CivilFEM supports progressive failure analysis for composites and other elastic materials. The material is assumed to be linear elastic up to the point of failure. Failure is indicated by the failure criteria described in the previous section. When failure occurs, the element stiffness is degraded. The material will not heal; the damaged elements keep the degraded properties after unloading.

 

This model uses a selective degradation of the moduli depending on failure mode. The moduli are decreased gradually when failure occurs. Within an increment, it attempts to keep the highest failure index less than or equal to one. Whenever a failure index F larger than one occurs, stiffness reduction factors ri are calculated based upon the value of the failure indices. The incremental contribution to the total reduction factor is calculated as:

 

 

This is done differently for different failure criteria as described below. Six such reduction factors are stored and updated. They are then used for scaling the respective material modulus according to:

 

 

The Poisson’s ratios are scaled in the same way as the corresponding shear modulus.

 

For the maximum stress criterion the reduction factors are calculated separately from each separate failure index: r1 is calculated from the first failure index , r2 is calculated from the second

failure index, etc. Thus, there is no coupling of the different failure modes for these criteria.

 

For the failure criteria which only have one failure index: Hoffman and Hill, all six reduction factors are decreased in the same way, using the smallest of the ri values.