Evaluation of Fiber-Bridging Effect on Model I Interlaminar Fracture Toughness of Unidirectional CFRP Based on Measurements of Crack Opening Profiles

For evaluating the mode I crack-tip stress intensity factor, KItip, of cracks with fiber bridging in fiber-reinforced plastics, a method based on measurements of crack opening profiles is proposed. The values of KItip and the bridging stress intensity factor, KI bridge, can be separately determined with good accuracy by the boundary element calculation based on measured crack opening profiles. This method was applied to the growth of interlaminar cracks in unidirectional carbon/epoxy laminates, Toray T800H/# 3631, under mode I monotonic loading. The measured value of the crack opening displacement of interlaminar cracks with fiber bridging was smaller than that without fiber bridging. The critical value of the applied stress intensity factor, KI ap, c, increased with crack extension, while the critical value of KI tip obtained from the proposed method took a constant value, KI tip, c, during crack growth. Therefore, KI tip is the true crack-driving force and the true resistance of crack growth is given by a material constant, KI tip, c for the laminates with fiber bridging.