| Summary: | Hollow particle filled polymeric materials called syntactic foams are used as core materials in sandwich composite structures. Syntactic foams find applications in aeronautical and space structures and therefore demand careful study and testing before they can be put to service. In the first part of this thesis work, syntactic foams are fabricated by varying the volume fraction of microballoons and also their density. Four different densities of microballoons are used ranging from 0.22 g/cc to 0.46 g/cc. The volume fraction of the microballoons is varied from 30% to 65%. A set of 3-point bending tests are conducted on these foam samples to determine their fracture toughness. It has been found that fracture toughness decreases with increase in volume fraction of the microballoons. As the microballoon density increases the fracture toughness also increases. From these current and previous studies it is found that the optimum volume fraction of microballoons is around 30%. Scanning Electron Microscopy analysis shows that at low volume fractions of 30% the failure mechanism is primarily due to the formation of micro cracks. These secondary micro cracks provide a toughening mechanism which is the reason for higher fracture toughness at this low volume fraction. As the volume fraction of microballoons increases due to the reduction in inter-particle distance, debonding occurs and the samples fail at much lower loads resulting in low fracture toughness values. In the second part of the study, samples are fabricated by incorporating two types of rubber particles. The volume fraction of the rubber particles is maintained constant at 2% and microballoon volume fraction at 63%. Load deflection curves show some limited plastic deformation just before the specimen fractures. Reinforcing with rubber increases the density by 15% and the fracture toughness by 35%. Rubber reinforcement also improves the crack propagation properties by changing the fracture pattern to the ductile mode. There is strong adhesion between the rubber particles and the matrix material. Micrographs show the rubber particles fractured resulting in an increase of the facture toughness.
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