Effects of ageing on mechanical properties of stiffened syntactic foam core sandwich composites for marine applications

Foam core based sandwich composite materials are extensively used in marine sectors because of its high strength and stiffness to weight ratios. The structural materials used for marine applications should possess good damage tolerance capability. Hence, it is essential that these materials shall be tested for their residual mechanical properties when exposed to marine environment. In the present work, syntactic foam is prepared by uniform mixing of dry fly ash cenosphere and phenolic resin in equal proportions. Syntactic foam is further stiffened by integrating it with honeycomb structure during manufacturing. Sandwich composites are developed with core of syntactic foam (with and without honeycomb structure) and face skins of glass/epoxy composite. Sandwich coupons are prepared in two batches; one being subjected to ageing in natural sea water and other under accelerated environment. Both aged and unaged coupons are subjected to mechanical tests to determine their residual properties under compression, flexure, and low-velocity impact as per ASTM standards. Results showed that the moisture absorption is significant up to about 60 days beyond which it is marginal. The saturation level was attained for an immersion period of about one year, at which the material exhibited significant damage, at the interfacial regions of core-skin, cenosphere-phenolic resin, and fiber-matrix. Ageing of sandwich composites under sea water and accelerated environment has shown detrimental effect on their mechanical properties. However, the extent of degradation in properties due to ageing can be reduced by the incorporation of resin impregnated honeycomb structure in syntactic foam. Microscopic features of aged coupons are also investigated to predict mode of damage due to ageing.