Experiments and material model for high strain rate and large strain impact on polyurethane
I am currently working on modelling water droplet and solid impact on different types of polyurethane coatings. Here we reach quite high strain rates, as the impact speed is in the range of 100-170 m/s, and we expect the strain rate and amplitude dependence to be significant. Using DMA master curve data, we have had some success in fitting a prony series that can be input in Abaqus, however it only works for some of the materials and is quite simplified.
We have tensile curves at different strain rates and max strain, however i believe such curves do not go near the strain rates we reach in the actual impacts. We also have DMA data both as temperature and frequency sweeps along with master curves, however here we do not reach the high strain rates we see in the experiments. I have tried to use MCalibration to fit different material models to the data but without much success – even when waiting more than a full day for the optimization to run. I am starting to think that we might be taking the wrong approach here, and would be grateful for some advice.
Do you have a suggestion on what kind of experiments makes most sense to use for fitting a material model for the above described problem? We are planning on trying to do cyclic tests similar to those mentioned as “smart testing” in some of the tutorials here on PolymerFEM. As for material model, I have been closest with the TNV model although I could not get a very nice fit, but what kind of material model would you suggest?
Thank you in advance,
Kristine Munk Jespersen
Hello Kristine, That sounds like an interesting project! What is the max strain and strain rate that you expect to see?
DMA is a reasonable start, but if the strains are large then it may not be sufficient.
If you like, I can review your MCalibration files. Just send them to support at PolymerFEM.com.
Thank you very much for your answer.
We just checked the strains and strain rate for our model for a soft PU modelled using a prony series for the viscoelastic properties and tensile test data for the hyperelasticity. Here we obtain a minimum principal strain of -0.093 and maximum principal strain of 0.05 with the maximum strain component being the shear strain with a value of 0.131. Regarding the strain rate, the maximum was also obtained in shear with a max value of 8533 1/ms. The maximum strain rates in the 11, 22, 33 directions were 4452, -7124, and 1532 1/ms, respectively.
Of course they would be affected by the use of a different material model, however it can give an idea, i hope. Would you think we should include more information than the DMA data to obtain realistic results?
Hi Kristine, so the max strain is about 0.1 (that is, 10%) - got it. Can you let me know the approximate impact time for the water droplet?
Note that the Abaqus reported max strain rate is often much higher than the actual strain rate due to nodal vibrations that are inherent in explicit FE simulations.
The impact time from it touches till it leaves the surface is in the range of 0.02-0.03 ms. We have confirmed this both by experiments and Abaqus simulations.
Thank you for the information on the strain rates reported in Abaqus - they did seem quite large and the max values are quite localized so it might make sense to consider an average based on max strain and time, as you might be inferring.
By the way, regarding sending the MCalibration files, I will send some of them early next week after i have cleaned them up and chosen the cases that makes sense to start with.