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[Solved] Cohesive elements in the Split Hopkinson Pressure Bar modeling

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csuznxg
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Hi, I am doing the modeling about dynamic Brazil split for rock material on the Split Hopkinson Pressure Bar (SHPB). I inserted cohesive elements (defined by *MAT_COHESIVE_MIXED_MODE, same density with solid elements) into the rock model for crack propagation modeling. But the results showed that the stress wave spreading in the model is slower than the model without cohesive elements. Thus,it takes more time until the stress wave spread to the transmission bar. At this time, the reflected stress wave in the incident bar has loaded to the rock sample several times.

Why cohesive elements have a negative effect on stress wave propagation?

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csuznxg
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SHPB model

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Negative Volume
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Hey @csuznxg

I apologize for not helping with this. I was just reading a paper about characterizing soft tissue using physical material tests through a split hopkinson pressure bar and it clicked in my brain that I remember seeing a post on this. 

To start, did you solve this issue?

If not, I'm assuming that you are using either ELFORM 19 or 20 which work with cohesive elements. I don't work with this type of solid, but from reading I looks like these behave like 2D elements and the deformation is defined as relative displacements instead of strain. Maybe that is why you are seeing differences in stress propogation? 

https://www.dynasupport.com/howtos/element/cohesive-element-formulation

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csuznxg
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@negativevolumeThank you so much for replying. And I am sorry for I didn't update this post timely. This issue is addressed by reducing the mass (density) of the cohesive element. For static or quasi-static problem, the density is not important. However, if the cohesive element has the same density with the solid element in a dynamic analysis, the cohesive element introduces excessive mass into this system, thus which has a negative effect on the stress wave propagation.

In a 3D model, the cohesive element consists of 8 nodes (but two adjacent nodes has same coordinate). If the cohesive element deformed, it will be extended to a 3D element.

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Negative Volume
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@csuznxg Great to hear you got it figured out!

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