## 3 Results and Discussion
### Influence of Rib Height on SIF of Crack Tip  
The bending capacity, weight, dimension space and bearing capacity of the integral panel are directly affected by the rib height, so the reasonable rib height is very important, which is usually in the range of 25 mm-60 mm. The influence of rib height on the SIF at the crack tip was analyzed by using the integral plate finite element model with central through crack. Using the method of controlling a single variable, keep the dimensions of other parameters (\(L\) = 120, \(W\) = 100, \(t_{m}\) = 5, \(d\) = 80, \(t\) = 4) of the panel unchanged, and set the ribbed height \(h\) as 10 mm, 20 mm, 30 mm, 40 mm and 50 mm respectively. The load on the model is \(M\) = 7.81 \(\times\) 10\({}^{6}\) N-mm. The specific parameters are shown in Table 4.  
The SIFs at the crack tip under each group of parameters with different rib heights were obtained by finite element calculation, as shown in Figure 6.  
Figure 5: Stress intensity factor (SIF) curves for crack lengths of (**a**) 5 mm, (**b**) 10 mm, (**c**) 15 mm, (**d**) 20 mm, (**e**) 25 mm, and (**f**) 30 mm. It can be seen from Figure 6a that the trend of SIF with crack depth under different rib heights are relatively similar, with the SIF increases approximately linearly with the change of skin thickness. With the increase of the rib height, the slope of the curves decreases sharply, the curves tend to be flat, and the \(K_{I}\) value at the crack tip decreases as a whole. It can also be seen from the figure that the curves of h10 and h20 are particularly steep and when the corresponding normalized distance of h10 curve is 0.65, the \(K_{I}\) value even reaches 0, indicating that at this point, the crack tip is under compressive stress in the direction perpendicular to the crack surface. When the rib height reaches 30 mm, the slope of the curve becomes smaller, and continues to increase the rib height, the slope of the curves changes little. The above result shows that when the rib height is small, both the crack tip \(K_{I}\) value and the maximum \(K_{I}\) value decrease rapidly with the increase of rib height. However, when the rib height increases to a certain height, the effect of increasing the rib height on reducing the \(K_{I}\) value of the crack tip is no longer obvious. By comparing the six figures in Figure 6, 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 curve are almost the same. The above results show that the effect of rib height on the change velocity and amplitude of \(K_{I}\) value at each point is consistent during the crack propagation. The above analysis shows that when the rib height is small, the increase of the rib height can significantly reduce the integral crack tip \(K_{I}\) value, and the integral panel's fracture resistance can be enhanced. Figure 7 shows the comparison between the curve of the maximum SIF at the crack tip with rib height when the crack length is 5 mm and the influence of rib height on the SIF of the integral plate structure in the literature. The left ordinate is the SIF value of the FEM result, in MPa\(\cdot\)mm\({}^{*}\)1/2. The right ordinate is the SIF value of the reference result, in MPa\(\cdot\)mm\({}^{*}\)1/2. Although the units of the two curves are not consistent, the comparison of the two curves shows that the influence of rib height on the stress intensity factor in this study is consistent with the conclusion in the literature. As you can see from Figures 6 and 7, when the crack length a = 5 mm and rib height h increases from 10 mm to 30 mm, \(K_{I}\) value decreases by 80%. When the rib height is large, the effect of increasing the rib height on the decrease of the crack tip \(K_{I}\) value is weak, when h increases from 30 mm to 50 mm, \(K_{I}\) value only decreases by 50%, and this effect does not change with the crack propagation.