Comparison of compressive properties of layered syntactic foams having gradient in microballoon volume fraction and wall thickness

The existing functionally graded syntactic foams (FGSFs) are based on creating a gradient of microballoon (hollow particles) volume fraction along the length or thickness of the composite to achieve a variation in density and mechanical properties. However, such an approach has several limitations. Variation in volume fractions of the constituents leads to the possibility of warping or localized swelling of FGSF if it is exposed to varying temperature and moisture conditions. It can also lead to premature fracture due to nonuniform distribution of stress along the microballoon gradient and existence of resin/microballoon rich sides in the material. Hence, a FGSF material needs to be created that is independent of the volume fraction variation. The present study attempts to address this requirement. A FGSF is fabricated that is based on the microballoon wall thickness variation along the specimen length. In this novel approach the volume fraction is available as an additional parameter for controlling the properties of the FGSF. The FGSFs based on volume fraction gradient (VF-type) and on microballoon wall thickness gradient (RR-type) are fabricated in a layered structure, and evaluated for compressive properties. It is observed that the VF-type FGSFs show a sharp drop in stress, on the order of 40-60%, after the peak compressive strength value. However, such feature is not observed in RR-type FGSFs leading to the possibility of gaining better control over strength and energy absorption. The compression of such foams could be continued to 60-75% strain based on the microballoon volume fraction in their structure. The total energy absorption was found to be three to five times higher in RR-type FGSFs compared to VF-type FGSFs and plain syntactic foams.