# Evaluation of Combat Helmet Behavior under Blunt Impact
## 1 Introduction  
One of the most significant challenges of the defense industry for improving the protective capacity against different load states is focused on the combat helmet. During the different conflicts in which the U.S. Army has participated since 2000, a large number of traumatic brain injuries (TBIs) have been observed as a result of more than 360,000 injuries, where 82% were classified as minor (mTBIs) [1; 2]. Soldier activity in combat scenarios supposes a high exposure of the head, where it is easy to suffer falls, collisions, or even traffic accidents or failed parachute jumps, and in all cases, the most typical type of injuries are mTBIs, in which brain diffuse injuries, concussions, and contusions are the most common consequences [3; 4]. However, it is not a trivial task to improve the protective performance of a combat helmet without losing the real purpose of using composite materials, which is a decrease in weight. Traditionally, combat helmets have to be subjected to ballistic standards such as STANAG 2920 [5] or NJI 0106.01 [6] and, consequently, they are analyzed at high impact velocities for the different ammunition [7; 8; 9; 10; 11; 12], also under blast loadings [13; 14; 15; 16; 17; 18]. However, the military industry has not focused on analyzing thebehavior of the combat helmet under low-velocity impact. This type of analysis is more common in the motorbike or bicycle industry because helmets must satisfy the requirements established by the ECE R22.05 [19] or EN 1078 [20] standards, respectively. Few studies have analyzed the performance of the combat helmet under low velocity impacts and its response over the user. Thus, our study is focused on verifying the performance of a current combat helmet configuration and improving the knowledge of combat helmet response under blunt impacts  
[PERSON] et al. [21] carried out experimental low velocity impact tests of a combat helmet mounted on a combined NOCSAE head model with two different types of necks, i.e., HIII and EuroSid-2 dummy. They made impacts in the rear and lateral areas at 3 m/s, obtaining data related to the maximum longitudinal and rotational accelerations measured at the center of gravity of the head model. The data served as input for a validated simulated injury monitor (SIMon) of the brain for analyzing brain strain. They designed this methodology to analyze the consequence of rotational accelerations on brain damage. [PERSON] et al. [22] developed a combined combat helmet design improvement methodology that took into account different parameters such as the size of the foams, the tension of the straps, the impact location, or the impact velocity. This methodology combined a validated finite element model with experimental tests that simulated the combat helmet drop tower tests. From the results, peak linear acceleration was obtained. This parameter was used as input for a statistical model to optimize helmet design. In 2012, [PERSON] and [PERSON] [23] realized an investigation for the Natick Soldier Research, Development and Engineering Centre in which they tested different foam materials and configurations for the design of combat helmets. They developed a finite element model in LS-Dyna, validated previously with experimental test, which combined different foam configurations, and subjected the helmet to different velocity and impact locations. [PERSON] and [PERSON] [24] analyzed the performance of different combat helmets used by the U.S. Army, i.e., the advanced combat helmet (ACH), and Personnel Armor System for Ground Troops (PASGT). Impact series tests were carried out at two impact velocities, three ambient temperatures, and seven different impact locations, and the peak linear acceleration produced from each one was obtained. The results obtained for both helmets were compared with the recommended threshold for maximum acceleration. In a study carried out by [PERSON] et al.