Frontiers of Engineering Reports on Leading Edge Engineering from the 1996 NAE Symposium on Frontiers of Engineering (1997) / Chapter Skim
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Innovations in Materials and Processes
Innovations in Materials and Processes
Pages 73-82
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From page 75... ...
Processes now exist for making these cellular materials from almost any solid material, including polymers, metals, ceramics, and glasses. Their cellular structure gives rise to stress-strain curves with a characteristic shape: initial linear elasticity, caused by cell wall bending, is followed by a period of roughly constant stress, resulting from cell collapse, and then a final sharp increase in stress at the point at which opposing cell walls touch and the material densities (Figure 2~.
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From page 76... ...
, we have used the models developed for engineering honeycombs and foams. For instance, wood is much stiffer and stronger when loaded along the grain than across it; we have found this anisotropy arises, in part, from the composite nature of the cell wall in wood and, in part, from its cellular structure: loading along the grain axially compresses the cell walls 4.0 mE At ~ 30 on en LL on LLI 2.0 on on ~ 1 0 o o POLYETHYLENE |/ l l _ _ t=M PRECISION -L>/ l l =4 an_ .
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From page 77... ...
About 10 percent of hip fractures and 50 percent of vertebral fractures are thought to be the result of the activities of daily living rather than a sudden impact such as a fall, and understanding how this progressive damage occurs in osteoporotic bone is essential for evaluation of fracture risk. Here, we briefly describe models for the mechanical behavior of cellular materials, compare the models with data, and note some of the remaining questions to be answered.
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From page 78... ...
Source: Gibson and Ashby, 1988 lyzing the response of a unit hexagonal cell. The elastic moduli, the compressive strength, and the brittle fracture toughness depend on the properties of the solid from which the honeycomb is made, the volume fraction of the solid (raised to some power)
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From page 79... ...
79 to Q i, .
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From page 80... ...
The required bending stiffness is the constraint; it depends on both the flexural rigidity of the faces as well as the shear rigidity of the foam core, which can be written in terms of the core density using the models described above. The bending stiffness constraint equation is then solved in terms of the core density, and this is substituted into the weight equation.
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From page 81... ...
The belief now is that current processes can be modified to produce more-complex shaped components. Current foam core panels usually use either expanded polystyrene or rigid polyurethane foam cores; their structural application is limited by the low creep and fire resistance of the polymer foam cores.
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From page 82... ...
In a further development, the scaffold can be designed to carry bioactive drugs, such as epidermal growth factor, which act to increase tissue growth. Porous scaffolds for peripheral nerves, cartilage, bone and bone marrow currently are being studied by a number of researchers (Ellis and Yannas, 1996; Paige et al., 1996~.
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