## 4 Discussion
The first peak is correlated with the failure of the composite tube; the second one is related to the collapse of the chiral lattices. The curves representing the chiral structure and filled tube showed similar patterns because of the progressive crush of the hollow composite tube. The use of chiral structure as a filling material for hollow tube increased the PCF and MCF magnitudes in uniaxial loading, as presented in Table 8. An increase in PCF is an undesirable situation for crashworthy structures but the trend in the increase of MCF is more than the PCF. Thus, the CFE value, also known as the load uniformity ratio, reached a value of one, which is an improved performance index. In energy absorption, chiral fillers exhibited higher performance with a 10% increase. However, the addition of extra mass using a chiral structure as a filling material to the configuration provides a small increase in the absorbed energy, although it lowered the SEA value by more than half. In the case of the lateral (radial) crush of composite tubes, two different energy-absorbing processes occur when the load is applied. One of them is related to the flattening of the tubes cross-sections curvature; the other one is about the presence of hinge lines [47]. The two energy absorption processes could be observed for both the hollow and the chiral-lattice-filled tubes (Figure 13). Figure 12: Deformed under uniaxial load. (**A-C**) Hollow and chiral; (**D-F**) lattice-filled composite tubes. Figure 13: Undeformed (**A-1**), 12.5 mm deflection applied (**A-2**), and 25 mm deflection applied (**A-3**) lattice-filled composite tubes. During the lateral compression, both elastic and plastic deformations occur. After the load was removed at the end of the test, it was noticeable how the tube's wall returned close to its original shape. In the last stage of the chiral-filled tube, the diameter of the compressed tube in the uniaxial direction was 25 mm, and it became 44 mm after releasing the load. The final dimensions of the deformed tubes (hollow and chiral lattices-filled) are almost the same (Figure 14). In contrast to the axial compression case, significant in-plane deformation of the chiral lattices auxetic behavior is observed, and the chiral structure does not expand under lateral loads. As seen in Figure 13, the lateral contraction of chiral fillers was experienced, and gaps were formed between composite walls and chiral structures. In Figure 13A-2, the deformation pattern of chiral structure was observed. Nodes started to rotate and ligaments were bent. In Figure 13A-3, it is seen how densification phase started and lateral contraction is increased, in which gaps in the cell was contracted and filled with material. From the observation of Figure 11b, the compression of the hollow composite tubes gives a fairly regular and smooth characteristic load-displacement curve. The shape of the same load-displacement diagram is more irregular in the case of the chiral-filled tubes due to the progressive deformation of the cells in the chiral structures. The lateral crush of the chiral structure and hollow tube do not exhibit distinct peaks or a pre-crushing phase. However, the combination of chiral and hollow tubes enabled the pre-crushing stage and two explicit peaks. Moreover, the onset of the densification phase pulled back to a smaller displacement. According to Table 9, the chiral filler led to a very high increase in the PCF and MCF and decreased CFE values by almost 20%. The absorbed energy by the combination of the chiral structure and hollow tube was higher than the sum of the energies absorbed by constituent elements tested separately. Remarkably, the SEA values were more than three times higher in comparison with that of the hollow tube. This is a clear indication of the negative Poisson's ratio effect of the chiral structures, which creates a large equivalent densification and stiffening effects during transverse compression loading, with only a limited amount of effective mass provided by the filling of the tube.