Hi, I am currently trying to obtain parameters for a filled polybutadiene rubber for use in finite element analysis. I am using a PNM with eight-chain and Yeoh hyperelastic models and the Bergstrom-Boyce flow model. The general fit is pretty good and the peak force looks great but the problem lies with the initial gradient. Here is a picture to help you understand my point:
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You can see that the loading curve shape, especially the initial section isnt quite right. Is this due to my selection of material models or is there a parameter which controls the loading shape??
Any help you be greatly appreciated. Thanks.
The experimental stress-strain curve seems to have very slow tangent stiffness at small strains, and then reach an almost constant slow around a strain of 1.5% (or so). My guess is that that behavior is caused by slack in the experimental setup and not a reflection of the true material behavior. Are you sure that the behavior is real?
My guess is that your material model (PNM) is just fine.
PS. The MCalibration software has a feature that can be used to compensate for initial slack, or you can eliminate it manually.
-Jorgen