Mixed-mode (I+ 11) propagation of delamination fatigue cracks in unidirectional graphite/epoxy laminates

Abstract

Mixed-mode propagation behavior of interlaminar fatigue cracks was studied with unidirectional graphite/epoxy laminates, T800H/ 3631. Mixed-mode fatigue tests were conducted by the mixed-mode bending method under a stress ratio of R=0.2. The mixed-mode ratio, Gr/Gn, was 0.030, 0.19, 0.57 and 1.37. When the crack propagation rate da/d2V, was correlated to the maximum value of the total energy release rate, Gmax, the propagation rate increased as the Gi/Gn value increased. For each mixed-mode ratio, the crack propagation rate was given by a power function of the maximum value of the energy release rate in the region of rates above 10-9m/cycle. The exponent of the power function decreased as the Gi/Gn value decreased. Below da/dX= 10-9 m/cycle, there exists the threshold for fatigue crack propagation. At the threshold, the relation between the mode I and mode II components of the maximum energy release rate, Gmax and dHmax, is given by the following equation: GImoxth + GHmax/Giimaxth=l, where GImaxth and GImaxth are the threshold values of the maximum energy release rate for pure mode I and pure mode II loadings.