Gradient syntactic foams: Tensile strength, modulus and fractographic features

Syntactic foams are new age polymer composite materials. They find use not only in aerospace but also in naval applications as marine utility components. Due to their low density, lesser ingression by water and ability to withstand compressive strengths they are considered for sub-sea applications. Dispersing hollow glass microballoons in matrices like, for instance, thermosetting epoxy resins yields on curing the syntactic foam slabs. Due to the differences in densities of epoxy resin and the glass hollow microballoons, the former being heavier than the latter, the resins have a tendency to settle down. This settling leads to a gradation in the density across the height of the cast sections. This effect, however, is more conspicuous in rectangular slabs of higher thickness, for e.g., in 25 mm size mould used in this work. Hence, to study such distribution differences, 3 mm thick sections are sliced from the different sections along the thickness part of the slab from which tension test coupons are made and tested. From the data, both tensile strengths and moduli are calculated. These increased from 23.8 to 41.9 MPa and 2 to 2.47 GPa, respectively, as the volume% of microballoons in the sliced sections decreased from 43.9 to 25.9. Fractographic features, observed on the tension-failed samples, show that higher microballoons' bearing samples (i.e., samples from the top portion of the slabs' thickness) have formations of cavities. The lower microballoons' bearing sample show well marked matrix deformation features. These and other findings are analyzed and the features observed are correlated with the mechanical test results.