Viscoelastic Properties of Foam Under Hydrostatic Pressure and Uniaxial Compression

Foam is a lightweight material suitable for aerospace applications for load bearing structures or noise reduction media. The microstructure of the foam, which is constructed with cell ribs, allows its unique mechanical properties. In this work, commercial polyurethane foams with a pore size on the order of a few hundred microns were subjected to quasi-static hydrostatic and uniaxial compression at low strain rates, as well as dynamic sinusoidal loading for studying their loss tangent and storage modulus. The identified incremental negative modulus depends on deformation modes, and it is been shown hydrostatic compression may trigger the negative bulk modulus mode, while uniaxial compression may not. The use of negative modulus in composite materials may lead to extreme high damping and high stiffness materials. Furthermore, by finite element calculations on a dodecahedral unit cell with different elastic constant, it is found that high elastic constant of the cell ribs may give rise to larger negative stiffness effects, when the cell in under hydrostatic compression.