Growth of interface cracks on consecutive fibers: On the same or on the opposite sides?

The growth of fiber/matrix interface cracks (debonds) located on consecutive fibers along the through-the-thickness (vertical) direction is studied in glass fiber-epoxy UD composites. Debonds could appear, along the vertical direction, on the same or on opposite sides of their respective fibers. Determining which configuration is the most energetically favorable to debond growth is the objective of this paper. To this end, two different families of Representative Volume Elements (RVEs) are developed: the first implements the classic condition of coupling of the vertical displacements to model a unit cell repeating symmetrically along the vertical direction; the second uses a novel set of boundary conditions, proposed here by the authors, to represent a unit cell repeating anti-symmetrically along the vertical direction. The model is analyzed in the context of Linear Elastic Fracture Mechanics (LEFM) and the Mode I and Mode II Energy Release Rate are evaluated to investigate crack growth. The calculation is performed using the Virtual Crack Closure Technique (VCCT) in the framework of the Finite Element Method (FEM). It is found that Mode I dominated propagation is favored when debonds are located on the same sides of their respective fibers; while for larger (Mode II-dominated) debonds, Mode II ERR is higher when they lie on the opposite sides. No interaction effect is present when at least two fully bonded fibers are located between the partially debonded ones.