REDIPhE Project Objectives

REDIPhE Project aims at developing new protective devices by evaluating the effects they have on the injury mechanisms inside the human head. This is possible thanks to the use of bio-faithful models in virtual simulations of impacts. One of the main objectives of the project is the development of three, very detailed, models of human head, for woman, man and teenager. Once the mechanism of brain injury is properly described, protection technologies can then be developed to aid in their prevention. The three head models are coupled with innovative helmets to assess the helmet performance.

Before addressing the novel aspects of the proposal for safety helmets, let us summarize some contribution to innovative helmets provided by the research group of the principal investigator in recent years.

The part of the helmet that dissipates the highest portion of impact energy is the energy absorbing liner, component number 2 in the figure 1(a) above, so it is reasonable to assume that such a component will be the candidate to the most effective innovations. A few examples of possible innovative ideas are shown in figure 3. Figure 3(a) [10] presents a helmet with a transparent shell so that it is possible to see the innovative energy absorbing liner. In this case the polystyrene foam has been substituted by a structural layer made of plastics, that dissipates energy by inducing buckling in the conical elements supporting the shell.

Figure 3(b) [11] shows instead a new way to combine different materials, polystyrene and aluminium honeycomb, to increase energy dissipation in the liner. The two materials do not simply work in series but they introduce a new dissipative mechanism when friction develops between aluminium and polystyrene as the former penetrates in the latter. Figure 3(c) [12] shows a new concept that involves a liner subdivided in several layers so that sliding, taking places between adjacent layers, can reduce the rotational acceleration applied to the head.

Figure 3 shows just three possible improvements, many more can be proposed, but their effectiveness has to be assessed not only by evaluating the peak linear or rotational acceleration or the acceleration time history, but also by considering the strain and the strain rate induced by impacts in the tissues of the human head.

Previous studies on simple models [13] and on functionally graded materials with varying yield stress [14] have shown that an engineered liner can decrease the transmitted force to the head in comparison to a conventional material. In particular it has been suggested that [14]:

–           the peak linear acceleration is reduced with increased contact area, therefore the first contact zone should be reasonably compliant to increase the contact area,

–           again, the peak linear acceleration, in the case of liners made of foam, may be reduced, with respect to that obtained with an equal mass homogeneous liner, when a higher inner liner density and a lower outer liner density are used;

–           finally the peak linear acceleration can be reduced by using the energy absorption capabilities of the liner more efficiently, by optimizing the balance between absorbed energy and contact force. This is achieved when the foam is in the late plateau stage.

So the indication of the research carried out in recent years is that the liner should not be simply made by a homogeneous materials, but it should be a carefully designed structure, possibly with varying properties across the thickness, capable to optimize energy dissipation. Stiffness and yield stress should vary along the thickness of the liner to increase the dissipation of energy and reduce the force transmitted to the head. Ideally, functionally graded foams could improve the energy absorption of the helmet, in reference [15] Di Landro et. al carried out several experimental tests on helmet liners’ foams and proposed to use functionally graded foams in order to improve helmets’ protection. In another experimental study, Gupta [16] showed that a functionally graded structure could increase the energy absorption up to 500% comparing to a structure with uniform material properties through the thickness.

Moreover, Cui et. al. [17] showed that the protection capability of helmets could be improved by means of using liners with varying mechanical properties through the thickness. However the manufacturing processes to make such foams are not straightforward. Even for the most commonly used foams in safety helmets (such as EPS) which, as energy dissipating materials, have been known for several decades, a precise control of density and mechanical properties still escapes the producers, as quality control of helmet manufacturers reveals.

In this moment in time the most promising way to manufacture liners with controlled mechanical properties through the thickness is 3D printing of lattice structures. The new liner we propose is a polymeric hierarchical lattice structure. The word ‘hierarchical’ indicates that the mechanical properties of the structure are varying in a controlled way according to the designer’s project.

Figure 4: example of hierarchical lattice structure.

In recent years, thanks to advances in 3D printing technology, complex hierarchical lattice structures can be designed and manufactured [18]. Hierarchical lattice structures can be designed to have varying mechanical properties by having different cell sizes through thickness, as shown in figure 4. Another advantage is that the cell size can be different at different points in order to optimize the helmet performance for points which are statistically more susceptible to receive impacts. Moreover, such structures can be designed to respond to the normal and tangential loading differently, therefore, an optimized hierarchical lattice can be achieved in order to mitigate the linear and rotational acceleration of the head, simultaneously.

Our proposal aims at developing new protective devices by evaluating the effects they have on the injury mechanisms inside the human head. The main objectives of the project are:

• Development of three, very detailed, models of human head, for man, woman and teenager
• Development of innovative hierarchical lattice liners for safety helmets
• Virtual assessment of the performance of innovative helmets by evaluating the protection they provide based on the strains in the brain tissues of the human head models.

The most important outcome of the project will be a new generation of helmets, designed according to human needs and not only to pass conventional standard tests, which will provide increased protection to the public and save human lives.