Static, dynamic and microstructural characterization of basalt short-fiber reinforced hybrid syntactic foams

Sandwiched structures with foam core and fiber reinforced composite skins may have a great potential in pipe and pressure vessel structures due to their lightweight, competitive material cost, and thermal and acoustic insulation effects. As core materials, syntactic foams have gained significant importance in sandwich structures due to their high energy absorption, specific compressive strength, better damage tolerance, and low moisture absorption. A major concern of the syntactic foam, as core material, is its brittleness (low toughness). One way to improve toughness or ductility is to incorporate high density fibers into the foam system. The fibers employed might be short or long depending upon the potential application of the composite. It is also noted that the short distributed fiber reinforced composites exhibit wider applicability and more isotropic characteristics. In the current study, short basalt fibers were used as reinforcing phase to toughen a syntactic foam material with a microballoon (glass) volume fraction of 40%. Three different basalt fiber reinforced syntactic foam (BFRSF) structures were fabricated and tested. Quasi-static compressive, notched three-point bending tests were performed on the three types of foams and the results were compared. Further, to characterize the dynamic mechanical properties of the short fiber reinforced syntactic foams, impact and dynamic mechanical analysis were conducted. In order to measure the global fracture behavior (fracture toughness) and local fracture behavior on the notched three-point bending test specimen, two inclinometers were employed to record the rotation of the notched beam. Based on a moment-rotation based formulation, the energy release rate (ERR), was calculated, which reflects the fracture toughness of the syntactic foam material. Meanwhile, a CCD camera was incorporated to capture the crack tip opening displacement (CTOD). With the measured local crack tip separation and the J-integral theory, the local fracture behavior (cohesive law) was determined. In this study, the test results showed that the compressive strength slightly decreased with inclusion of short basalt fibers. The notched three point bending test indicated that even with a very low basalt fiber volume fraction (0.25% and 0.5%), there was dramatic increase in the syntactic foam's tensile strength, ductility and toughness. Low velocity impact tests also demonstrated that the impact energy absorption and the maximum load bearing capability can be improved significantly. Finally, dynamic mechanical analysis (DMA) was conducted to characterize the dynamic mechanical properties and damping properties.