DELAMINATION FRACTURE - EFFECT OF FIBER ORIENTATION ON FRACTURE OF A CONTINUOUS FIBER COMPOSITE LAMINATE

Delamination fracture toughness (DFT) of continuous fiber reinforced resin matrix composites is of significant interest because failure by delamination is considered to be the weak link in the fracture process. In this investigation, the effect of fiber orientation on delamination fracture toughness was examined. A modified version of the monolithic short bar test specimen (the hybrid short bar, HSB) was used to determine DFT in relationship to the orientation of the reinforcing fibers. The fracture toughness was determined for a graphite/epoxy composite laminate in which the crack direction-fiber orientation (CD/FO) angle varied from 0 to 90-degrees. The fracture toughness increased as the CD/FO angle increased. The observed increase in fracture toughness as a function of fiber misorientation was attributed to deviant crack tip behavior. Misorientation of the fibers promoted crack tip bifurcation, deflection, and twisting. Such deviant behavior increased mixed mode fracture character locally at the crack tip; and as a consequence, the near field stress intensity at the crack tip was reduced. Since the local crack tip driving force is reduced, the applied load must be increased to advance fracture in the laminate, hence, an apparent increase in the DFT is observed. Results showed that the fracture toughness increased by nearly a factor of 2 as the CD/FO angle increased from 0 to 90-degrees.