S = 2 * eta * D

where S is the extra-stress tensor, D is the rate of deformation tensor, and eta is the viscosity.

The temperature dependence of the viscosity is denoted by a function H(T), where T is the temperature.
H(T) comes as a factor in the viscosity law which is written as follows :

eta (g,T) = eta (g * H(T)) * H(T)

For the "Arrhenius shear stress" law, the function H(T) is written as follows :

H(T) = exp [ alpha / ( T - T0 ) - alpha / ( Talpha - T0 ) ]

where alpha is the ratio of the activation energy to the Blotzman constant, Talpha is a reference temperature,
and T0 is a scaling temperature when non-absolute temperature scales are used.

This law differs from the original Arrhenius law as follows : a time-temperature equivalence has been introduced
by scaling the shear rate g by the temperature dependence H(T). The same scaling factor H(T) therefore affects
the shear rate in the first equation above.

By default, alpha = Talpha = T0 = 0.
 
 

A high value of the parameter alpha leads to strong non-linearities. A Picard iteration is recommended
instead of a full Newton iteration. In case the program diverges, an evolution scheme (on alpha) must be used.

The units for alpha, Talpha and T0 are :
 

parameter	mass	length	time	temperature
alfa	-	-	-	1
talfa	-	-	-	1
t0	-	-	-	1