**1. Introduction**

Traumatic Brain Injury (TBI) can occur when the head is suddenly impacted by an object and the reaction forces cause internal tissue damages and alter the normal brain function. Traumatic Brain Injury (TBI) is a major contributing factor to a third (30.5%) of all injury-related deaths in the United States. About 75% of TBIs that occur each year involve concussions or other forms of mild injuries [1].

Recently, worldwide the number of people affected with war related TBI has increased due to terrorism, civil and military conflicts [2, 3]. To minimize the severe illness and mortality resulting from blasts, vehicle crashes, and projectiles, several types of head protective equipment with different material options have been proposed since the end of the 19th century. Starting from the 1960s, multi-layers composite materials became a preferred option for personal armor applications, resulting in improved body armor with lightweight, good protection, flexibility, and improved comfort [4]. On the other hand, padding has been used for improving energy absorption in protective structures, packaging systems, sports equipment, handheld devices, as well as comfort and support systems. Particularly, the interior of motor vehicles has been identified as an area where severe head and neck/spinal injuries can occur in frontal, side, rear, roll over, or oblique impacts. Hence, there is a critical need to reduce occupant injuries, including potential head injury. Several researchers have investigated head impacts with the roof, pillars (A-Pillar, B-Pillar), and support structures [5–9]. For instance, Friedman and Nash [8] have proposed preventing head contact with the vehicle interior through interior padding and increased headroom to prevent serious injury during rollover crashes. Lim [9] investigated the energy absorption characteristic of foam and plastic paddings used for vehicle interior and the head injury performance. Results showed that depending on the type of materials and countermeasure space, the energy absorption and the resulting head injury varied.

However, despite evidence of correlations among impact energy, materials, and head acceleration, all of the above research did not present the influence of padding material and geometry variations on the skull-brain relative motion and the resulting strain and stress values.

In this study, simplified and complex head models with various geometries and materials including the skull, cerebrospinal fluid (CSF), muscle, and neck were considered to better understand and predict the behavior of each part and their effect on the brain response during the impact scenario. The effect of padding thickness was also considered to further analyze the interaction of the vehicle structure and the head. Particularly, the response of the head was evaluated based on the peak and rate of acceleration, strain, and stress at various locations in the brain.
