This dissertation defines the material properties of human skin appropriate for evaluating skin injuries due to blunt head impacts. These properties are revealed through a series of stretch tests on excised skin specimens. Constitutive models are provided for five human skin samples taken from different donors. The constitutive relationships are derived from biaxial step-relaxation tests. In each test, loads and displacements are measured in two directions as one edge of a specimen is extended and the other three edges are constrained. The overall elastic stress response of skin is shown to be nonlinear and best described by an exponential strain energy function. Moreover, viscoelasticity is observed in skin with significant decay occurring abruptly. Therefore, a quasi-linear viscoelastic formulation is chosen to describe the constitutive relationship. The five constitutive fits reveal how skin differs from one donor to the next. Significantly more anisotropy appears in some samples than in others. Differences in the skin moduli and the length of the low-stress “toe region” are also highlighted.

A multi-step skin excision procedure is also established to assure the test reference state is consistent with the physiological in vivo state. A curve-fitting procedure is illustrated in which the exponential constitutive coefficients are found from biaxial test data. Using an iteration procedure, low strain rate test data is fit to the model in such a way that the resulting QLV constitutive coefficients may be applied to high strain rate conditions.

Also provided are the material parameters of skin for use in finite element computer analysis. A simplified QLV model appropriate for soft tissues is available in LS-Dyna, a nonlinear finite element code. The LS-Dyna model applies only to isotropic materials, but in certain cases skin is shown to behave as an isotropic material. A procedure is provided in which material coefficients for the simplified LS-Dyna model may be extracted from the more complicated QLV human skin model.