History
The isostrain assumption states that all crystals of the body deform exactly as the entire body.
It is often also referred to as »full constraints (FC) Taylor« assumption as Taylor (1938) first applied it for the prediction of the deformation behavior of polycrystals.
In the framework of finite strain, the isostrain assumption can formally be written as equality of the deformation gradient of each grain $g$ with the average deformation gradient of the body $ \cal{B} $
\begin{equation}
\label{eq: isostrain}
\tnsr F^{g} = \bar{\tnsr F}\quad \forall \; g \in \cal{B}
\end{equation}
or in rate form
\begin{equation}
\label{eq: isostrain rate}
\dot{\tnsr F^{g}} = \dot{\bar{\tnsr F}}\quad \forall \; g \in \cal{B}
\end{equation}
at any time.
Stress response
Typically, a different stress $\tnsr P^g$ will be required in each grain $g$ to obtain the prescribed deformation.
This, for instance, can be due to anisotropy or different constitutive behavior (strength) of the grains.
The isostrain scheme, therefore, usually violates stress equilibrium among the constituent grains.
grain average (default)
The stress at the material point is calculated as the average of the stresses of all grains:
\begin{equation}
\label{eq: stress average}
\bar{\tnsr P}= \sum_{g=1}^{N}\nu^g\tnsr P^g
\end{equation}
with $\nu^g = 1/N$ the (constant) volume fraction of grain $g$.
This is the default behavior for the isostrain homogenization scheme.
grains in parallel
In case all grains act in parallel, the stress at the material point is taken as the sum of the stresses of all grains:
\begin{equation}
\label{eq: stress sum}
\bar{\tnsr P}= \sum_{g=1}^{N}\tnsr P^g.
\end{equation}
Material configuration
Parameters
To select the isostrain homogenization scheme and set the above parameters use the following (caseinsensitive) naming scheme in a material.config file:
key 
value 
comment 
type 
isostrain 

Ngrains 
$ N $ 
number of grains (of equal volume) at material point 
mapping 
avg mean average 
stress calculation according to \eqref{eq: stress average} 

sum parallel 
stress calculation according to \eqref{eq: stress sum} 
Outputs
key 
output 
(output) Ngrains 
report $N$ at material point 
Boolean flags
flag 
comment 
/echo/ 
copy whole section to output log 
References
 [1]
 Taylor, G. I.
Plastic strain in metals
J. Inst. Metals 62 (1938) 307324