Relação entre o limite de viscoelasticidade linear e o limite de escoamento de um material elastoviscoplástico

Abstract

The yielding of elastoviscoplastic materials is usually associated to two critical quantities that must be surpassed for the material’s breakdown: the yield stress and the yield strain, which define the material’s yield point. Futhermore, the viscoelastic nature of these materials gives raise to another transition induced by shearing, from the linear to the non-linear viscoelastic range. This work aims to elucidate the role of the linear-to-nonlinear transition on the yielding of a commercial elastoviscoplastic material, a hair gel, in rotational rheometers. The material was observed to leave the linear viscoelastic region at critical stresses and strains smaller than those associated to the yield point in constant shear rate, creep and oscillatory stress amplitude sweeps experiments. Besides that, the methods proposed in literature to characterize the yield point led to yield stresses and strains that varied significantly among the types of experiments and among the characteristic time-scales of each one of the experiments performed. The critical quantities that define the linear viscoelastic limit, on the other hand, showed a smaller variation not only with the types of experiments performed, bur also with the different time scales of each one of the experiments performed. Finally, the linear viscoelastic limit evaluated through the shear rate, creep and oscillatory stress amplitude sweeps testes was in the same order of magnitude of the strain reversibility limit measured in recovery experiments. Therefore, one can deduce that irreversible deformations start to take above the linear viscoelastic limit, which can thus be understood as an alternative measure of the yield point for elastoviscoplastic materials.