# Development of an Eco-Friendly Head Impact Protection Device
## 2 Material and Methods  
Three commercial headbands with distinct price tags and material densities and three cork agglomerates were tested. In order to perform mechanical tests, more specifically uniaxial compressions, a representative piece of material was extracted from the commercial headbands, as depicted in Figure 1. Cork agglomerates in three different densities were provided by Amorim Cork Composites (ACC, Mozelos, Portugal). The padding materials present in commercial headbands ranged from polyurethane (PU) and polyethylene (PE) foams to Vinyl Nitrile (VN) or Expanded Polypropylene (EPP). In order to not disclose brand names, commercial headbands were denoted as Headband A, B and C, whereas cork agglomerates kept commercial denominations NL10, 20 and 25. A summary of material densities (average from three measurements) is given in given in Table 4.  
Three samples from each product were tested. They were removed preferably from the forehead area of the headbands; firstly, because the product area is generally larger there; secondly, from Table 3, it is the head zone where impact likeliness is also bigger. At first, a quasi-static uniaxial compression test was performed using a Shimadzu universal testing machine. Given the limited size of the samples extracted it was not possible to follow any standard. Nevertheless, the parameters were chosen carefully as shown in Table 5.  
Figure 1: Commercial headbands and cork agglomerates. Figure 2 presents uniaxial compressive strain-stress curves for all materials under analysis. Tests were repeated at least three times. It became evident that after the initial elastic deformation, the so-called plateau stage was developed under lower stresses for synthetic foams A, B and C. NL10, given its lower density, was the cork agglomerate yielding lower plateau stresses and later densification strain. In fact, NL10 showed compatible mechanical response compared to commercial headbands. NL20 and NL25 showed more stiff behaviour, exceeding stress values of 1 MPa for very low strain values, which were not compatible with an application where impact energy must be sustained at low-stress levels. In fact, NL20 and NL25 were discarded as potential materials for the headband device. By setting a threshold of 50% strain, it was possible to conclude that cork could absorb more energy per unit volume than the synthetic foams used in the market headbands, but at the expense of larger reaction \begin{table}
\begin{tabular}{l c} \hline \hline
**Sample** & **Density (kg/m\({}^{3}\))** \\ \hline
**Agglomerate Cork:** & \\ NL10 & 140.00 \\ NL20 & 193.53 \\ NL25 & 234.12 \\ \hline
**Commercial Headband:** & \\ A & 64.90 \\ B & 309.00 \\ C & 218.00 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Materials densities. Figure 2: Stress-strain curves in quasi-static uniaxial compression test. forces. Subsequently, NL10 was tested against synthetic foams in dynamic conditions. Similarly to quasi-static tests, dynamic tests did not follow any standard due to the samples' limited thickness. Dynamic loading was carried out in a guided drop tower apparatus (Figure 3). The 20 kg impactor was raised to the desired height. An encoder monitored the positions and a uniaxial load cell was connected to the movable system, transmitting reaction force data to the acquisition software. Finally, a Butterworth type filter eliminated part of the noise yielding a smoother curve. Figure 4 presents the dynamic stress-strain curves. Tests were repeated at least three times. Compared to quasi-static ones, the stress-strain curves presented a higher stress level for the same strain values, which is common in cellular materials due to their strain-rate sensitivity between quasi-static and dynamic levels. However, the way each material response varied with the deformation speed was interesting: it was possible to check that material from headband B was much more sensitive to strain-rate than the others.