Deflecting mode-I cracks in anisotropic materials

Growing cracks in specimens of natural fiber reinforced composites exhibit a pronounced crack deflection in spite of a single mode-I crack tip loading. To explain this crack growth behavior, new crack deflection criteria are introduced considering the orthotropy of the fracture toughness or crack resistance, respectively, and the elastic anisotropy of these materials. A bifurcation point is predicted in terms of a critical anisotropy ratio, beyond which a mode-I loaded crack will depart from a straight extension, exhibiting an instant kinking. In this paper, the J-integral vector criterion and the maximum tangential stress criterion are exploited, yielding slightly different bifurcation points. The interpolation of measured directional fracture toughnesses appears to play a crucial role for the accurate prediction of a crack path. The point of bifurcation of crack deflection is even uniquely determined by the interpolation coefficient for incipient mode-I cracks in transverse direction. The postulated crack deflection theories are verified, comparing the theoretical predictions to experimental results of crack initiation and growth experiments in compact tension specimens of 45 fiber reinforced composites with a thermoplastic matrix exhibiting different crack resistance orthotropies.