Effect of Internal Ribs on Fatigue Performance of Sandwich Panels with GFRP Skins and Polyurethane Foam Core

This paper investigates the cyclic fatigue behavior of sandwich panels composed of glass fiber reinforced polymer (GFRP) skins connected by longitudinal $Z$-shape ribs and low-density polyurethane foam core. Eighteen $\mathrm{1,143}\times 635\times 78\text{-}\mathrm{mm}$ panels with ribs of two different flange widths, including three control static tests, were tested in fatigue under fully reversed loading ($R=-1$) and fully unloaded ($R=0$) conditions, to maximum loads ($P_{\text{max}}$) of 20–70% of their ultimate static strength ($P_{\text{ult}}$). Fatigue life curves were established and compared with those of panels without any ribs. It was shown that internal ribs increased static strength and stiffness by 80 and 66%, respectively, but were only beneficial within the low cycle fatigue range. Under $P_{\text{max}}$ of 50% $P_{\text{ult}}$, number of cycles to failure ($N_f$) at $R=-1$ was only 6% of $N_f$ at $R=0$ in the ribbed panels. Fatigue failure consistently initiated by flange debonding of the rib from skin. In panels loaded to higher than $P_{\text{ult}}$ of their nonribbed counterparts, final shear failure of foam core occurred simultaneously with rib debonding, whereas in panels loaded below this level, additional fatigue life was attained after rib debonding, until core shear failure, but at significantly lower stiffness. To achieve an $N_f$ of 2M cycles, $P_{\text{max}}$ should not exceed 35 and 21% of $P_{\text{ult}}$ at $R=0$ and $-1$, respectively. A three-dimensional (3D) failure surface (Haigh diagram) was established for the ribbed panels. It predicts fatigue life for a given mean and amplitude loads.