Directional damage gradient modeling of fiber/matrix debonding in viscoelastic UD composites

Delayed damage due fiber/matrix debonding can be observed in many unidirectional fiber-reinforced composites, for instance those made with Epoxy-based polymeric resins. As the role of the constitutive matrix is to deform and support stresses primarily in shear, adequate viscoelastic modeling must be adopted. We have recently shown that such a phenomenon can be described within the integrity-basis formulation of transverse isotropy that allows for a neat decomposition of the mechanical response into fiber-directional, transverse, and pure shear parts, the latter being used here to capture the aforementioned viscoelasticity. For the modeling of the degradation by debonding, we use in this work an approach based on continuum damage mechanics. More specifically, two ingredients are involved: (i) the use of the gradient of the damage quantity within a higher-order formulation and, (ii) the damage gradient is herein projected toward the direction of the fibers. Indeed, it is mainly observed that debonding paths mostly propagate along the reinforcements. One then speaks of a directional damage gradient formulation.