[lmoresi]

The exponential (ETD) Maxwell integrator formed alpha = exp(-dt/tau) and phi from float(viscosity)/float(modulus). A viscosity written as an expression of position or of a field (a damage band, a temperature-dependent law) has no float; the Maxwell time fell to None, which the coefficient update reads as the viscous limit (alpha = phi = 0). The model silently lost its elastic response wherever the viscosity was not a plain number (measured on a box fault model: first-step end velocity 0.007, the viscous value, against 0.061).

What changes:

• _etd_maxwell_time(eta, mu), used by all three callers (ViscoElasticPlasticFlowModel, TransverseIsotropicVEPFlowModel, TransverseIsotropicVEPSplitFlowModel): a number when both parameters are numbers, otherwise the ratio of the parameters' own expression atoms, so the kernels evaluate it pointwise and the units system scales it.
• ddt._update_exp_values(..., dt_symbol=...): the symbolic coefficients take the step as the dt_elastic atom, so a new step reaches the kernels as a constant (no recompile, no stale coefficient); phi is -expm1(-x)/x; the Maxwell time and the modulus are floored with uw.maths.functions.vanishing, so a vanishing modulus is the elastic limit as on the numeric path.
• tests/test_1053_ve_etd_variable_maxwell_time.py (level_2, tier_b): a numerically uniform expression (1 + 1e-12 x) must reproduce the constant to 1e-10 at order 1 and 2, across a changing step, and for the transversely isotropic model; a weak band is more compliant than the uniform block and far from the viscous limit of the same banded model. All five fail on development; the step-change test also fails when the step is frozen into the kernel as a number. 5 passed serial and on mpirun -n 2.

The fix first appeared on feature/tidal-shell (9e97670); it is ported here so it does not wait on that branch, and extended after review.

Internal adversarial review, two rounds (findings and evidence in the comment below). Left open, all predating this branch: a dt_elastic that cannot be read as a number still reads as the viscous limit; ETD's Maxwell time uses the raw shear_viscosity_0, not the yield-modified viscosity; ETD-2 under a step traction gives a velocity that alternates in sign.

Checklist (UW3 Style Charter — docs/developer/UW3_STYLE_CHARTER.md)

• I have read the Style Charter and followed it over the surrounding code (S2)
• No drive-by refactors, renames, or "while I was here" cleanups (S9)
• Bug fixes ship the regression test, written first, with level_* and tier_* markers (S8)
• No hedging names (maybe_ / try_ / do_) and no commented-out code (S3, S4)
• Every exception swallow states its sanctioned failure mode in a comment (S4)
• New data access uses .array / mesh.X.coords — no with ....access(...), no mesh.data (S7)
• Parallel safety considered — np2/np4 checked where swarm/mesh/solver dispatch is touched (S11)
• No pixi.toml / pixi.lock or other dependency changes riding in an unrelated PR
• AI-assisted work carries the attribution line below

Underworld development team with AI support from Claude Code

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[lmoresi]

Internal adversarial review — two rounds, against builds with and without the fix.

Round 1 (on the first port):

• Blocking, fixed: the symbolic coefficients took the step as a number frozen into the kernel; a change of step left alpha on an earlier step (Stokes ETD-1 1–2 %, ETD-2 4–8 %, NS 77 %). Now the dt_elastic atom — step-change cases agree with the constant to ≤ 1.6e-12 (Stokes/NS, ETD-1/2, nodal/Eulerian/integration-point histories), including the exact viscous value at dt = 1e6.
• Blocking, fixed: the test was outside every CI glob (test_109*); now test_1053, check_test_coverage.py OK (355 files).
• Fixed: dimensional modulus + varying viscosity raised SympifyError; alpha now evaluates to exp(−0.125) in a units model (0.8824969), quantity vs quantity×(1+1e-12x) ≤ 1.8e-12.
• Fixed: the experimental TransverseIsotropicVEPSplitFlowModel kept the silent fallback; one helper for the three callers (Charter §5).
• Fixed: test gaps (order 2, step change, TI) and loose tolerances.

Round 2: no blockers. Nit fixed: phi = 0/0 where mu = 0 and lost digits as x → 0 — now -expm1(-x)/x with the modulus floored by vanishing.

Attacks that failed: constant ↔ expression switches mid-run (identical to the constant run both ways); a mesh-variable viscosity changed between solves (exact viscous scaling); recompiles (none on a step change: 0.5–1.0 s vs 0.4–0.9 s per solve); TI-VEP order 1/2 (≤ 1.2e-13); mpirun -n 2 (5 passed); all five tests fail on development.

Not changed here (predates the branch): an unreadable dt_elastic still reads as the viscous limit; ETD's tau uses the raw shear_viscosity_0; ETD-2 under a step traction alternates in sign.

No open issue covers this bug.

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[lmoresi]

Post-review commit 189aa88: ETD coefficients from the scaled parameters

Found while reviewing the stacked multi-material branch. When η, μ and dt carry units, float(par.sym) is the magnitude in each parameter's own units: 1e21 Pa s / 3.2e7 Pa is a Maxwell time in seconds, against a step of 100 kyr. α came out as 1 − 3e-12, so the model was purely elastic. The base read the same magnitudes, so this predates the PR; the symbolic branch, through the JIT constants, was already scaled.

_etd_maxwell_time and _etd_timestep now read the non-dimensional value (unwrap_expression(..., mode="nondimensional")) through one helper, _nondimensional_float. No float(….sym) reads of a parameter remain in constitutive_models.py.

New test: test_dimensional_parameters_give_the_nondimensional_maxwell_time, for the isotropic and TI models, expects α = e^-0.1. It failed before the fix (α = 0.9999999999968) and passes after; test_1053 passes in full (7 tests). The BDF path was already correct with units (test_1064, and the reviewer's probe matched the plain-number run to 2.6e-14).

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