The REDIPhE innovative idea

The REDIPhE innovative ideas for improved safety helmets were developed in the Path Engineering of the project. The part of the helmet that dissipates the highest portion of impact energy is the energy absorbing liner. Some preliminary studies have already been carried out by the principal investigator and his research group in collaboration with the Swedish company Swerea. Figure 5 shows a sample of hierarchical lattice structure still within the 3D printer used to print it. The lattice structure is yellow and pink and it is apparent that the size of its five cells varies, being the biggest cell at the bottom and the smallest at the top.

Figure 5: a sample of a hierarchical lattice structure within the 3D printer that printed it. It was printed by Siamak Farajzadeh Khosroshahi, PhD student of Prof. Galvanetto temporarily working at Swerea, the Swedish Research Institute for Industrial Renewal and Sustainable Growth.

A hierarchical lattice structure has different cell sizes at different rows as shown in Figure 5 and each cell size provides particular mechanical properties, therefore, the entire structure has varying mechanical properties through the thickness. As it is shown in Figure ‎6 a material like EPS (usually used in helmet liners) crushes uniformly through thickness but a lattice structure, with varying mechanical properties through the thickness, starts to crush from the weakest layer and all layers would crush one after the other from the weakest layer to the strongest one. Such a structure provides varying stiffness and yield stress through the thickness, therefore a helmet’s liner with such a mechanical properties could reduce the transmitted force to the head centre of gravity according to reference [13].

Figure ‎6 – Difference in crushing mechanism between a homogeneous material (top row) and a hierarchical lattice structure (bottom row), compression is increasing from left to right.

It is important to stress that the lattice structures are made of a homogeneous material, the variability of the mechanical properties across the thickness is due to the structural design of the energy dissipative layer and it appears as a global property of the whole layer. All struts of the structure are made of the same material, but their different length and cross-section provide the required variability in mechanical properties.

Preliminary studies of the constitutive behavior of the lattice structures have been performed to define the input parameters to be used in finite element commercial software, as shown in figure 7. Then a simplified model of a helmet was equipped with an innovative liner made with the hierarchical lattice structure and virtual impact tests were simulated for the two cases of simplified helmet with innovative liner and with a state of the art equal mass EPS liner, as shown in figures 8-9-10.

Figure 7: FEM simulation of a test on a sample of hierarchical structure.

Figure 8 – Section view of the head-form and EPS liner (left), Section view of the head-form and the lattice liner (middle) and impact condition for assessment of the concept helmet (right).

Figure 9 – Stress distribution in the EPS liner during the impact.

Figure 10 – Stress distribution in the lattice liner during the impact.

Figure 11 – linear and rotational accelerations of the headform’s center of gravity.

Figure 11 shows that both linear and rotational accelerations are considerably reduced in the model with hierarchical lattice liner. The very promising results shown in figure 11 require a much deeper investigation, to be carried out within the proposed project. (Figures 8 to 11 are still unpublished work obtained by the PhD student of the Department of Industrial Engineering Siamak Farajzadeh Khosroshahi).