# Ballistic Head Protection in the Light of Injury Criteria in the Case of the Wz.93 Combat Helmet
## 3 Formulation of the Problem  
Based on the literature review [2; 23; 24; 25; 26], it can be stated that the selected parameters for the requirements of bodily injury criteria (including the head) have been developed for car accidents [27; 28; 29]. Figure 4: The mechanism causing blunt force brain injuries. Figure 5: Evolution of helmets from World War II to the present day: (**a**) US Army; (**b**) Polish Army. Adopting these for ballistic impacts requires some correlations. Although there are similar physical characteristics in both cases (a car accident and a ballistic impact), predictions regarding injuries are different. The application of parameters, such as head injury criteria (HIC) or acceleration [14; 30; 31; 32; 33], when assessing a head injury is not the ultimate criterion. The relationship between energy level and injury severity when determining the blunt criterion (BC) [34; 35; 36; 37] should also be taken into account when considering a ballistic impact. The main problem with defining the BC parameter is determining the local curvature of a striking projectile and a head (which greatly complicates the estimation of this criterion [38]) and the probability of determining the extent of trauma suffered in accordance with the Abbreviated Injury Scale (AIS). This scale ranges from 1 to 6 (see Table 3), where 1 is a minor injury seldom requiring medical treatment, and 6 stands for a 100 percent probability of death. Any injury greater than AIS level 3 is considered life-threatening. When a helmet protecting a head is impacted by a projectile, kinetic energy is transferred within a small area and the helmet simultaneously undergoes deformation. Propagation of the deformation (in the form of deflection) in the rear part of a helmet is defined as a BFD, and may be brought into contact with a part of the skull, causing extensive load [39; 40]. Shock waves affecting the helmet surface transfer the load further inwards and indirectly influence the skull. The significance of this influence is the result of the deformation value of the helmet's shell [41]. As the BFD parameter increases, the risk of serious head injuries in the form of TBIs [42; 43] and skull fractures also increases. This type of helmet is designed to reduce traumatic brain injuries (TBIs), and the criterion for assessing the protective properties of this TBI-based helmet relates to medical aspects of the brain injury. Some studies [44; 45] have undertaken in-depth analysis of the correlation between head kinematics and the response of brain tissue. This effect often manifests itself after a long delay (see Figure 3) and requires a neurosurgical assessment. In this article, the TBI parameter has been accentuated due to its importance in a comprehensive assessment of head injury. Authors [46; 47; 48] have indicated that such injuries require analysis using a medical approach, but this article is limited to the identification in mechanical and biomechanical terms, leaving aside the medical aspect. The first studies [49; 50] focused on determining the value of the force required to cause a skull fracture with deflection off the frontal bone. The author of the work [49] carried out a preliminary study involving shooting at the helmet surface with 9 mm ammunition in order to estimate the BC for a head. Initial results confirmed assumptions made about the possibility of experiencing a blunt head injury from BFD impact. However, this problem requires accurate analyses because much uncertainty remains. Estimating the BC for a head constitutes a complex problem. Determining the scope of a helmet's BFD (the value specified in the National Institute of Justice (NIJ) Standard for Ballistic Helmets) as the contact geometry (the curvature of contact between a helmet and a projectile) poses considerable problems associated with determining the actual diameter of the surface [51] (see Figure 6).