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In current Finite Element (FE) head models, brain tissue is commonly assumed to display linear viscoelastic material behaviour. However, brain tissue behaves like a non-linear viscoelastic solid for shear strains above 1%. The main objective of this study was to study the effect of non-linear material behaviour on the predicted brain response. We used a nonlinear viscoelastic constitutive model, developed on the basis of experimental shear data presented elsewere. First we tested the numerical implementation of the constitutive model by simulating the response of a silicone gel (Sylgard 572 A&B) filled cylindrical cup, subjected to a transient rotational acceleration. The experimental results could be reproduced within 9%. Subsequently, the effect of non-linear material modelling on computed brain response was investigated in an existing three-dimensional head model subjected to an eccentric rotation.

Linear viscoelastic material parameters of porcine brain tissue and two brain substitute materials for use in mechanical head models (edible bone gelatin and dielectric silicone gel) were determined in small deformation, oscillatory shear experiments. Frequencies to 1000 Hertz could be obtained using the Time/Temperature Superposition principle. Brain tissue material parameters (i.e., dynamic modulus (phase angle) of 500 (10°) and 1250 Pa (27°) at 0.1 and 260 Hz, respectively) are within the range of data reported in literature. The gelatin behaves much stiffer (modulus on the order of 100 kPa) and does not show viscous behavior. Silicone gel resembles brain tissue at low frequencies but becomes more stiffer and more viscous at higher frequencies (dynamic modulus (phase angle) 245 Pa (7°) and 5100 Pa (56°) at 0.1 and 260 Hz, respectively).