Fix Newman-Raju G22 term in surface crack SIF for a > c - #4531
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The a > c branch of surface_stress_intensity_factor computed the G22 coefficient as 0.14*(c/a)*1.5 where the published Newman-Raju equation (and the sibling a <= c branch) uses 0.14*(c/a)**1.5 — multiplication in place of exponentiation. Currently benign in outputs because bending_stress is hardcoded to zero so H2 never enters the stress intensity factor, but wrong the moment bending is enabled. Also adds the previously-missing test coverage of the a > c branch, with expected K values computed independently from the Newman-Raju equations (the existing tests only exercised a < c at phi = pi/2). Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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Closes #4530
Overview
One-token fix in
process/models/cs_fatigue.py: the a > c branch ofsurface_stress_intensity_factorcomputed G22 as0.14e0 * c_a * 1.5e0where the Newman-Raju equation (and the a <= c branch) requires0.14e0 * c_a**1.5e0.Changes
process/models/cs_fatigue.py:*→**in the G22 term.tests/unit/models/test_cs_fatigue.py: add two a > c parametrised cases (phi = pi/2 and phi = 0) totest_surface_stress_intensity_factor, with expected K values computed independently from the Newman-Raju equations (derivation in the test comment). The a > c branch previously had zero test coverage.Notes for reviewers
bending_stressis hardcoded to 0 inside the function, so H2 (which contains G22) never reaches the returned K today — the existing golden values are untouched and regression outputs are unaffected. The fix removes a landmine for whenever bending is parameterised.bending_stressto a parameter (default 0) so H1/H2 become testable directly.Found during an independent audit of v3.4.2.
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