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Polymer Foam Testing and Modeling

Technical Challenge: 
Polymer foams are complex materials that exhibit drastic sensitivity to applied loading and strain rates. As applied strain rate increases, polymer foam materials stiffen more than bulk polymers, presenting challenges in both testing and modeling. Additionally, foams can exhibit large differences in stress relaxation at different strain rates. Accurately capturing the response of foams is critical to modeling the material for use in Finite Element (FE) simulations, enabling design engineers to optimize foam components.

Veryst Solution:
Veryst Engineering has developed high strain rate testing and material modeling capabilities to characterize and implement models in commercial FE software. As an illustration of the strain rate dependence of a urethane foam, we tested the material in compression over five decades of strain rate. Figures 1 and 2 show the results from the tests, demonstrating the extreme strain rate dependence – the material is 100 times stiffer at 1000/s compared to 0.001/s.

Figure 1. Low strain rate compression tests of a Urethane foam.

Compression test results for high strain rate tests.

Figure 2. High strain rate compression tests of a urethane foam.


Selecting and calibrating a material model to the experimental data is difficult due to this extreme strain-rate dependence. Veryst has developed the PolyUMod® Responsive Elastomer Foam (REF) material model specifically to capture the response of this class of materials. Figures 3 and 4 show the results of the model calibration from MCalibration®. The calibrated model is in excellent agreement with the experimental data, and the material model is available for use in Abaqus/Explicit and LS-DYNA.

Material model calibration results for low strain rate tests.

Figure 3. Results from the material model calibration for the low strain rate tests.  Experimental data are solid lines while simulated data are dashed lines. 

Material model calibration results for high strain rate tests.

Figure 4. Results from the material model calibration for the high strain rate tests. Experimental data are solid lines while simulated data are dashed lines.


Veryst recommends validating a calibrated material model with test data that uses a complex stress state. For soft materials like this urethane foam we recommend performing a ball impact test. The test is then simulated with the calibrated material model, and the results are compared to the experimental data. Figure 5 shows a video of the effective strain rate in the material during the FE simulation. The sample experiences extremely high strain rates, over 10,000/s.

 

 Figure 5. LS-DYNA simulation showing the effective strain rate during the ball impact test.

 

 

 

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