## 3 Results and Discussion  
### Influence of Rib Spacing on SIF of Crack Tip  
In order to control the quality of the integral wall panel and the complexity of assembly, the spacing of the ribs on the panel must be reasonable, which is usually in the range of 70 mm to 140 mm [27]. The influence of rib spacing on SIF at the crack tip was analyzed by using a finite element model of integral panel with center penetration crack. Using the method of controlling a single variable, keep the dimensions of other parameters (\(W=100\) mm, \(t_{m}=5\) mm, \(h=30\) mm, \(t=4\) mm) of the panel unchanged, and set the ribbed spacing \(d\) as 60 mm, 70 mm, 80 mm, 90 mm and 100 mm respectively. The specific parameters are shown in Table 2. The bending moment of the integral wall plate in unit mm length is M\({}_{0}=4.88\times 10^{4}\) N\(\cdot\)mm, and the bending load of the integral wall plate model under different length is shown in Table 3.  
The SIFs at the crack tip of different length under each group of parameters were obtained by finite element calculation, data were normalized along the \(Z\) direction of the skin thickness, and positions of '0' and '1' are indicated in Figure 4. In particular, '0' in Figure 5 represents the point is the intersection of the crack tip and the outer surface, and '1' represents the point is the intersection of the crack tip and the inner surface of the integral panel. The ordinate is the SIF value at the crack tip, in MPa\(\cdot\)mm\({}^{1/2}\). It can be seen from Figure 5a that when the crack length is 5 mm, the change trend of SIF with skin thickness under different rib spacing are relatively similar, and the change trend of stress intensity factor with thickness is approximately linear. With the increase of rib spacing, the slope of the curves increases gradually, the curves tend to be steep. The \(K_{I}\) value of the area below the skin thickness center increases, while the \(K_{I}\) of the area around the skin thickness center almost remains the same, and the \(K_{I}\) of the area above the skin thickness center decreases. The result shows that the minimum \(K_{I}\) value of the crack tip decreases with the increase of the rib spacing, while the maximum \(K_{I}\) value of the crack tip increases with the increase of the rib spacing. By comparing and observing the six figures in Figure 5, it can be found that \(K_{I}\) value of the crack tip increases as a whole with the crack propagation, and the ratio of increase is close to each other, and the shape and slope of the curves change very little. The above results show that the effect of rib spacing on the change velocity and amplitude of \(K_{I}\) value at each point is consistent during the crack propagation. The above analysis shows that the \(K_{I}\) value at the crack tip increases with the crack propagation and the increase of rib spacing, and the crack resistance of the integral panel decreases. When a = 5 mm and ribbed spacing d increased from 60 mm to 100 mm, \(K_{I}\) value increased by 16%, indicating that ribbed spacing has a great impact on SIF at the crack tip of components. However, this effect tends to \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Groups**} & **Model Length** & **Model Width** & **Skin Thickness** & **Rib Spacing** & **Rib Height** & **Rib Thickness** \\  & **_L_(mm) & **_W_(mm) & \(t_{m}\)(mm) & **_d_(mm) & **_h_(mm) & **_h_(mm) \\ \hline
1 & 120 & 100 & 5 & 60 & 30 & 4 \\
2 & 140 & 100 & 5 & 70 & 30 & 4 \\
3 & 160 & 100 & 5 & 80 & 30 & 4 \\
4 & 180 & 100 & 5 & 90 & 30 & 4 \\
5 & 200 & 100 & 5 & 100 & 30 & 4 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Structural parameters of integral wall panels with different rib spacing.