Reply To: Negative eroding contact energy in impact testing

[j2001]

Hi Reno,

Yes, you're right, I meant negative eroded internal energy. 

Regarding the context of the model, I have no shell elements in the model, only a rectangular deformable (MAT_143) wood projectile striking a rigid cylindrical target attached to a spring, both using solid elements,. The model is supposed to be measuring the contribution of spring stiffness + "lumped mass" inertia to overall contact force history. I've added SLDTHK offsets to the ERODING_SURFACE_TO_SURFACE contact, and I'm using SOFT = 2 since it's wood impacting on metal (and the meshes are a bit different since one is rectangular and the other is circular). I'm also using CONTACT_AUTOMATIC_SINGLE_SURFACE for the projectile self-contact, with SOFT = 1, with some adjustments to SOFSCL and SFSA in that contact definition. I've tried a lot of things from changing stiffnesses, changing the value of VDC, changing hourglass control parameters, to make it such that hourglass energy is low and interpenetrations aren't present. I've never been able to solve the negative eroded internal energy issue, however. A problem I had at the beginning was that the entire frontal layer of elements would erode upon impact, which is why I increased SLDTHK and penalty stiffness scale factors in the first place. 

Also, that internal energy screenshot I attached originally was from MATSUM, of the projectile. I did find it strange that the total internal energy in MATSUM was lower than the internal energy without considering eroded energy. 

If I turn off the erosion criteria for the material (IVOL = 0 for wood), I get an error termination after a short while (negative volume and out of range velocities), but the eroded internal energies that result from the few deletions that happen before that are still negative. Worth noting that the internal energies of the projectile that aren't deleted remain positive throughout the simulation, though. From what I can see, everything else looks fine but the eroded internal energy being negative really perplexes me, since I've used this same exact projectile to impact a deformable shell target, and the shell's eroded internal energy was positive while the projectile's eroded internal energy was sometimes very slightly positive and sometimes very slightly negative. In this solid-solid contact simulation it's consistently negative, however. 

As for the version of LS-DYNA, I'm using R15.0.2 for the solver (it says smp_d_R15.0.2_.....), which should be quite recent as I downloaded that from the website earlier this year. 

Since receiving this, I've tried:

• Setting DAMPING_PART_STIFFNESS to 0.2, to which I got the same results.
• Removing VDC
• Increasing VDC on the contact definition to a large number like 60
• Setting the self-contact definition back to default
• Scaling contact stiffness on the eroding contact definition
• Changing hourglass modes between 5 and 6, with varying QM parameters according to the crash guidelines file you attached

From which I got varying other results (like contact force or timestep) but energy was still increasingly negative the more elements eroded from the wood model. Is it something to do with the failures possibly being due to negative volume (negative volume causing negative internal energy)? Let me know your thoughts.