Tese: An experimental study of the validity of the Von Mises yielding criterion for elasto-viscoplastic materials
Aluno(a) : Luiz Umberto Rodrigues SicaOrientador(a): Paulo R. de Souza Mendes e Roney Thompson
Área de Concentração: Termociências
Data: 29/09/2017
Link para tese/dissertação: http://doi.org/10.17771/PUCRio.acad.52741
Resumo: In the first part of the present research, we are going to reinforce that normal stress differences are not negligible and should be taken into account in material properties evaluation. It is usual practice in rheology to measure the apparent yield stress in simple shear flow. In these measurements, the apparent yield stress is identified as the maximum value of the shear stress below which no irreversible flow occurs. Then, the thus determined apparent yield stress is used in conjunction with the von Mises criterion in any complex flow. The von Mises criterion compares it with the intensity of the deviatoric stress tensor. It happens that for simple shear flow the intensity of the deviatoric stress is composed of both the shear stress and the normal stress differences, but the contribution of the latter is never considered in the experimental determination of the apparent yield stress. In view of assess the importance of the contribution of the normal stresses to the apparent yield stress, a sequence o standard constant shear stress tests were performed for each material, estimating the critical stress which represents the mean value obtained between the stress values of the curves in which the material flows and does not flow with an accurate tolerance. After that, a proposed test to evaluate the proper values of the normal stress differences was performed for each material, giving the value of N1-N2 at the critical stress value previously obtained. Following, with those results the intensity of the second invariant of the deviatoric stress tensor was evaluated for two cases considering and neglecting N1 and N2 for each material. It was observed that for some materials the normal stress contribution is much larger than the shear stress contribution. Furthermore, the validity of the von Mises yielding criterion for elasto-viscoplastic materials was evaluated analyzing the following aspects: (i) if the Bauschinger effect is negligible, and (ii) if the direct relation in which the magnitude of the apparent yield stress in shear flow is sqrt{3} lower than the apparent yield stress in tension/compression. For this purpose, in order to generalize the study for different flow conditions, constant volume squeeze flow and traction tests were performed evaluating the compressive and tensile critical stresses. After that, the intensity of the deviatoric stress tensor was calculated for each flow and material. Besides, no-slip and partial slip boundary conditions were considered for the constant volume squeeze flow, and as a remarkable result, it was possible to obtain the static equilibrium state for several materials estimating the compressive apparent yield stresses. The author is not aware of the existence of these experimental results. As the most important conclusion, the von Mises yielding criterion was considered not accurate representing yielding of elasto-viscoplastic materials. However, it stills an amazing approximation for metal and widely used in civil engineering.