Unusual mechanical response of carbon black-filled thermoplastic elastomers

• Unusual mechanical phenomena are observed in TPEs under cyclic loading.
• Novel constitutive equations are derived in finite viscoplasticity.
• Ability of the model to predict these phenomena is confirmed by simulation.

Observations are reported on carbon black-filled thermoplastic elastomer in multi-step uniaxial tensile cyclic tests with various strain rates at room temperature. Experimental data reveal several unusual features of stress–strain diagrams: (i) fading memory of deformation history in cyclic tests with increasing maximum elongation ratios, (ii) transitions from simple to mixed to inverse relaxation and creep in specimens subjected to stretching and subsequent retraction down to various minimum stresses, and (iii) strain rate-induced acceleration of inverse relaxation and creep. A constitutive model is developed for the time- and rate-dependent response of polymer composites under an arbitrary three-dimensional deformation with finite strains. Adjustable parameters in the stress–strain relations are found by fitting the experimental data. Ability of the constitutive equations to describe the mechanical behavior of thermoplastic-elastomer composites under cyclic deformation and to predict the observed phenomena is confirmed by numerical simulation.