Viscoelastic Properties of Hybrid III Head Skin

The biofidelity of the Hybrid III headform in impact is largely dependent on local head geometry and viscoelastic mechanical properties of its polymer skin. Accordingly, for accurate simulation of the ATD headform in computational models, a quantitative understanding of the mechanical properties of skin material is required at a variety of strain rates and strain amplitudes. The objective of this study was to characterize the head skin material of the Hybrid III test dummy for finite deformations and at moderate strain rates for blunt impact simulation using finite element models Head skin material from a single ATD was tested using uniaxial compression. A viscoelastic constitutive model with separable temporal and elastic responses was used to characterize the nonlinear and viscoelastic material behavior. Model parameters were determined using data from a series of relaxation tests up to 30% compressive strain (step displacement and hold for 60 s), and the model was validated using constant strain rate tests (up to 100% 1/s). Rate dependence, hysteresis, and nonlinear elastic material behavior were seen during the testing. The results showed that the relaxation response of the material was strain independent up to 20% strain, with some strain- dependence seen at the 30% strain level. A two-term Ogden hyperelastic model was used for the instantaneous elastic function, and a Maxwell viscoelastic model with five time constants was used for the reduced relaxation function. The temporal response of the material was fit with time constants of 0.01, 0.1, 1, 10, and 100 seconds. The 0.1 second and 0.01 second time constants were associated with relatively large relaxation coefficients, which was noteworthy given that many blunt head impacts occur over these time scales [1-2]. The model was validated up to 30% compressive strain using the constant strain rate tests.