Here is a super simple example, two surfaces tied on the edge loaded with gravity, the model is definently not happy
type or **
** Surfaces ++++++++++++++++++++++++++++++++++++++++++++++++
**
*Surface, Name=Internal_Selection-1_Shell_part-1_to_Shell_part-2-1_Master, Type=Element
Internal-1_Internal_Selection-1_Shell_part-1_to_Shell_part-2-1_Master_S2, S2
*Surface, Name=Internal_Selection-1_Shell_part-1_to_Shell_part-2-1_Slave, Type=Element
Internal-1_Internal_Selection-1_Shell_part-1_to_Shell_part-2-1_Slave_S4, S4
Internal-1_Internal_Selection-1_Shell_part-1_to_Shell_part-2-1_Slave_S3, S3
*Surface, Name=Internal_Selection-1_Shell_part-1_to_Shell_part-2-2_Master, Type=Element
Internal-1_Internal_Selection-1_Shell_part-1_to_Shell_part-2-2_Master_S4, S4
Internal-1_Internal_Selection-1_Shell_part-1_to_Shell_part-2-2_Master_S3, S3
*Surface, Name=Internal_Selection-1_Shell_part-1_to_Shell_part-2-2_Slave, Type=Element
Internal-1_Internal_Selection-1_Shell_part-1_to_Shell_part-2-2_Slave_S1, S1
**
** Physical constants ++++++++++++++++++++++++++++++++++++++
**
**
** Materials +++++++++++++++++++++++++++++++++++++++++++++++
**
*Material, Name=S185
*Density
7.8E-09
*Elastic
210000, 0.28
*Expansion, Zero=20
1.1E-05
*Conductivity
14
*Specific heat
440000000
**
** Sections ++++++++++++++++++++++++++++++++++++++++++++++++
**
*Shell section, Elset=Internal_Selection-1_Shell_section-1, Material=S185, Offset=0
1
**
** Pre-tension sections ++++++++++++++++++++++++++++++++++++
**
**
** Constraints +++++++++++++++++++++++++++++++++++++++++++++
**
*Tie, Name=Shell_part-1_to_Shell_part-2-1, Position tolerance=0.05, Adjust=No
Internal_Selection-1_Shell_part-1_to_Shell_part-2-1_Slave, Internal_Selection-1_Shell_part-1_to_Shell_part-2-1_Master
*Tie, Name=Shell_part-1_to_Shell_part-2-2, Position tolerance=0.05, Adjust=No
Internal_Selection-1_Shell_part-1_to_Shell_part-2-2_Slave, Internal_Selection-1_Shell_part-1_to_Shell_part-2-2_Master
**
** Surface interactions ++++++++++++++++++++++++++++++++++++
**
**
** Contact pairs +++++++++++++++++++++++++++++++++++++++++++
**
**
** Initial conditions ++++++++++++++++++++++++++++++++++++++
**
**
** Steps +++++++++++++++++++++++++++++++++++++++++++++++++++
**
**
** Step-1 ++++++++++++++++++++++++++++++++++++++++++++++++++
**
*Step
*Static
**
** Boundary conditions +++++++++++++++++++++++++++++++++++++
**
*Boundary, op=New
** Name: Fixed-1
*Boundary
Internal_Selection-1_Fixed-1, 1, 6, 0
**
** Loads +++++++++++++++++++++++++++++++++++++++++++++++++++
**
*Cload, op=New
*Dload, op=New
** Name: Gravity-1
*Dload
Internal_Selection-1_Gravity-1, Grav, 999, 0, -1, 0
**
** Defined fields ++++++++++++++++++++++++++++++++++++++++++
**
**
** History outputs +++++++++++++++++++++++++++++++++++++++++
**
**
** Field outputs +++++++++++++++++++++++++++++++++++++++++++
**
*Node file
RF, U
*El file
S, E
**
** End step ++++++++++++++++++++++++++++++++++++++++++++++++
**
*End step
paste code here