Delamination as the main defect created during drilling of composite laminate is principally a crack nucleation and propagation phenomenon. The fracture-based investigation is performed to identify the significance of different modes involving in this process. The sensitivity analysis is implemented to evaluate magnitude and importance of each mode. As a result, mode I is a dominant mode while drill point removes the material; however, the crack continues to propagate under pure mode III for a while after drilling due to contact of flutes with spalls. This paper investigates the crack formation process for wide variety of drilling conditions and tool geometries. It is demonstrated that although mode III contributes, its minor effect might be neglected if comparing with fracture mode I. Therefore, it may be vanished as a tool design strategy. It is indicated that chisel edge plays a great role in crack propagation under major mode I; therefore, any further design approach which limits or eliminates opening action of chisel edge decreases delamination significantly. Material removal starting from hole perimeter as well as implementing small predrilled holes (such as action of primary cutting lips in step drill) are examined as solutions based on this approach.

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