Merge pull request #96 from boschresearch/80-slope-estimator

Fix evaluation of slope k and scatter TN by evaluating only fractures…

CHANGELOG.md

@@ -6,6 +6,11 @@ In this file noteworthy changes of new releases of pyLife are documented since
 
 ## pylife-2.1.1
 
+### Breaking changes
+
+* Change fracture load levels used for slope `k_1` and scatter `T_N` estimation. Now only fractures in the `finite_zone` are used for estimation and not all fractures (in `finite_zone` and `infinite_zone`). Change based on findings in DIN50100:2022-12.
+
+
 ### New features
 
 * Add option to manually set `fatigue_limit` for Woehler curve estimation. (#73)

src/pylife/materialdata/woehler/elementary.py

@@ -73,6 +73,7 @@ def analyze(self, **kwargs):
         """
         if len(self._fd.load.unique()) < 2:
             raise ValueError("Need at least two load levels to do a Wöhler analysis.")
+        self._finite_fractures = self._fd.finite_zone.loc[self._fd.finite_zone.fracture == True]
         wc = self._common_analysis()
         wc = self._specific_analysis(wc, **kwargs)
         self.__calc_bic(wc)
@@ -117,8 +118,8 @@ def __calc_bic(self, wc):
         self._bic = (-2 * log_likelihood) + (param_num * np.log(self._fd.num_tests))
 
     def _fit_slope(self):
-        slope, lg_intercept, _, _, _ = stats.linregress(np.log10(self._fd.fractures.load),
-                                                        np.log10(self._fd.fractures.cycles))
+        slope, lg_intercept, _, _, _ = stats.linregress(np.log10(self._finite_fractures.load),
+                                                        np.log10(self._finite_fractures.cycles))
 
         return slope, lg_intercept
 
@@ -129,7 +130,7 @@ def _transition_cycles(self, fatigue_limit):
         return 10**(self._lg_intercept + self._slope * (np.log10(fatigue_limit)))
 
     def _pearl_chain_method(self):
-        self._pearl_chain_estimator = PearlChainProbability(self._fd.fractures, self._slope)
+        self._pearl_chain_estimator = PearlChainProbability(self._finite_fractures, self._slope)
 
         TN = functions.std_to_scattering_range(1./self._pearl_chain_estimator.slope)
         TS = TN**(1./-self._slope)

tests/materialdata/woehler/test_analyzer.py

@@ -178,9 +178,7 @@ def test_woehler_endur_zones(data, fatigue_limit_expected):
 ])
 def test_woehler_endur_zones_conservative(data, fatigue_limit_expected):
     fd = woehler.determine_fractures(data, 1e7).fatigue_data
-    print(fd.fatigue_limit)
     fd = fd.conservative_fatigue_limit()
-    print(fd.fatigue_limit)
     assert fd.fatigue_limit == fatigue_limit_expected
 
 
@@ -193,10 +191,10 @@ def test_woehler_endure_zones_no_runouts():
 def test_woehler_elementary():
     expected = pd.Series({
         'SD': 362.5,
-        'k_1': 7.0,
-        'ND': 3e5,
-        'TN': 5.3,
-        'TS': 1.27,
+        'k_1': 9.88,
+        'ND': 417772,
+        'TN': 6.78,
+        'TS': 1.21,
         'failure_probability': 0.5
     }).sort_index()
 
@@ -208,10 +206,10 @@ def test_woehler_elementary():
 def test_woehler_elementary_initialize_with_determined_fractures():
     expected = pd.Series({
         'SD': 362.5,
-        'k_1': 7.0,
-        'ND': 3e5,
-        'TN': 5.3,
-        'TS': 1.27,
+        'k_1': 9.88,
+        'ND': 417772,
+        'TN': 6.78,
+        'TS': 1.21,
         'failure_probability': 0.5
     }).sort_index()
 
@@ -223,10 +221,10 @@ def test_woehler_elementary_initialize_with_determined_fractures():
 def test_woehler_elementary_initialize_with_pandas_dataframe():
     expected = pd.Series({
         'SD': 362.5,
-        'k_1': 7.0,
-        'ND': 3e5,
-        'TN': 5.3,
-        'TS': 1.27,
+        'k_1': 9.88,
+        'ND': 417772,
+        'TN': 6.78,
+        'TS': 1.21,
         'failure_probability': 0.5
     }).sort_index()
 
@@ -255,13 +253,41 @@ def test_woehler_elementary_only_one_load_level():
         woehler.Elementary(fd).analyze().sort_index()
 
