# A Review of Validation Methods for the Intracranial Response of FEHM to Blunt Impacts
## 3 Prominent FEHM and Their Validation
### Replication of Empirical Scenarios in Numerical Environment  
In order to validate FEHM, empirical conditions must be replicated in the numerical environment, namely impacts need to be generated in simulations. A number of methods have been applied to existing FEHM, depending on the experimental data available and the properties of the FEHM in question. Two methods of recreating the impact conditions from [PERSON] et al. [34] are predominantly used. One directly applies impact properties to the FEHM being validated, either via the force history [74; 82; 88] or the head acceleration response [25; 44] provided by [PERSON] et al. [34] for experiment 37. Alternatively, a simple model of the cylindrical impactor is created and impacts the FEHM at the relevant velocity (9.94 m/s for test 37) [77; 78; 80; 81; 86]. However, the impactor is padded and the corresponding material details are not provided. Researchers have modelled the padding as a linear elastic material [73; 80; 86], rubber [81] or foam [77; 47], and adjusted the properties until the impact force produced matches the force-history provided by [PERSON] et al. [34]. The resulting padding properties are published for WSUBIM (elastic modulus \(E=49\) MPa) [73], SUFEHM (Strasbourg University Finite Element Head Model) (\(E=13.6\) MPa, [PERSON]'s ratio \(\
u=0.16\)) [80] and YEAHM (YEt Another Head Model) (\(E=6\) MPa, \(\
u=0.16\)) [86]. Despite the differences in elastic moduli, simulations of experiment 37 on all three FEHM report matching the peak contact force measured empirically. However, WSUBIM uses 7.9 kN as the reference value [72] while SUFEHM and YEAHM state/plot 6.9 kN. Figure 1a, adapted from Figure 1 of [PERSON] et al. [34], demonstrates a peak value of 6.9 kN, in agreement with [PERSON] et al. [80] and [PERSON] et al. [86]. However the 7.9 kN value is listed in a table in [PERSON] et al. [34] (Series I in Table III), introducing some confusion about the matter. [PERSON] et al. [72] further used the frontal pressure response to validate the material choice, rather than the model response, and matched the 141 kPa peak empirical value to within 10% (154 kPa, +9%). This pressure value is agreed on between the plots and tables in [PERSON] et al. [34]. The Wayne State University Brain Injury Model (WSUBIM) [72; 73] and GHBMC [44] had their validation extended beyond experiment 37 to all six cases in the first series of experiments from [PERSON] et al. [34]. The impactor properties were adjusted to change the conditions experienced by WSUBIM. GHBMC scaled the acceleration response provided for test 37 so the peak value matched that given for each experiment and applied that directly to the FEHM skull, defined as rigid. Subsequent experimental work which had FEHM validation in mind during their procedures, including [PERSON] et al. [38], [PERSON] et al. [35; 36], monitored the motion of the skull and provide complete kinematics at the centre of gravity (CG) of the specimen's head. Detailed motion data are similarly provided for the controlled cadaver and in vivo impacts. The kinematics can be applied directly to the FEHM as boundary conditions on an equivalent point in the skull, provided this is defined as rigid. This allows the motion resulting from the impact to be recreated without the need to consider the impactor properties or contact behaviour. Furthermore, as the complete motion of the head is replicated, the constraining properties of the neck do not need to be considered, an aspect thought to be influential for longer duration impacts [79; 99]. This has therefore been the primary method of recreating empirical conditions from [PERSON] et al. [38], [PERSON] et al. [35], [PERSON] et al. [36]and in vivo experimental conditions in the numerical environment. Of the models listed in Table 4, only [PERSON] et al. [78] used an alternative method during the validation of the Nanjing FEHM. [PERSON] et al. [78] continued to model external impactors throughout the validation of the GHBMC.