AEM [6]. In addition to the immersion or passing of the solution, in practical maintenance scenarios, operations such as physical rubbing to remove fouling substances may be required. If such additional operations are performed, the deterioration of the AEM will inevitably progress.