Flyash is incorporated in glass fibre reinforced epoxies to study their response to the filler addition. Low cost of flyash can reduce the overall cost of the component. Only very low volume fractions of filler are investigated in the present study. To obtain further clarification of the observed phenomenon, another abundantly available low cost material, calcium carbonate is incorporated in one set of the specimens. Compressive strength of the material is found to decrease, whereas steep increase in impact strength is observed by introduction of very small quantity of fillers. Specimens containing calcium carbonate are tested for impact properties only. Effect of specimen aspect ratio on the compressive strength values is also studied by testing specimens of three different aspect ratios. Scanning electron microscopic observations are taken to develop a better understanding of the phenomena taking place in the material system at microscopic level.

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Microballoons (hollow particles) of the same outer radius but with five different inner radius values are used to fabricate five types of syntactic foam slabs. These five types of slabs are used as the core material to fabricate sandwich composites. Three- and four-point bending and short beam shear strength tests are carried out to characterize the flexural behavior of syntactic foam core sandwich composites. The effect of change in microballoon radius ratio (ratio of the inner to the outer radius) on the flexural properties of the sandwich composites is also studied. The results show that in three- and four-point bending tests, the failure is governed by tensile properties of the foam core and the strength is not affected by the microballoon radius ratio. Shear failure takes place in short beam shear tests, which makes the microballoon radius ratio an important factor in determining the strength of the sandwich composite.

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A novel approach for changing the density of syntactic foams, while keeping the microballoon (hollow particles) volume fraction constant, is adopted in this work. This is achieved by selecting microballoons of the same size but with different wall thickness. Five types of microballoons are selected to fabricate syntactic foams. All the types of microballoons have about 40 mm mean particle sizes, but different wall thicknesses. This approach allows to maintain the same volume fractions of constituents and interfacial area while changing the density of syntactic foams. The fabricated syntactic foams are tested for their compressive properties in accordance to the ASTM D 695-96 standard. The results of the experimental investigation show a strong dependence of the compressive properties and the fracture characteristics of syntactic foams on the microballoon wall thickness. Shear cracking followed by cracking under secondary tensile stresses has been observed as the fracture mode. The present approach is found to be more effective than changing the microballoon volume fraction to change the syntactic foam density as it considerably increases the strength to the weight ratio.

Use of syntactic foams as core materials gives several distinct advantages over traditionally used core materials. Syntactic foams have an excellent combination of compressive strength, low density, low radar detectability and low moisture absorption coefficient among others. The present work aims at studying the behavior of sandwich-structured composites containing syntactic foam as core material under three-point bending conditions. Flexural and short-beam shear tests are conducted, where large (16 : 1) and small (5 : 1) aspect ratio (span length/thickness Downloaded from ratio) specimens are tested, respectively. It is observed that the specimen failure mode changes completely with the change in the aspect ratio. Specimens are found to fracture under the effect of shear stresses in the smaller aspect ratio specimens, whereas compressive stresses lead to the fracture in higher aspect ratio specimens. The observations of fracture features are correlated with the test data and the loaddisplacement curves obtained in the tests. A method of analysis is also presented for syntactic foams and the sandwich structures containing syntactic foam as core material.