Fatigue Life Estimator with S-N Curve
What's on screen
Two panels. Top panel is the S-N curve (Wöhler diagram) on log-log axes showing stress amplitude vs cycles to failure. A red operating point marks your current stress level, with dashed lines to both axes.
All four material curves are drawn simultaneously (active one bold, others faded). Bottom panel is the Goodman diagram showing the safe operating region for combined alternating + mean stress, with the Goodman line and Soderberg line.
4 sliders:
σa (10-600 MPa): Stress amplitude. Drag right and watch the red dot climb the S-N curve, life drops. Cross the endurance limit and life goes from infinite to finite instantly.
σm (0-500 MPa): Mean stress. Non-zero mean stress reduces fatigue life. The Goodman diagram dot moves right, approaching the failure line. This is why pre-loaded bolts and pressurized vessels need mean stress correction.
Kf surface finish (0.40-1.00): Rough surfaces have lower Kf, reducing Se. A polished part (Kf=1.0) survives much longer than a machined one (Kf=0.7).
Kd size factor (0.50-1.00): Larger parts have lower fatigue strength. A 50mm shaft has lower Kd than a 10mm shaft.
Key slider experiments::
Start with steel at σa = 150 MPa. Status = INFINITE. Now drag to 250 MPa (just above Se = 240). Life drops from ∞ to ~2M cycles. That cliff is the endurance limit.
Switch to aluminum. Even at 80 MPa, life is finite (~500M cycles). Aluminum never has infinite fatigue life. This is why aircraft get mandatory inspections.
Set σa = 200 MPa on steel. Life = ∞. Now drag σm from 0 to 200 MPa. Watch the Goodman dot slide right toward the failure line. Life drops because mean stress makes fatigue worse.
Drop Kf from 0.85 to 0.50 (rough surface). Watch Se plummet and the status change from INFINITE to FINITE. Surface finish is one of the biggest real-world fatigue killers.