Abstract:
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Sandwich materials are structures made of two stiff skins and a softer lightweight
core in between them. The function of the core is to separate the face sheets
without significantly increasing the weight, thus increasing the moment of in-
ertia of the structure. This makes sandwich structures convenient and eficient
to resist bending and buckling loads while keeping a light weight, especially
for applications where the weight is a critical factor (such as naval, aerospace,
the transport industry and sporting equipment).
For applications requiring resistance to impact loading (such as ship hulls,
armors, helmets. . . ) sandwich materials have good capabilities in absorbing
energy. The face sheets are designed to resist penetration/fracture, while
the core is comprised of a material able to deform at a controlled level of stress
providing isolation and damping of vibrations.
The possible modes of failure of a sandwich panel are: face yielding or
fracture, face and core indentation, face wrinkling (local buckling of the face
sheets), core failure (usually in shear), and failure in the bonding between the
core and the face sheets. This last one is the most diffcult to analyze because
delamination is affected by the nature of the interface between the two ma-
terials and also depends on the kind of adhesive being used. Oftentimes the
adhesive is stronger than the core itself, so the bond is not a problem unless
an interfacial crack appears which may propagate and separate the structure.
Functionally graded materials (FGM) are characterized by the gradual vari-
ation of the composition over the volume, which implies a gradual variation
of the mechanical properties. This concept can be used in a sandwich struc-
ture in order to enhance its performance and better tailor its properties.
More precisely, the concept of FGM can be used in sandwich structures to im-
prove the interface between the face sheets and the core, making it a smooth
transition between materials, in order to mitigate the bonding failure and de-
lamination at the interface.
Multi-material Additive Manufacturing has created the opportunity of
building structures that contain diferent materials with very dissimilar me-
chanical properties integrated in the same body. This technology can allow
manufacturing complex parts while controlling at a microscopic scale the lo-
cal composition of the material being printed, which can be used to generate
functionally graded materials. In this project, the device used is a Strata-
sys Objet260 Connex3 which can print blending up to three diferent base
materials in the same part, although only a two material gradation has been
studied.
The objective of this work is to design, manufacture, and model the me-
chanical behavior of functionally graded sandwich beams, in order to obtain
optimized combinations of stifness and damping. For this purpose, the vis-
coelastic mechanical properties of the base 3-D printed materials (i.e. Ve-
roCyan and TangoBlack+) have been obtained experimentally using varioustechniques. Three diferent designs of graded sandwich structures have been
proposed and modeled: linear graded face sheets, quadratic graded face sheets
and quadratic graded core. |