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The allowable defect size for a helicopter rotor hub made of high strength glass fiber reinforced plastic (GFRP) was evaluated based on the damage tolerant design method. The threshold values of the energy release rate range, ΔGth, for fatigue crack propagation were established under the pure and mixed modes of I, II and III for GFRP. An initial delamination crack was located at various positions in rotor hub. The energy release rate range, ΔG, was calculated for each position using the finite element method. The allowable defect size was determined from the threshold values and the energy release rate range, ΔG. The maximum energy release rate ranges of mode II and III, ΔGII and ΔGIII, for an internal elliptic crack were generated at the crack front edge in the centrifugal direction, and in the perpendicular direction. The maximum value of ΔGII is large than that of ΔGIII, but the threshold value of mode III, ΔGIIIth, is smaller than that of mode II. This suggests that, for an internal elliptic crack, the energy release range at the crack front edge in the centrifugal direction is critical for damage tolerant design. The region where the allowable defect size was determined to be the smallest corresponded to the position that showed the peak shear stress in the absence of cracking, which, for the hub in this study, was the center of the plate thickness at the thickness change division.

Damage Tolerant Evaluation of Interlaminar Delamination in a High Strength GFRP Helicopter Rotor Component