Estudo dos limites de escoamento e de viscoelasticidade linear de material tixotrópico com tensão limite de escoamento

Abstract

Yield stress materials are characterized by presenting predominantly elastic behavior for low mechanical stress and predominantly viscous behavior for high mechanical stress. The yield stress is usually used to characterize the transition between the behavioral states and this value may be related to the linear viscoelastic limit, which defines the nonlinearity region of stress and strain amplitudes in oscillatory tests. Therefore, these transitions were experimentally studied from different rheological tests with bentonite suspensions. Critical values of shear stresses and strains were obtained from flow curve, flow start-up, creep, oscillatory stress amplitude sweep and recovery experiments for different resting times. The shear stresses obtained by the flow curve test presented a minimum value for a critical shear rate, which suggests a maximum limit of the occurrence of the shear banding. It was proposed a methodology to obtain the yield stress from the flow curve taking into account the occurrence of shear banding. The yield stress extrapolated to the limit of low shear rates was also obtained. The two approaches to obtain the yield stress from the flow curve provide the dynamic yield stresses values associated with a stress required to keep the material flowing. The dynamic yield stresses presented smaller values than the static yield stresses, obtained from the experiments of flow start-up, creep and crossover of the dynamic modules in an oscillatory stress amplitude sweep. Static yield stresses work with the stresses required to initiate the flow of a previously resting material, those are increased with resting time. The results of critical strains showed less influence of resting time and the characteristic time of the experiment than the critical stresses. Linear viscoelastic limits were obtained from the Fourier transformation of the strain results of oscillatory stress amplitude sweep tests. The linear viscoelastic limits presented similar results for the elastoplastic regions, obtained by the recovery test, showing that the linear viscoelastic limit and the elastic limit are intrinsically linked.