We are using OptiStruct 2026, with Implicit-Explicit Chaining (CNTNLSUB)
to continue an Explicit (NLEXPL) impact subcase from a preceding Implicit
(NLSTAT) subcase containing 24 PRETENSION bolt loads on a non-symmetric
wheel rim structure.
ISSUE:
We observed that simply changing TTERM (with all other settings, including
DTMIN, SRCHDIS, and NLOUT, held fixed) causes the results to diverge from
a very early stage (within ~0.3-0.5% of the impact contact time), even
though repeating the same TTERM value twice yields identical results
(confirming the solver is deterministic).
ISOLATED TESTING PERFORMED:
Using simplified flat-plate models, we confirmed:
- Pure Explicit (no chaining, no pretension): no divergence
- Chaining with no pretension (empty Implicit step): no divergence
- Chaining with a single simple pretensioned CROD (100N): no divergence
- Full wheel geometry, pure Explicit (no chaining, no pretension): no divergence
- Full wheel geometry, chaining + 24 pretensioned CBEAM spokes (1000N each):
divergence reproduced
This suggests divergence requires BOTH the chaining mechanism AND complex
multi-body (24-way) pretension coupling. We also note that this feature is
labeled "(beta)" in the 2025 Release Notes.
QUESTIONS:
1. Is this a known limitation of the Implicit-Explicit Chaining (beta)
feature when combined with multiple (>1) PRETENSION loads?
2. Could this be related to TTERM(SUB) vs TTERM(TOT) default behavior
when not explicitly specified in the Explicit subcase?
3. Does INISTRS support importing an initial stress/strain state
(including PRETENSION-generated JOINTG states) into a standalone
NLEXPL subcase, as an alternative to CNTNLSUB chaining? If so, is
this a more stable/mature pathway for combining bolt pretension
with subsequent explicit impact analysis?
4. If neither CNTNLSUB chaining nor INISTRS is currently robust for
this use case, what is Altair's recommended workflow for combining
multi-bolt pretension with subsequent explicit dynamic impact
analysis in OptiStruct?
Model info: OptiStruct 2026, 24 CBEAM spokes with PRETENSION=1000N each,
S2S contact (MORIENT=NORM, CLEARANCE=0.2, SRCHDIS=0.2), DTMIN=1E-07,
Mass Chg.%=0.84% (verified acceptable).