 
+def test_woehler_elementary_set_fatigue_limit_low():
+    expected = pd.Series({
+        'SD': 350.0,
+        'k_1': 9.88,
+        'ND': 590904,
+        'TN': 6.78,
+        'TS': 1.21,
+        'failure_probability': 0.5
+    }).sort_index()
+    fd = woehler.determine_fractures(data, 1e7).sort_index().fatigue_data.set_fatigue_limit(fatigue_limit=350.)
+    wc = woehler.Elementary(fd).analyze().sort_index()
+    pd.testing.assert_series_equal(wc, expected, rtol=1e-1)
+
+
+def test_woehler_elementary_set_fatigue_limit_high():
+    expected = pd.Series({
+        'SD': 376.,
+        'k_1': 7.07,
+        'ND': 191572,
+        'TN': 4.18,
+        'TS': 1.22,
+        'failure_probability': 0.5
+    }).sort_index()
+    fd = woehler.determine_fractures(data, 1e7).sort_index().fatigue_data.set_fatigue_limit(fatigue_limit=376.)
+    wc = woehler.Elementary(fd).analyze().sort_index()
+    pd.testing.assert_series_equal(wc, expected, rtol=1e-1)
+
+
 def test_woehler_probit():
     expected = pd.Series({
-        'SD': 335,
-        'TS': 1.19,
-        'k_1': 6.94,
-        'ND': 463000.,
-        'TN': 5.26,
+        'SD': 339.3,
+        'TS': 1.2,
+        'k_1': 9.88,
+        'ND': 802898.,
+        'TN': 6.78,
         'failure_probability': 0.5
     }).sort_index()
 
@@ -270,6 +296,21 @@ def test_woehler_probit():
     pd.testing.assert_series_equal(wc, expected, rtol=1e-1)
 
 
+def test_woehler_probit_set_fatigue_limit():
+    expected = pd.Series({
+        'SD': 340.4,
+        'TS': 1.21,
+        'k_1': 7.1,
+        'ND': 386721.,
+        'TN': 4.18,
+        'failure_probability': 0.5
+    }).sort_index()
+
+    fd = woehler.determine_fractures(data, 1e7).sort_index().fatigue_data.set_fatigue_limit(fatigue_limit=376.)
+    wc = woehler.Probit(fd).analyze().sort_index()
+    pd.testing.assert_series_equal(wc, expected, rtol=1e-1)
+
+
 def test_woehler_probit_one_runout_load_level():
     fd = woehler.determine_fractures(data_one_runout_load_level, 1e7).fatigue_data
     expected = woehler.Elementary(fd).analyze()
@@ -283,9 +324,9 @@ def test_woehler_probit_data01():
     expected = pd.Series({
         'SD': 490,
         'TS': 1.1,
-        'k_1': 8.0,
-        'ND': 530e3,
-        'TN': 3.0,
+        'k_1': 5.2,
+        'ND': 298638.,
+        'TN': 1.93,
         'failure_probability': 0.5
     }).sort_index()
 
@@ -315,10 +356,10 @@ def test_woehler_probit_no_runouts():
 def test_woehler_max_likelihood_inf_limit():
     expected = pd.Series({
         'SD': 335,
-        'TS': 1.19,
-        'k_1': 6.94,
-        'ND': 463000.,
-        'TN': 5.26,
+        'TS': 1.20,
+        'k_1': 9.88,
+        'ND': 897649.,
+        'TN': 6.78,
         'failure_probability': 0.5
     }).sort_index()
 
@@ -327,6 +368,21 @@ def test_woehler_max_likelihood_inf_limit():
     pd.testing.assert_series_equal(wc, expected, rtol=1e-1)
 
 
+def test_woehler_max_likelihood_inf_limit_set_fatigue_limit():
+    expected = pd.Series({
+        'SD': 333.6,
+        'TS': 1.22,
+        'k_1': 7.1,
+        'ND': 446054.,
+        'TN': 4.18,
+        'failure_probability': 0.5
+    }).sort_index()
+
+    fd = woehler.determine_fractures(data, 1e7).fatigue_data.set_fatigue_limit(fatigue_limit=376.)
+    wc = woehler.MaxLikeInf(fd).analyze().sort_index()
+    pd.testing.assert_series_equal(wc, expected, rtol=1e-1)
+
+
 def test_bic_without_analysis():
     fd = woehler.determine_fractures(data, 1e7).fatigue_data
     we = woehler.MaxLikeFull(fd)