A total fatigue life model for mode I delaminated composite laminates

Fracture mechanics based total fatigue life model for delaminated composite structures is presented. The model includes the delamination growth in three domains, namely subcritical, linear, and final fracture. The resistance increases due to matrix cracking and fiber bridging in the case of unidirectional composites; and tow splitting, separation, bridging and breaking in the case of woven/braided fiber composites were included through normalization of the applied load (GImax) by the instantaneous resistance value (GIR). The proposed method was applied to mode I loaded woven roving glass/vinyl ester delaminated composite. The ASTM standard mode I fracture test was conducted to determine the resistance (GIR) as a function of delamination extension. The fatigue onset life test was conducted to determine the threshold energy release rate, GIth. Constant amplitude cyclic opening displacement fatigue test was conducted to establish the delamination growth rate (da/dN) as a function of maximum cyclic energy release rate (GImax). The total life delamination growth rate was found to be da/dN=Am(GImax/GIR)(1−(GIth/GImax))D1/(1−(GImax/GIR))D2da/dN=A(GImax/GIR)m(1−(GIth/GImax))D1/(1−(GImax/GIR))D2 where the material constants A, m, D1, and D2 were 0.1, 5.4, 8 and 2, respectively. This equation was verified for a block loading and found to accurately predict the delamination length.