# A Review of Validation Methods for the Intracranial Response of FEHM to Blunt Impacts
## 1 Introduction  
Mild Traumatic Brain Injury (mTBI) is widely known to have a significant cost in society [1; 2; 3] but, compared to injuries in other parts of the body, is not well understood. As such, head impacts and injuries are continuously researched, particularly in relation to public and automotive safety and sporting injuries. Over the past two decades or so, Finite Element Head Models (FEHM) have become a key tool in investigating the intracranial effects of blunt force impacts and studying potential injury risk factors. Numerous FEHM have been developed, representing the brain and surrounding tissues in a number of ways, the most prominent of which have been comprehensively reviewed [4; 5; 6; 7]. Background theory around modelling the human head is also well documented [8; 9], however there are still challenges associated with modelling the human head, particularly relating to the representation of the brain tissue and the interface between the brain and the skull. Brain tissue has complex mechanical properties and presents significant challenges during testing. As such, identification of its mechanical behaviour and subsequent constitutive modelling is an extensively discussed topic [10; 11; 12; 13; 14]. In relation to FEHM, the result of this is that a range of viscoelasticand hyperelastic material models have been implemented to govern the behaviour of the brain. Within a single model type, characteristic material properties can vary by an order of magnitude and it has been shown that both the type of material model [12; 15] and the material properties within [16; 17] can have significant effects on the impact responses of FEHM. The treatment of the interface between the brain and the skull also has a significant influence on the impact response experienced by the brain. The complex system of membranes and cerebral spinal fluid (CSF) forming this interface is necessarily simplified in FEHM. While research has been conducted into including genuine fluid representations [18; 19; 20], the CSF has generally been modelled as a combination of contact algorithms and \"fluid-like\" solids [5; 6; 7]. As with the brain tissue, various factors in the CSF/brain-skull interface representation have been shown to affect the impact response of the brain in FEHM [21; 22; 23; 24; 25]. As a result of these uncertainties, there is a lack of a \"gold standard\" or uniformly accepted way of representing the major components of FEHM. Furthermore, design choices such as the level of tissue differentiation [21], the overall size [21; 26] and geometry [27; 28; 29] of the model and the simulation environment and element formations used [30] affect the FEHM performance characteristics. Validation of the complete assembly is therefore essential, especially when considering the intracranial response of models; the key point of interest when investigating mTBI and concussion type injuries using FEHM. However, despite following similar validation processes, studies comparing the responses of FEHM have found the reactions of different models to similar impacts to be incomparable [17; 31; 32; 33]. This review aims to assist researchers in making informed choices during the development and validation of future FEHM by establishing the range of possible tools available to them. Furthermore, by including research efforts and discussions around the methods previously applied during FEHM validation and approaches to assessing model performances, this review hopes to encourage more consistency in validation protocols, moving towards best practices within the materials available. While other factors in FEHM may cause differences in behaviour, collating the information currently available around FEHM validation may improve understanding of the contribution of validation procedures in this. Previous reviews [4; 5; 6; 7] and instructional texts [8; 9] have largely focused on providing information and opinions on the properties within FEHM.