The model is designed for transversely isotropic elastic material defined by five elastic material constants. Typical example is a carbon fiber tow. Axis represents the material principal direction. The orthotropic material constants are defined as
(197)  
(198)  
(199) 
Material orientation on a finite element can be specified with lcs optional parameter. If unspecified, material orientation is the same as the global coordinate system. lcs array contains six numbers, where the first three numbers represent a directional vector of the local axis, and the next three numbers represent a directional vector of the local axis with the reference to the global coordinate system. The composite material is extended to 1D and is also suitable for beams and trusses. In such particular case, the lcs has no effect and the 1D element orientation is aligned with the global component.
The index symbolizes six components of stress or strain vectors. The linear softening occurs after reaching a critical stress , see Fig. 9. Orientation of cracks is assumed to be orthogonal and aligned with an orientation of material axes [3, pp.236]. The transverse isotropy is generally lost upon fracture, material becomes orthotropic and six damage parameters are introduced.

The compliance material matrix , in the secant form and including damage parameters, reads
Damage occurs when any out of six stress tensor components exceeds a given strength
(201) 
(202) 
The point of damage initiation is never reached exactly, one needs to interpolate between the previous equilibrated step and current step to achieve objectivity.
The evolution of damage is based on the evolution of corresponding strain
. A maximum achieved strain is stored in the variable . If
the damage may grow so the corresponding damage variable may increase. Desired stress is evaluated from the actual strain
(203) 
(204)  
(205) 
The damage initiation is based on a trial stress. It becomes necessary for higher precision to skip a few first iteration, typically 5, and then to introduce damage. A parameter afterIter is designed for this purpose and MinIter forces a solver always to proceed certain amount of iterations.
allowSnapBack skips the checking of sufficient fracture energy for each direction. If not specified, all directions are checked to prevent snapback which dissipates incorrect amount of energy.

Borek Patzak