Increased energy absorption and load result when the contact area between an object and a foam cushion is less than the foam area because of an increased foam deformation volume (i.e. “Load Spreading”). The deformed volume is trapezoidal (i.e. not prismatic) and is a function of the foam dimensions and the object geometry. The load spreading effect is greatest when the ratio of the object area to the foam thickness is smallest; a common configuration in commercial packages (Figure 1).

‘Soft Impact Limiters’ such as polyurethane foams and aluminum honeycombs are being studied to assist the Sandia National Lbiatory's Transportation Base Technology Program. The aim of this research is to study the mechanical behavior of these materials, which are being used as impact absorbers in nuclear waste transportation containers.

A series of tests were performed along various loading paths using an Instron, servo-controlled, multi-axial loading machine and Soiltest triaxial testing apparatus. Static tests included uniaxial tension, uniaxial compression, triaxial compression, hydrostatic compression and fracture toughness testing. The purpose of using different loading paths was to generate an extensive test data which is being used to develop constitutive models for these materials, under a separate research program.

Dynamic tests were conducted at strain rates of 100 strains/sec., to generate experimental data relevant to accident situations. These tests were conducted on an instrumented Charpy impact testing apparatus. Results of these tests were subsequently used to conduct scale-model tests of transportation casks of different industrial designers.

Different densities of Polyurethane foams, aluminum honeycombs, and corrugated aluminum honeycombs were tested in different orientations. The paper discusses the experimental program, instrumentation and test results for the nuclear waste transportation industry and other potential applications.

Numerical calculations of disposal room configurations at the Waste Isolation Pilot Plant (WIPP) near Carlsbad, NM are presented. Specifically, the behavior of either crushed salt or a crushed salt-bentonite mixture, when used as a backfill material in disposal rooms, is modeled in conjunction with the creep behavior of the surrounding intact salt. The backfill consolidation model developed at Sandia National Laboratories was implemented into the SPECTROM-32 finite element program. This model includes nonlinear elastic as well as deviatoric and volumetric creep components. Parameters for the models were determined from laboratory tests with deviatoric and hydrostatic loadings. The performance of the intact salt creep model previously implemented into SPECTROM-32 is well documented.

Results from the SPECTROM-32 analyses were compared to a similar study conducted by Sandia National Laboratories using the SANCHO finite element program. The calculated deformations and stresses from the SPECTROM-32 and SANCHO analyses agree reasonably well despite differences in constitutive models and modeling methodology. These results provide estimates of the backfill consolidation through time. The trends in the backfill consolidation can then be used to estimate the permeability of the backfill and subsequent radionuclide transport.

We discuss scaling theories for the elasticity and tensile fracture of random central force spring networks with bond dilution disorder. Effective medium theory works quite well for elasticity but needs very new ingredients to be even qualitatively correct for tensile fracture. A novel “extreme scaling theory” predicts a dilute limit singularity and a size effect in the tensile strength. These predictions are supported by numerical simulations.

We extend the above arguments to networks with distributions of bond disorder, and compare the central force network theories to models currently used in the study of rigid cellular materials.

Aerogels are a unique type of ultrafine cell size, low density foam. Traditional aerogels are inorganic, but the synthesis of organic aerogels has also been reported. In all cases, solution chemistry can be used to tailor the structure and properties of the resultant aerogels. This study examines the microstructural dependence of the compressive mechanical properties of silica, resorcinol-formaldehyde, carbon, and melamine-formaldehyde aerogels.

We discuss scaling laws for scalar and vector transport properties of, and fracture processes in, disordered materials. Random resistor networks, and elastic and superelastic percolation networks are used to model the disordered material. While scalar transport properties of such systems (e.g. conductivity or diffusivity) obey universal scaling laws near the percolation threshold, vector transport properties (e.g. elastic moduli) may not follow such universal laws, and the critical exponents characterizing such scaling laws may depend on the microscopic force laws of the system. On the other hand, fracture processes in such systems appear to obey universal scaling laws. In particular, the external stress F for the fracture of the system scales with its linear size L as F ˜ Ld-1/(ln L)ψ, where d is the dimensionality of the system and ψ is a small critical exponent (ψ ≃ 0.1). Moreover, as the macroscopic fracture point of the system is approached, the ratio of various elastic moduli of the system approaches a universal fixed point, independent of the microscopic details of the system. Finally, the distribution of fracture strength in a randomly reinforced system, or in a system near its percolation threshold with a broad distribution of elastic constants, is in the form of a Weibull distribution, rather than the recently-proposed Gumbel distribution.

Though the volume fraction (or %) porosity is commonly used as the exclusive porosity variable to describe the porosity dependence of physical properties, it is not sufficient. The average minimum solid area, i.e., of the bond or sintered area between sintering particles, or the minimum web or strut cross-sections is the most appropriate second porosity parameter. Pore shape-stress concentration effects have, at best, limited direct effect on mechanical properties, but generally correlate with minimum solid area effects. Methods of combining effects of different types of porosity within the same body are important and result in quite reasonable descriptions of mechanical properties across a broad range of porosities.

The volume of foams used in packaging is enormous. Proper design requires identifying the right material and selecting the right density for each particular application. A new approach to package design is presented in the form of “the Packaging Selection Diagram”, from which the optimal density of a cellular material can be obtained once the maximum permitted stress of the packaging is known. This approach offers greater generality and simplicity than the existing methods such as the Janssen factor or the Energy Absorption Diagram.