Two-dimensional fatigue debonding in GFRP/balsa sandwich panels

An experimental investigation of the two-dimensional (2D) debonding behavior of GFRP/balsa sandwich panels with embedded circular disbonds at the face sheet/core interface subjected to out-of-plane fatigue loads was conducted. The constant amplitude fatigue experiments were performed under load control and different R-ratios. Two face sheet configurations were investigated: a pure woven ply layup (SPA) and a combination of continuous filament mat (CFM) and woven plies (SPB). For the latter, the CFM layers (which are prone to develop fiber-bridging) were placed above and below the woven plies. In contrast to one-dimensional (1D) beam-like fatigue fracture experiments, decreasing crack growth rates and stable crack propagation were achieved under load control as a result of the 2D growth of the disbonds. The fatigue fracture performance obtained under R = 0.1 for the SPB configuration was less efficient than for the SPA configuration owing to the fiber-bridging crushing associated with high fatigue amplitudes. Under higher R-ratios, a reduced amount of fiber-bridging crushing was observed, leading to a corresponding crack arrest and much longer life. The load-displacement hysteresis loops exhibited an increase in the cyclic stiffness within each fatigue cycle throughout the experiments. This stiffening was mainly caused by in-plane stretching stresses that developed in the debonded part of the face sheets whose magnitude and evolution were evaluated based on the in-plane strains monitored during the experiments.