# Evaluation of Impact Energy Absorption by Natural Fiber Composites in Motorcycle Helmets
## 2 Experimental Method for Impact Absorption
### Impact Absorption Test  
The gravitational acceleration exerted on the test head, along with the helmet, during a guided free-fall test, was used to assess the helmet's impact absorption capabilities. During the test, the helmet assembly was dropped from specific heights, achieving an impact speed of (7.00 \(\pm\) 0.15) m/s on a flat surface and (6.00 \(\pm\) 0.15) m/s on a hemispheric surface. The ABNT standard NBR 7471:2015 regulates the maximum allowable acceleration to ensure user safety. Notably, this maximum acceleration is expressed in multiples of \"\(g\)\", where \"\(g\)\" is equivalent to 9.80665 m/s\({}^{2}\). The regulations stipulate that acceleration must not exceed 300 \"\(g\)\" at any point and must not surpass 150 \"\(g\)\" for more than 5 ms [31]. The ABNT NBR 7471:2015 standard aligns closely with the testing principles outlined in UNECE Regulation 22.06 [32], which defines uniform requirements for the approval of protective helmets and visors for motorbike and moped users. Both standards assess the helmet's impact absorption capacity using a guided free-fall test that measures the gravitational acceleration exerted on a test head equipped with the helmet. However, each standard incorporates adaptations to address the specific conditions and regulatory requirements of its respective region. The absorbed energy was calculated following the procedures outlined in ECE Regulation No. 22.05 [33]. The acceptance criteria, originally established in ECE Regulation No. 22.05 and later incorporated into the NBR standard, include a maximum resultant acceleration of 275 g (ECE) or 300 g (NBR) and a maximum HIC value of 2400, which is exclusive to the ECE criteria. During the impact absorption tests, the absorbed energy was evaluated based on the acceleration data recorded at the center of gravity of the head form during the impact. The assessment included the calculation of the Head Injury Criterion (HIC) using Equation (1):  
\[HIC=\max_{t_{1},t_{2}}\left((t_{2}-t_{1})\left[\frac{1}{t_{2}-t_{1}}\int_{t_{ 1}}^{t_{2}}a(t)\,dt\right]^{2.5}\right) \tag{1}\]  
where \(a(t)\) represents the acceleration measured in multiples of the acceleration of gravity (\(g\)) and \(t_{1}\) and \(t_{2}\) represent the time limits used for the calculation. The results comply with the acceptance criteria established by the standards. This approach ensures both the standardization and reliability of the impact test results. The experimental tests were conducted using a vertical impact testing machine, model MAU 1006, manufactured by AD Engineering, located in Bergamo, Italy. Figure 3 shows that this equipment is equipped with an integrated computer system that automatically records test results, including the acceleration of helmet head assembly, expressed in units of \(g\) (9.81 m/s\({}^{2}\)), and the impact duration in milliseconds (ms). The testing machine handles the entire measurement and processing steps, including the application of Equation (1), and yields the acceleration value corresponding to the HIC in units of \(g\). In this procedure, a test head with a circumference of 62 cm and a mass of 6.1 kg (\(\pm\)0.18 kg) was employed, chosen for its larger size and mass to simulate more severe impact conditions and represent a critical scenario for evaluating impact energy absorption. This impact head is equipped with a triaxial piezoelectric accelerometer positioned at its center of gravity, featuring an integrated amplifier and power source, along with three low-pass filters that have a cutoff frequency of 2 kHz and a nominal sensitivity of 1 mV/(m/s\({}^{2}\)), as illustrated in Figure 4.  
The test was conducted from a standard height of 50 cm, commonly utilized in interlaboratory comparisons. The drop begins from point \(P\), located at the upper rear of the test head. This area is of utmost importance because it is a location that might be severely damaged during testing.