Polyurethane resins and foams are finding extensive applications. Seat cushions and covers in automobiles are examples of these materials. In the present work, hollow alumina particles are used as fillers in polyurethane resin to develop closed-cell syntactic foams. The fabricated syntactic foams are tested for compressive properties at quasistatic and high strain rates. Strain rate sensitivity is an important concern for automotive applications due to the possibility of crash at high speeds. Both the polyurethane resin and the syntactic foam show strain rate sensitivity in compressive strength. It is observed that the compressive strength increases with strain rate. The energy absorbed up to 10% strain in the quasistatic regime is 400% higher for the syntactic foam in comparison to that of neat resin at the same strain rate. 1. Introduction A class of closed cell porous composites called syntactic foam is prepared by dispersing hollow filler particles in a matrix medium [1, 2]. The mechanical (compressive, tensile, and flexural) [3, 4], thermal (coefficient of thermal expansion (CTE) and thermal conductivity) [5–7], and electrical (dielectric constant) [8–10] properties of syntactic foams can be tailored based on the volume fraction and the wall thickness of the reinforcing hollow spherical filler particles. Epoxy, vinyl ester, polyester, and bismaleimide resins are examples of polymers that have been used previously in syntactic foams [2, 11]. Lightweight metals such as magnesium and aluminum are also extensively used in fabricating syntactic foams [12]. The density of metal matrix syntactic foams can be as low as 1？g/cm3, whereas the densities of commonly fabricated polymer matrix syntactic foams (PMSFs) are in the range of 0.4–0.8？g/cm3. Hollow glass microballoons (HGM), fly ash cenospheres, and carbon and polymer hollow spheres are widely used as fillers in syntactic foams [2, 11–13]. Interest in developing high performance syntactic foams has resulted in development of hollow particles of ceramics such as SiC [14, 15] and Al2O3 [16]. CTE of hollow glass microballoon (HGM)/vinyl ester syntactic foams was found to be 60.4% lower than that of the matrix resin. In comparison, vinyl ester matrix syntactic foam containing SiC hollow spheres has CTE up to 79.3% lower in comparison to the matrix resin [14]. Thus, with the development of the ceramic hollow spheres, properties of syntactic foam beyond the commonly used HGM can be obtained. Elastomeric matrices have not been commonly used in fabricating syntactic foams and only a few studies are available on
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