Influence of water on mechanical properties and fracture mechanism of CFRP/aramid honeycomb core sandwich beam

Carbon fiber reinforced facing/aramid honeycomb core sandwich beams were studied through three point bending test. In order to clarify the influences of water on the stiffness and strength, the dry and wet specimens were used. At the same time, the fractured specimens were inspected by a scanning electron microscope (SEM) and a scanning acoustic microscope (SAM) to observe the fracture surfaces and interior damages of the specimens, respectively, and the fracture mechanism was discussed. The experimental results showed the following. (1) With 6wt.% water absorption the flexural stiffness of the sandwich composites rises a little while the shearing stiffness falls by 11%. (2) The core shearing strength is more sensitive to water absorption than the facing one. For a 3wt.% absorption ratio, the facing strength and core shearing strength are 9% and 18% lower, respectively. However, the decrease has a tendency to level off over 4wt.% absorption ratio. (3) The bonding strength between the facing and the core is larger than the matrix. (4) The buckling axes observed in the compressive fracture surfaces are more seen in wet specimens than dry ones, which shows that the fibers are liable to buckle in wet specimens. (5) At a small span length (< 200mm), the wet specimens deflect more than the dry specimens, which is due to the degradation of the aramid fiber paper of the cell wall.