Documentation/Modules/Fatigue Assessment

Fatigue Assessment

Estimate life with S-N curves and load cycles

Standards catalog

Validation: indicative · Method band: advanced-numerics

Open calculator

Indicative method: Indicative closed-form or numerical model

Assumptions

  • Linear elastic material behavior unless noted otherwise.
  • User is responsible for load combinations and load factors per the selected design code.
  • Design standard (US/EU/ISO) sets unit defaults and screening check labels — not a full code worksheet.

Limitations

  • Professional screening / indicative workspace — does not replace a licensed PE or official code compliance review.
  • Where specialized evaluators are not implemented, checks map solver outputs to catalog templates for orientation only.

Engineering checks

CheckINDUSEUISO
Modified Goodman utilizationimplemented———
Estimated fatigue lifeimplemented———

Fatigue Assessment (fatigue)

Purpose

Estimate fatigue life and mean-stress-adjusted allowable alternating stress using S–N curves, Marin modification factors, and Goodman, Gerber, or Morrow mean-stress corrections. Supports rotating bending, axial, and torsion load types.

Physics & theory

Fatigue failure occurs below yield after many stress cycles. The S–N curve relates alternating stress amplitude to life . Endurance limit at cycles is modified by Marin factors: surface finish , size , load type , giving .

Mean stress reduces allowable alternating stress. Modified Goodman: . Gerber uses parabolic mean-stress locus; Morrow uses true fracture strength. Basquin log-linear relation between and cycles predicts finite life: .

Notch sensitivity and stress concentrations are not computed in this module — apply fatigue stress concentration factors to nominal stresses before entry when needed.

Governing equations

Numerical method

Closed-form Marin factors (Shigley Table 6-2), mean-stress correction, and Basquin life prediction (engine). Surface finish, size, load type, and method selectable. Infinite life flagged when after mean-stress correction.

Inputs

ParameterDescription
alternatingStress, meanStress,
ultimateStrength, enduranceLimitMaterial fatigue data
surfaceFinish, loadTypeMarin factors
characteristicDiameterSize factor (rotating bending)
meanStressMethodgoodman, gerber, or morrow

Outputs

  • Modified endurance limit, allowable alternating stress, predicted cycles to failure, infinite-life flag
  • Marin factor breakdown.

Design codes & checks

  • Indicative: Modified Goodman utilization, estimated fatigue life
  • ISO: ISO 12107 fatigue of metallic materials
  • US: ASME VIII-2 fatigue screening (reference)

Assumptions & limitations

  • Uniaxial stress state; multiaxial fatigue needs equivalent stress approaches.
  • No notch sensitivity unless user adjusts endurance limit.
  • Constant amplitude loading; variable amplitude needs Miner's rule extension.
  • No environmental corrosion-fatigue interaction.

Verification

References

  1. Shigley, J. E., & Budynas, R. G. Mechanical Engineering Design, 11th ed., Ch. 6.
  2. ISO 12107:2012. Metallic materials — Fatigue testing — Statistical planning.
  3. Dowling, N. E. Mechanical Behavior of Materials, 5th ed.
  4. Peterson, R. E. Stress Concentration Factors.
  5. Beer, F. P., et al. Mechanics of Materials, 8th ed. McGraw-Hill — foundational stress and deformation theory.
Maintainer note: Cycle-life models and assumptions require careful migration.