The mechanical behavior of twistless and twisted PET yarns used for tire cord construction has been investigated by means of monotonic and cyclic uniaxial tests conducted at room temperature. Under monotonically increasing strain, the force-strain curves exhibit a characteristic shape, with a change in concavity that appears at strain values below 2% and involves a stiffening behavior for higher strains. A new material model is here developed that includes, in addition to viscosity and plasticity, a nonlinear elastic behavior in the fiber direction aimed at capturing the stiffening behavior. This model is used in numerical simulation accounting for the local anisotropy, defined pointwise by the filament direction, which follows coaxial helices. The material parameters of the proposed constitutive model are identified using the experimental response of monotonic tests on twistless yarns and on a twisted yarn characterized by a specific construction. The model has been subsequently validated by simulating monotonic tests on twisted yarns of different constructions and cyclic tests on twistless and twisted yarns.
Faraci et al. (Fri,) studied this question.