Experimental evaluation and analytical prediction of effects of skin defect size on the long-term durability of bio-based balsa core sandwich structures

This work focuses on the characterization of the cyclic moisture absorption-desorption aging behavior of balsa wood core sandwich structures with two-dimensional defects in Glass-Fiber-Reinforced-Polymer (GFRP) skins. Experimental investigations and analytical predictions were compared to evaluate the effects of skin defect size on the moisture diffusion kinetics during long-term aging cycles, in terms of the moisture diffusion coefficients and maximum moisture content capacity. Two distinct protocols were employed to investigate the effects of hygroscopic aging history on the moisture diffusion kinetics, including complete-cycle and incomplete-cycle absorption conditions, respectively. The Dual Fickian model proves to be admissible in predicting the moisture absorption behavior, while the moisture desorption process is in good agreement with the classical Fickian model, regardless of the skin defect size. Furthermore, the results reveal that the main moisture diffusion coefficient D I of the Dual Fickian model in the first complete absorption cycle could be doubled with a twofold increase in the skin defect size, while the larger skin defect has resulted in a slight decrease in the moisture diffusion coefficient D during all desorption cycles, which is mainly due to the transport of more bound water. In addition, the predicted maximum moisture content capacity is nearly three times higher for all absorption and desorption cycles as the skin defect size is doubled.