Uniaxial equivalent stress amplitude functionality by KarasTomas · Pull Request #55 · faberorg/FatPy

KarasTomas · 2025-11-12T15:46:39Z

Pull request following issue #40 and is prepared for other stress based uniaxial fatigue criteria like #44 , #45 and #46.

codecov · 2025-11-12T15:49:25Z

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✅ All modified and coverable lines are covered by tests.

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Vybornak2 · 2025-12-19T08:49:48Z

typo in the file name iniaxial

Vybornak2 · 2025-12-19T08:54:20Z

+from numpy.typing import ArrayLike, NDArray
+
+
+def _validate_stress_inputs(


This function seems to implement DRY principle, but to me seems a bit clunky and hard to extend

reason might be, that it is trying to do multiple things, so it does not have 1 responsibility only, thus it branches in its logic and is harder to fit to use cases.

think it through if it could not be improved

Vybornak2 · 2025-12-19T11:15:52Z

+        for compressive-dominated loading conditions.
+
+    """
+    stress_amp_arr, mean_stress_arr = _validate_stress_inputs(stress_amp, mean_stress)


I would convert ArrayLike to np.ndarray here and check for positive values
It is more explicit and does not create too much boilerplate code

In this case validate function seems as overkill to me

I am not saying that we have to do that. Just going through cases of usage of validate function

Vybornak2 · 2025-12-19T11:20:49Z

+            "appropriate for compressive-dominated loading conditions."
+        )
+
+    return np.sqrt(stress_amp_arr * (mean_stress_arr + stress_amp_arr))


** 0.5 might be more readable than np.sqrt but it does not matter as much, just is more consistent with the mathematical formulas

Vybornak2 · 2025-12-19T11:33:28Z

+        ratio = abs_mean / material_param_arr
+
+        if np.any(ratio >= 1.0):
+            raise ValueError(


what about case, where user inputs real values, where some are simply too high, so some results are negative?
Is it okay to disallow user from calculating all values ?

I personally would rather raise warning and exclude it from this validation function

Vybornak2 · 2025-12-19T11:35:10Z

+        ("mean_equals_material", False, "Mean stress magnitude.*exceeds or equals"),
+    ],
+)
+def test_validate_stress_inputs_parametrized(


validate function requires more then it brings value

Vybornak2 · 2025-12-19T11:36:10Z

I would recommend using test classes to separate different test baste on function tested

Test class than would be more generic, where each test case within the test class repeats for each tested function

class TestFunctioon1: def test_feature1(): pass def test_feature2(): ... # now same thing for class TestFunction2

it puts a bit more structure into the test file, so it is easier to navigate
also you can run tests based on the test class and more

Vybornak2 · 2026-02-13T08:50:22Z

Additionally the things should be added / considered:

additional eq methods (please contact @MartinNesladek)
STW for HCF docstring should include usage requirements and reference to SWT LCF (for now create issue placeholder until implementation)
for each equivalent stress function should be specified what is it equivalent of (R = -1 standard or R=0, ....) and potentially other descriptive information
consider/consult whether we should raise error for some conditions when mean stress is lower then zero or rather return stress amp as is

KarasTomas · 2026-03-17T11:28:33Z

On top of equivalent stress amplitude functionality requested by issues, several other functions were added based on this source: Mean stress effect in stress-life fatigue prediction re-evaluated

Use-case info blocks in docstrings still need to be revised.

pavkukula

SWT's validity condition σ_a > |σ_m| is incorrect — it rejects R≥0 loading, which is the canonical SWT case; should be σ_a + σ_m > 0.

invalid = (stress_amp_arr + mean_stress_arr) <= 0

pavkukula · 2026-04-25T13:39:34Z

+            rules for the input arrays.
+
+    Raises:
+        Warning: If mean stress exceeds half of the ultimate tensile strength.


