[IMPL] Implement stress-life prediction function based on the Morrow mean-stress correction

[MartinNesladek]

ℹ️ General Information

Component Name: Morrow

Component Location: core/stress_life/damage_params/uniaxial_stress_eq_amp/

Suggested Python Name: calc_stress_eq_amp_morrow

FABER WG Relation: 4.1

Brief Description: Uniaxial equivalent stress amplitude based on Morrow line

Priority: 2

Technical Complexity: 2

Estimated Effort: 2

Dependencies: -

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Implementation Details

📋 Specification

Using the Morrow mean-stress correction, compute the value of equivalent stress amplitude, $\sigma_{aeq}$, in MPa for given stress values $\sigma_a$ and $\sigma_m$ representing a single load cycle.

Mathematical Formulation

$$ \displaystyle\sigma_{aeq}=\frac{\sigma_a}{1-\frac{\sigma_m}{\sigma_{true}} } $$

$$  \displaystyle\sigma_{aeq}=\frac{\sigma_a}{1-\frac{\sigma_m}{\sigma_{true}} } $$

Inputs

1. Static tensile parameters

Parameter	Symbol	Type	Description	Units	Constraints
true_fract_stress	$\sigma_{true}$	array of floats	True tensile fracture stress	MPa	$>0$

2. Stress / Strain values

Parameter	Symbol	Type	Description	Units	Range
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
$\sigma_{aeq}$	array of floats	Equivalent stress amplitude by Morrow	-	$(-\infty; \infty)$

Expected Behavior

🔧 Implementation Guidelines

Function Signature

# Suggested function signature
def calc_stress_eq_amp_morrow(
    stress_amp: ArrayLike,
    mean_stress: ArrayLike,
    true_fract_stress: ArrayLike,
) -> NDArray[np.float64]:

Code Structure

Error Handling

✅ Validation & Testing

Test Cases

Test Case	Inputs	Expected Outputs	Notes
Example 1	$\sigma_{true} = 770 MPa; \sigma_a = 180 MPa, \sigma_m = 100 MPa$	$\sigma_{aeq} = 206.9 MPa$

Validation Criteria

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

📚 References & Resources

J. Draper: Modern Metal Fatigue Analysis. EMAS Publishing, 2008

📝 Technical Notes

Performance Considerations

Edge Cases to Handle

Issue a warning if $\sigma_m > \sigma_{true}$
An error handling should be implemented for $\sigma_m = \sigma_{true}$

Special Requirements