A Quasilinear Viscoelastic Model for Brain Tissue

The nonlinear viscoelastic behavior of brain tissue that was observed in oscillatory shear tests was modeled with a quasilinear viscoelastic constitutive relation. The dynamic viscoelastic behavior of brain tissue has been previously modeled with only linear models. The reader is referred to Darvish (2000) for a review of the previous models. Studies on live neural tissue (Thibault et al., 1990) and physical head models with brain surrogate (Margulies, 1987) indicate that brain undergoes finite deformation prior to traumatic injuries. Therefore, it is necessary to use the nonlinear theories to model its mechanical behavior in injurious loading conditions. Recently, the nonlinear viscoelastic behavior of brain tissue at finite strains has been characterized using the results of stressrelaxation tests (Prange et al., 1998). These models, due to the limitations of the test procedure, are not valid for short-time loadings (below 60 ms) that are observed in the events that might lead to traumatic brain injury, e.g., automotive crashes and ballistic injuries. Using the forced vibration method, the brain constitutive model can be improved by broadening its range of validity at the lower end down to about 1-ms (Darvish et al., 1998).