Different stiffness matrix of ChElementHexaCorot_8 since update to V9

[multibody]

Hi everyone,
I have just updated some code that was using pychrono 8 to pychrono 9 and noticed that the code is giving very different results. I found out that this is caused by the stiffness matrix of the ChElementHexaCorot_8. It looks to me like a bug has slipped in here. The stiffness matrix is off by a factor of 2. This corresponds to the error in simulation results in displacements.

I attached an pychrono example with code for pychrono 8 and pychrono 9 to reproduce the problem.

The stiffness matrix in pychrono 8 (latest commit: c5b8194 on 01.04.2023 at 20:53) gives the following stiffness matrix (first 4 entries):
[[49358974358.97437, 16826923076.923075, 16826923076.923075, -22435897435.89745, ...

The stiffness matrix in pychrono 9 (latest commit: 882fec0 on 31.10.2024 at 15:42) gives the following stiffness matrix (first 4 entries):
[[24679487179.487186, 8413461538.461537, 8413461538.461537, -11217948717.948725, ...

I hope this helps!
Christian

chrono_hex8_stiffness_matrix.txt