Foams Hyperfoam

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Foams Hyperfoam

A test plan to characterize foams typically looks like this: 

  1. Compression test with lateral strain measurement 
  2. Shear test 
  3. Tensile test 
  4. Density 

Elastomer foams are a lot like dense elastomers.  Large strain material models like hyperfoam models are best calibrated using multiple strain states to make certain that they are physically real and stable. 

Foams are different from dense elastomers in 2 significant ways. 

  1. Because they are somewhat compressible, they do not exhibit the massive increase in stress when constrained like dense elastomers do. 
  2. Foams may exhibit a significant strain-stress slope stiffness jump when most of the gas inside the foam is compressed. 

These experiments may be performed at multiple temperatures and measurement of thermal expansion is often added. 

Hyperfoam material models do 2 basic things: 

  1. They capture the stress changes that may occur from confinement.   
  2. They can represent large elastic strain behaviors.

 

Additional models may be combined with hyperfoam  material models to capture softening, viscoelastic effects, plasticity and fatigue. 

Polyurethane PU Foam compression specimen between test platens.

Foam Compression Tests

The foam compression test is a valuable experiment. 

It may be performed at various strain rates and with the inclusion of lateral strain measurement.  Combined with axial strain measurements, lateral strain provides a measure of compressibility.  The use of small strain Poisson’s ratio measure is usually not sufficient as the nature of the compressibility changes with increased axial compression. 

Foam compression data at multiple rates.
EPP Foam compression specimen between test platens.
EPP Foam compression specimen on high energy test system.

Foam Simple Shear Experiment

The foam simple shear experiment is a natural complement to a compression test for foams.   The experiment is designed to maintain a simple shear case by keeping the distance between the loading plates equidistant throughout the loading. In some cases, the material model calibration effort may benefit from a measure of the resulting lateral stress. 

Foam shear test for hyperfoam calibration.
Machined hard foam specimens.
Foam shear tensile test with lateral reaction force load cell for hyperfoam calibration.

Foam Tensile Test

A foam tensile test can be a simple experiment using the same non-contacting strain measuring and gripping that we use for rubber tests. 

However, in some cases, great care needs to be taken not to crush or alter the tensile specimen in gripping. At Axel Products, sometimes custom fitted holder cups are used to bind and hold the ends of the foam tensile specimen. 

Foam tensile test specimens glued into 3D printed holders.
Tested and potted foam tensile test specimens.

Foam Density

Measuring the density of every foam specimen is a good idea. 

It may be needed for analysis but as a laboratory matter, we want to know that there isn’t foaming variation at the specimen level. 

Scale to Measure mass and density.