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Published in Mark J. Mezger, Kay J. Tindle, Michelle Pantoya, Lori J. Groven, Dilhan M. Kalyon, Energetic Materials, 2017
Bahadir Karuv, Seda Aktas, Jing He, Hansong Tang, Constance M. Murphy, Suzanne E. Prickett, Dilhan M. Kalyon
In Equation 8.9, the − sign is used for negative shear stress τyz. The plane Couette flow shown in Figure 8.9 can be used to characterize the shear viscosity of viscoplastic fluids by applying either a constant apparent strain rate (strain-controlled rheometer) or a constant shear stress (constant-stress rheometer). For a no-slip condition, the motion of one of the walls at steady velocity Vw will impose a strain rate, Vw/H, to the fluid that is sandwiched between the two walls. However, as shown earlier, the flow-boundary condition for viscoplastic gels and concentrated suspensions is the wall slip condition (as depicted in Figure 8.8). Under the wall slip condition, the motion of the wall gives rise either to a strain rate that is smaller than the apparent strain rate, that is, dVz/dy<Vw/H, or to plug flow, dVz/dy=0, depending on whether the shear stress that is imposed, τyz, is greater or less than τ0, respectively [13,22–25].
Non-associated flow rules in computational plasticity
Published in G. Swoboda, Numerical Methods in Geomechanics Innsbruck 1988, 2017
In the elasto-viscoplasticity algorithm stress states exceeding the yield surface are permitted, and the viscoplastic strain rates at such a point are defined [5] by the flow rule () ε˙˜vp=γΦ(F)∂Q∂σ˜.
A viscoelastoplastic constitutive model of semi-crystalline polymers under dynamic compressive loading: Application to PE and PA66
Published in Mechanics of Advanced Materials and Structures, 2020
Lizhi Xu, Zhonghua Du, Jiangbo Wang, Chun Cheng, Chengxin Du, Guangfa Gao
Based on the properties of PE and PA66, a dynamic constitutive model is formulated to describe the response of semi-crystalline polymers. Figure 9 shows the structure of the constitutive model consisting of two groups of idealized mechanical parts. The first part consists of an elastic spring in parallel with a viscoelastic component composed of an elastic spring and a dashpot. It is well-known the first part is the standard Kelvin model which can describe the nonlinear viscoelastic response. The second part includes a switch of the stress threshold in parallel with a nonlinear dashpot component and captures the rate-dependent behaviors of the yield stress and plastic deformation. It can be understood that the switch of the stress threshold controls the function of the nonlinear dashpot characterizing the viscoplastic flow deformation. When the stress in the material exceeds the yield stress, the material enters into the irreversible viscoplastic flow deformation.