Composite materials are not perfectly brittle, instantaneous failure is not necessarily observed in all or any of the material directions. The stress/strain based failure models available for use in conjunction with the orthotropic model instantaneously set material stresses to zero on failure initiation, as described in Brittle Damage Model.

In this section we report developments [2] which model the softening behavior observed in some composites (see Figure 3.1: Typical In-Plane Stress-Strain Behavior of Kevlar-Epoxy). Maximum tensile and shear stresses in a cell are limited by failure surfaces used to define failure initiation. Different failure modes are considered each of which are described by a unique surface. To model the gradual reduction in the ability of a laminate to carry stress a crack softening approach is used. This reduces the maximum tensile stress that can be sustained in an element as a function of some measure of crack strain.