If mean stress exceeds DOUBLE of the ultimate tensile strength.

pavkukula · 2026-04-25T13:40:16Z

+    Raises:
+        Warning: If mean stress exceeds half of the ultimate tensile strength.
+        ValueError: If ultimate tensile strength is not positive.
+        ValueError: If mean stress is equal to half of the ultimate tensile strength,


not half, but DOUBLE

pavkukula · 2026-04-25T13:40:36Z

+    ratio = mean_stress_arr / (2 * ult_stress_arr)
+    if np.any(ratio == 1.0):
+        raise ValueError(
+            "Mean stress equals half of the ultimate tensile strength this would result"


not half, but DOUBLE

it will result in infinite eq stress amp, not zero

Adjacent string literals concatenate without inserting a space, so the actual message reads "…would resultin zero…"

pavkukula · 2026-04-25T13:47:09Z

+    invalid_condition = (walker_parameter_arr < 0) | (walker_parameter_arr > 1)
+    if np.any(invalid_condition):
+        raise ValueError("Walker parameter (γ') must be in the range (0, 1). ")
+


Walker is missing the σ_max > 0 guard — silently produces NaN
Walker is a generalization of SWT (it reduces to SWT at γ=0.5), so the same physical validity should hold.
Since γ is a non-integer, raising a negative (σ_a + σ_m) to a fractional power yields NaN. SWT guards against this; Walker doesn't.
Add the same σ_a + σ_m > 0 check as SWT.

pavkukula · 2026-04-25T13:47:44Z

+        for mean_stress, stress_amp in [(-100.0, 180.0), (100.0, 180.0)]:
+            result = calc_stress_eq_amp_swt(stress_amp, mean_stress)
+            expected = np.sqrt((stress_amp + mean_stress) * stress_amp)
+        assert_allclose(result, expected)


misplaced assertion

pavkukula · 2026-04-25T14:04:51Z

+        result = calc_stress_eq_amp_ASME(stress_amp, mean_stress, 700.0)
+        assert result.shape == (4,)
+
+    def test_invalid_yield_strength(self) -> None:


with pytest.raises: outside for loop only tests the first input
When ys=0.0 raises, the context manager exits and the loop is abandoned — ys=-500.0 is never executed.
Fix:
@pytest.mark.parametrize("ys", [0.0, -500.0])
def test_invalid_yield_strength(self, ys: float) -> None:
with pytest.raises(ValueError):
calc_stress_eq_amp_ASME(100.0, 50.0, ys)

pavkukula · 2026-04-25T14:05:47Z

+
+    ??? info "ASME Use-case"
+        The ASME criterion accounts for mean stress effects in high-cycle fatigue
+        by modifying the stress amplitude based on the yield strength using a linear


ASME (quadratic, square-root denominator) — docstring says "using a linear relationship".

pavkukula · 2026-04-25T14:06:02Z

+
+    ??? info "Bagci Use-case"
+        The Bagci criterion accounts for mean stress effects in high-cycle fatigue
+        by modifying the stress amplitude based on the yield strength using a linear


Bagci (4th-power) — docstring also says "using a linear relationship".

pavkukula · 2026-04-25T14:06:38Z

+from numpy.typing import ArrayLike, NDArray
+
+
+def calc_stress_eq_amp_ASME(


PEP 8 — calc_stress_eq_amp_ASME should be lowercase
Every other function uses _lowercase. ASME breaks both PEP 8 and the file's own convention. calc_stress_eq_amp_asme is consistent with calc_stress_eq_amp_swt.

