## 4 Discussion of Test Results
### SCB Test Results
#### 4.1.2 Characteristics of the Three Stages at Different Stress Ratios  
In order to more clearly observe the process of crack initiation and propagation and describe the strain distribution field of the specimen, digital image correlation (DIC) technology was adopted to analyze the strain distribution of the front face of the semi-circular specimen during fatigue process. The stages of crack initiation, propagation and accelerated destruction during the entire duration of the fatigue process can also be visualized using the DIC method. The transverse strain distribution fields at different stages of the fatigue process under stress ration 0.5 are shown in Figure 7. The strain distribution under other stress ratios are similar, and not discussed further in this paper.  
The results in Table 2 show that, with the increase of stress ratio, the loading cycles in each stage decreased gradually. Under the same stress ratio, the load cycles in stage II, that is, the crack stable propagation stage, were obviously greater than that of other stages. In order to better study the proportion of each stage over the whole fatigue life, the ratio of the load cycles in each stage to the total amount of loads cycles during the entire fatigue process was calculated as shown in Figure 8.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Stress Ratio**} & \multicolumn{4}{c}{**Load Cycles Acting on SCB Specimens**} \\ \cline{2-5}  & **Crack Initiation** & **Crack Propagation** & **Accelerated Destruction** & **Total Amount** \\  & **(Stage I)** & **(Stage II)** & **(Stage III)** & **\(x_{3}\)** \\  & **\(x_{1}\)** & **\(x_{2}\)**-\(x_{1}\)** & **\(x_{3}\)**-\(x_{2}\)** & **Total Amount** \\ \hline
0.5 & 814 & 19,836 & 1334 & 21,984 \\
0.6 & 491 & 4556 & 240 & 5287 \\
0.7 & 214 & 1659 & 92 & 1965 \\
0.8 & 130 & 428 & 55 & 613 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Statistics of load cycles in SCB fatigue tests.  
Figure 7: Surface transverse strain field under stress ratio 0.5.  
The proportion of the total life of ECC represented by the crack initiation stage (stage I) increased while the stress ratio increased. When the stress ratio was 0.5, the crack initiation stage was only 3.7% of the total life; when the stress ratio increased to 0.8, stage I became 21.21% of the total life. This phenomenon shows that ECC had better performance with cracks in it under lower stress ratio, the existence of PVA fibers enabled ECC to maintain its load-carrying capability even after cracks initiated and propagated. The crack propagation stage (stage II) of ECC accounted for a higher percentage of total life than the other stages, which indicates that ECC exhibits excellent working performance with cracks in it. With the increase in the stress ratio, the proportion of life in stage II also decreased, which indicated that the working performance of ECC with cracks decreased continuously under a high stress ratio. Especially when the stress ratio rose from 0.7 to 0.8, the proportion of life in stage II decreased from 84.4% to 69.8%. There was no significant relationship between the percentage of total life in the crack acceleration destruction stage (stage III) and the stress ratio, since the material lost its stability and bearing capability rapidly and was completely destroyed in a short time.