# Study on Impact Process of a Large LNG Tank Container for Trains
## 4 Results and Discussion
### Impact Force  
There are two kinds of impact forces: the first is the force of the twist lock on the corner fitting (\(F_{1}\)), and the second is the force of the LNG on the inner vessel (\(F_{2}\)). Figure 13 shows the variation in \(F_{1}\) over time at different initial velocities and initial clearance. As can be seen, when the initial velocity is 8 km/h, the twist lock repeatedly impacts the corner fitting at each initial clearance, producing several peak forces, with the positive peak values being significantly greater than the negative peak values. The maximum of \(F_{1}\) in direct relation to the initial clearance, increasing from 5.46 \(\times\) 10\({}^{5}\) N at 0 mm to 9.41 \(\times\) 10\({}^{5}\) N at 8 mm. However, its appearance time gradually moves from the second peak to the first peak. In addition, when the initial clearance is 8 mm, the magnitude and number of the peak of \(F_{1}\) vary at different initial velocities. The \(F_{1}\) significantly affects the axial stress at A07, which is shown in Figure 14 that when the force increases or decreases, the stress changes in magnitude accordingly. Figure 11: Location of A28 and A07 in the finite element model: (**a**) A28; (**b**) A07. Figure 12: Comparison between experimental and simulated values: (**a**) A28 with the impact vehicle velocity of 6 km/h; (**b**) A07 with the impact vehicle velocity of 8 km/h. Figure 15 shows the relationship between the \(F_{1}\) and the relative location between the twist lock and the corner fitting. When the relative locations are between \(-8\) and \(0\) mm, separation occurs, and when they are outside of that range, collision happens. Figure 14: The variation of axial stress at A07: (**a**) at different initial clearance; (**b**) relationship with the \(F_{1}\). Figure 13: The variation in \(F_{1}\) over time: (**a**) at different initial clearance; (**b**) at different initial velocities. Figure 15: Relationship between the \(F_{1}\) and the relative location between the twist lock and corner fitting. As can be seen from Figure 16, when the initial velocity is 8 km/h, the different initial clearance led to different velocities when the transport vehicle just collides with the corner fitting, which in turn affects the different magnitudes of the equal velocities and ultimately causes the different magnitudes of the first peak of \(F_{1}\). When the initial velocity is 8 km/h, Figure 17 shows the variation of the LNG impact force with time on the impact side head (back head) at different initial clearance. It can be seen that the back head is repeatedly impacted by the LNG at each initial clearance, resulting in multiple peak forces (such as A-D), and the maximum increases with the increase in initial clearance from \(-1.12\times 10^{5}\) N at 0 mm to \(-2.10\times 10^{5}\) N at 8 mm. In fact, the variation in LNG impact force is brought on by the \(F_{1}\). However, although the \(F_{1}\) changes sharply between positive and negative, the LNG impact force must remain negative. As in magnitude, the former is much larger than the latter. Figure 17d shows the variation in LNG free surface at different peak impact force times (split along the longitudinal symmetry plane). LNG separates from the front head and upwells along the back head as a result of inertia. LNG starts to surge along the axial direction of the tank container and upwell along the front head when it reaches the top of the inner vessel. When the initial velocity is 8 km/h, Figure 18 shows the variation of the tank container axial acceleration at the mass center over time at different initial clearance. The maximum acceleration in direct relation to the initial clearance, from 35.4 m/s\({}^{2}\) at 0 mm to 63 m/s\({}^{2}\) at 8 mm. It is noted that the maximum acceleration is greater than 4g inertial acceleration specified in the Chinese standard NB/T 47059-2017 Tank containers for refrigerated liquefied gas for the design of tank containers, which means if assessed by the impact, the specifications of the standard are not conservative.