Micromechanics study of foam composites using digital nanospeckle photography

Abstract

Foam composites are increasingly used in sandwich structures for ship construction. Typically the foam serves as the core material for a sandwich panel whose face sheets are made of fiber composites. Such a panel is light but strong when loaded transversely. Determining the mechanical properties of a composite foam is not a simple matter in that most traditional methods such as strain gauge or moire methods are not applicable. However in this paper we show that the mechanical property of foam is a function of the size of the specimen. We apply the digital speckle photography technique to map the deformation of foam composites at different length scales. Emphasis is placed on revealing the composite deformation at microscales. For this we need to use nanosized speckles for the mapping. Nanospeckles are created via a physical vacuum deposition process whereby a metal piece (either gold or copper) is vaporized to fall on the specimen surface. The speckle pattern is digitally recorded at different stages of loading either through a high resolution optical microscope or a scanning electron microscope. The specklegrams are then “compared” using an efficient algorithm using FFT. The result is a full field deformation vector map. We discovered a number of interesting deformation patterns that are unique to the material.