[IMPL] LCF uniaxial SWT criterion

[MartinNesladek]

ℹ️ General Information

Component Name: LCF uniaxial SWT criterion

Component Location: core/energy_life/damage_params/uniaxial_fatigue_criteria/

Suggested Python Name: SWT

FABER WG Relation: 4.1, 4.7

Brief Description: Smith-Watson-Topper (SWT) damage parameter for mean stress correction in strain-life.

Priority: 8

Technical Complexity: 2

Estimated Effort: 2

Dependencies: -

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Implementation Details

📋 Specification

For given stress and strain values representing a single load cycle compute the value of SWT parameter, $P_{SWT}$, in MPa. By solving the non-linear equation below, obtain the number of cycles to failure, $N$, as a final output.

Mathematical Formulation

$$ P_{swt} = \sqrt{E\cdot\varepsilon_a\cdot(\sigma_{m}+\sigma_{a})} $$

The value of N is found by solving the following non-linear equation using, e.g., the Newton's iterative scheme:

$$ P_{swt}^2 - \sigma_f^2\cdot(2N)^{2b} - E\cdot\varepsilon_f\cdot\sigma_f\cdot(2N)^{b+c} = 0$$

Inputs

1. Parameters of the e-N curve in the form of Manson-Coffin and Basquin equation

Parameter	Symbol	Type	Description	Units	Constraints
fat_strength_coef	$\sigma_f$	array of floats	Manson-Coffin and Basquin equation fatigue strength coefficient	MPa	$>0$
fat_ductility_coef	$\varepsilon_f$	array of floats	Manson-Coffin and Basquin equation fatigue ductility coefficient	-	$>0$
fat_strength_exp	$b$	array of floats	Manson-Coffin and Basquin equation fatigue strength exponent	-	$<0$
fat_ductility_exp	$c$	array of floats	Manson-Coffin and Basquin equation fatigue ductility exponent	-	$<0$
elastic_modulus	$E$	array of ints	Young's / Elastic modulus	MPa	$>0$

2. Stress / Strain values

Parameter	Symbol	Type	Description	Units	Range
strain_amp	$\varepsilon_a$	array of floats	strain amplitue	-	$(0;\infty)$
stress_amp	$\sigma_a$	array of floats	stress amplitude	MPa	$(0;\infty)$
mean_stress	$\sigma_m$	array of floats	mean stress	MPa	$(-\infty;\infty)$

Outputs

Parameter	Type	Description	Units	Range
N	array of ints	Estimated repetitions N of a given load cycle to failure	-	$(0;\infty)$

Expected Behavior

🔧 Implementation Guidelines

Function Signature

# Suggested function signature
def function_name():
    pass

Code Structure

Error Handling

✅ Validation & Testing

Test Cases

Test Case	Inputs	Expected Outputs	Notes
Example 1	$\sigma_f = 475.4 MPa; b = -0.078; \varepsilon_f = 0.612; c = -0.62; E = 162000 MPa, \varepsilon_a = 0.0135; \sigma_a = 290 MPa, \sigma_m = 10 MPa, N_0 = 1$	$P_{swt} = 810 MPa; N = 278$	$N_0$ is initial estimate of N
Example 2

Validation Criteria

• Mathematical accuracy verified against literature
• Edge cases handled appropriately
• Output format matches specification

📚 References & Resources

• S. Suresh: Fatigue of Materials, Cambridge University Press, 1998

📝 Technical Notes

Performance Considerations

Edge Cases to Handle

Condition $\sigma_a > |\sigma_m|$ should be checked.

Special Requirements

[Vybornak2]

@MartinNesladek

Is negative mean stress correction applicable also for LCF variant?
Here is HCF mean stress corrections Comment I am refering to.

@itomac
If @MartinNesladek confirms this, than HCF mean stress corrections Comment needs to be implemented here as well.

[MartinNesladek]

@MartinNesladek

Is negative mean stress correction applicable also for LCF variant? Here is HCF mean stress corrections Comment I am refering to.

@itomac If @MartinNesladek confirms this, than HCF mean stress corrections Comment needs to be implemented here as well.

In LCF variant of SWT, the negative mean stress should be treated the same way as in its HCF version. Note that the LCF version is actually the most general formulation from these two, since both elastic and elastic-plastic $\varepsilon_a$ can be used as an input.