On the other hand, ASME in upper letters is highly established abbreviation... I don't have strong opinion on that.

pavkukula · 2026-04-25T14:18:20Z

+    invalid_condition = (walker_parameter_arr < 0) | (walker_parameter_arr > 1)
+    if np.any(invalid_condition):
+        raise ValueError("Walker parameter (γ') must be in the range (0, 1). ")
+


Invalid_condition handling accepts endpopints 0,1 even these degenerate walker (gamma=1 => no MSE; gamma=0 => σ_aeq = σ_a + σ_m)
Fix: raise ValueError("Walker parameter (γ') must be in the range [0, 1].")

pavkukula · 2026-04-25T14:24:38Z

ASME is more strict than Bagci/Gerber for no good reason
ASME raises for ratio >= 1.0 (combined). Bagci and Gerber raise only for ratio == 1.0 and merely warn for ratio > 1.0. The trouble: Bagci with σ_a=100, σ_m=600, R_e=500 returns σ_aeq ≈ -93 MPa with a warning — a non-physical negative equivalent amplitude that the caller can easily miss. Either align ASME's strict policy across all three (preferred — Papuga's §4 explicitly excludes such cases anyway, requiring σ_m < 0.75·R_e for these methods), or document why they differ.

pavkukula · 2026-04-25T14:25:53Z

ratio == 1.0 is a fragile float comparison
Exact float equality is brittle when inputs come from upstream computation rather than test literals.
np.isclose(ratio, 1.0) or ratio >= 1.0 (then warn separately for > 1.0) would be more robust.

pavkukula · 2026-04-25T14:35:24Z

+        expected = 180.0 / np.sqrt(1.0 - (100.0 / 500.0) ** 2)
+        assert_allclose(result, expected)
+
+    def test_array_inputs(


That tests one specific combination: two arrays of identical shape (4,) plus a scalar. It exercises a sliver of broadcasting — the trivial case where the two arrays already match. But the docstring promises much more.
Things that should work according to the docstring but are never tested:

Scalar σ_a, array σ_m → result shape (3,)
calc_stress_eq_amp_goodman(150.0, np.array([0.0, 100.0, 200.0]), 700.0)

1-D σ_a, 2-D σ_m → broadcasts to 2-D result
sa = np.array([100.0, 200.0]) # shape (2,)
sm = np.array([[0.0, 50.0], [100.0, 150.0]]) # shape (2, 2)
calc_stress_eq_amp_goodman(sa, sm, 700.0) # should give shape (2, 2)

Array of UTS values too (e.g., comparing materials)
calc_stress_eq_amp_goodman(150.0, 100.0, np.array([500.0, 600.0, 700.0]))

pavkukula · 2026-04-25T14:39:35Z

Strongly compressive σ_m is silently accepted by Goodman/Linear/Morrow/Soderberg/Smith
E.g., Goodman with σ_a=100, σ_m=-1000, R_m=500 returns σ_aeq = 33.3 MPa (less than σ_a) — mathematically defined, physically dubious. Worth at least a Note: in the docstrings; an optional σ_m > -R_m-style sanity check would also be reasonable.

Initial version of SWt for HFC

c7ef1e8

KarasTomas added 3 commits November 13, 2025 10:15

other eq stress functions and input validation

e5f37a2

tests proposition

c313975

ruff formating fix

9498061

KarasTomas added the WIP Work In Progress label Nov 24, 2025

KarasTomas requested a review from Vybornak2 November 24, 2025 15:22

Vybornak2 reviewed Dec 19, 2025

View reviewed changes

added walker

6ecfd77

KarasTomas added 6 commits February 21, 2026 16:50

Added ASME and other methods based on paper in ASME issue

1e2496e

Alphabetical order of functions

17dd2dd

reference added

f7f5f3d

implementation of tests

83f7322

excess condition removed for SWT

7601533

documentation error fix

56500bd

KarasTomas mentioned this pull request Mar 20, 2026

[IMPL] Equivalent stress amplitude functions #60

Open

5 tasks

pavkukula self-requested a review April 25, 2026 12:46

pavkukula reviewed Apr 25, 2026

View reviewed changes

		from numpy.typing import ArrayLike, NDArray


		def _validate_stress_inputs(

		from numpy.typing import ArrayLike, NDArray


		def calc_stress_eq_amp_ASME(

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