[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: -

---

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

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

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

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

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	$\epsilon_f$	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	$\epsilon_a$	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	$\sigma_f = 475.4 MPa; b = -0.078; \epsilon_f = 0.612; c = -0.62; E = 162000 MPa, \epsilon_a = 0.0135; \sigma_a = 290 MPa, \sigma_m = 10 MPa, N_0 = 1$	$P_{swt} = 810 MPa; N = 278$	$N_0$ 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