Influence of moisture on the mechanical behavior of a natural composite.

The effects of moisture on the mechanical properties of the spicules of the sponge Euplectella aspergillum have been investigated. Determinations were made with the aid of a dynamic mechanical analyzer in both the static and dynamic modes, as well as imaging of the failed surfaces with scanning electron microscopy. For comparison purposes, melt-grown glass fibers of similar diameters were also studied in both distilled water and seawater. That exposure reduced both the stiffness and strength of the spicules. In addition, the energy required to achieve complete failure decreased in moist environments. The data for the wet spicules in both aqueous media showed decreasing values of energy dissipated until catastrophic failure compared to dry samples. The strength of wet glass decreased when compared with the dry condition, and the elastic modulus was also reduced. The most marked influence of moisture was seen in the damping effects in moist spicule samples that were nearly an order of magnitude larger than the damping of dry spicules. This effect was attributed mainly to plasticization of the thin organic layers.