## 4 Results and Discussion  
The specimens were subjected to three different kinetic energy levels (2 kJ, 3 kJ and 6 kJ). Total impact mass used was 78 kg to keep the inertia effect constant during the study. The force-displacement and energy-displacement curves for the different kinetic energy levels are shown in Figure 7. Once the upper platen contacted the top of the specimen (initiator), the specimen started to compress, internal ply delamination started, and the specimen suffered from splaying away from the center. The load increased until there was 12 mm of the crushing displacement. Beyond 12 mm, there was no more increase in the load, the load flattened out as the specimen was crushed and started to decrease gradually. The initial increase in the load appeared to be linear followed by a flattening of the load as the specimen was crushed; this can be attributed to the initiator. The impact speed did not have a significant effect on the force or even the initial force. From the energy-displacement curves, the energy increased as crushing speed increased and the responses for different speeds were consistent. The energy increased until the maximum level then started to rebound due to the non-deformed closure plate which absorbed the energy and then released it. This finding confirmed what has been reported by [PERSON] [41], as well as [PERSON] and [PERSON] [42], who studied many tubes subjected to different crushing speeds. They mentioned that the energy absorption increased linearly as crushing speed increased. [PERSON] [24; 40] also studied CFRP square tubes under dynamic crushing loads. He found that energy absorption increased with the crushing speed. Figure 6: Top-hat geometry, boundary conditions and layup. The predicted force-displacement results from the finite element analysis are shown in Figures 8-10. For each speed value, three experiment tests were conducted for repeatability purpose. The initial results were almost the same as the experimental values until 14 mm (2 and 3 kJ) and 18 mm (6 kJ) of the crushing stroke when the load increased rapidly. After exceeding these displacements, the load was slightly lower than experimental values and it can be clearly observed in Figure 10. After that, the load tended to behave almost the same as experimental behavior. The peak loads in predicted results increased slightly with increasing kinetic energy (speed). Load-displacement curves produced an oscillating result, so the filter has been applied to compare with the experimental curves. An example of progressive crushing from the finite element model is depicted in Figure 11. There is good correlation between the finite element model and the experimental tests when the energy absorption is compared with the displacement as shown in Figures 8-10. Table 2 shows the comparison between experimental and numerical simulations of key crashworthiness parameters. The maximum difference was found to be at 13% between the experimental and FEA results. This is within the acceptable range for computational / finite element analysis [43; 44]. To add to this, the comparison was made between the average values of the experimental results and the FEA results. Besides this, for complex and non-linear problems such as the crushing of composite structures, errors below 20% are acceptable because the composite material failure models used in modelling are not exact models. Active research is still ongoing in this aspect [45; 46]. Figure 8: Hat-shaped crush at kinetic energy of 2 kJ: (**a**) Dynamic experimental and F.E load vs. displacement curves, (**b**) Dynamic experimental and F.E energy vs. displacement curves. Figure 7: Example of dynamic force-displacement curves for different impact energy levels.