Nonlinear finite elements/Homework 11/Solutions

Given:

For small strains, the strain tensor is given by

{\boldsymbol {\varepsilon }}={\frac {1}{2}}\left[{\boldsymbol {\nabla }}\mathbf {u} +({\boldsymbol {\nabla }}\mathbf {u} )^{T}\right]\qquad {\text{or}}\qquad \varepsilon _{ij}={\frac {1}{2}}(u_{i,j}+u_{j,i})~.

In classical (small strain) rate-independent plasticity we start off with an additive decomposition :of the strain tensor

{\boldsymbol {\varepsilon }}={\boldsymbol {\varepsilon }}^{e}+{\boldsymbol {\varepsilon }}^{p}\qquad {\text{or}}\qquad \varepsilon _{ij}=\varepsilon _{ij}^{e}+\varepsilon _{ij}^{p}~.

Assuming linear elasticity, we have the following elastic stress-strain law

{\boldsymbol {\sigma }}={\boldsymbol {\mathsf {C}}}:{\boldsymbol {\varepsilon }}^{e}\qquad {\text{or}}\qquad \sigma _{ij}=C_{ijkl}\varepsilon _{kl}^{e}~.

Let us assume that the

J_{2}

theory applies during plastic deformation of the material. :Hence, the material obeys an associated flow rule

{\text{(1)}}\qquad {\dot {\boldsymbol {\varepsilon }}}^{p}={\dot {\gamma }}{\frac {\partial f({\boldsymbol {\sigma }},\alpha ,T)}{\partial {\boldsymbol {\sigma }}}}

where

{\dot {\gamma }}

is the plastic flow rate,

f

is the yield function,

T

is the temperature, and

\alpha

is an internal variable.