Koroyd has an immediate loading curve meaning a large amount of energy is absorbed from the moment of impact. Note the ‘stress plateau’ for Koroyd is completely straight (the bright green line) compared to the EPS foam stress plateau which constantly increases (the grey line).
As Koroyd is a welded tube structure, when compressed, the tubes buckle/crush starting from one end. As the compression continues, the structure’s resistance to compression remains constant, allowing for efficient absorption of energy up until densification.
In comparison, EPS foam’s stress plateau increases because Expanded Polystyrene is made with beads. As the beads get more compressed against each other, the material has a higher resistance to compression (it gets stiffer and could pass on more forces as a result). Therefore, the load needed to continue the compression increases. The energy from a typical accident is fixed and does not increase. At some point, an EPS foam becomes too stiff to absorb the energy.