Fatigue Endurance Limit

S_{e} a pp r o x 0.5 S_{u} q u a d (t e x t s t ee l, r e v er se d b e n d in g, N t o in f t y)

S–N curve: stress amplitude vs cycles to failure. Many steels show an endurance (fatigue) limit Se ≈ 0.4–0.5 Su for polished reverse bending. Aluminum has no true limit — use Sf at 5×10⁸ cycles. Surface, size, reliability, and mean-stress (Goodman) modify Se.

$S_{e}$ (psi): Endurance limit
$S_{u}$ (psi): Ultimate strength
$N$: Cycles

Step-by-step solved example

Steel Su = 80 ksi, polished reverse bending. Estimate Se.

S–N idea: amplitude vs cycles. Steel endurance ~ 0.5 Su.

1. Rule of thumb
Se ≈ 0.5 Su = 40 ksi.
$S_{e} = 40 ksi$

Answer: 40 ksi

10 practice questions

0/10 correct

1.Endurance limit is the

2.For steels Se is often estimated as

3.Aluminum S–N typically

4.Rougher surface finish

5.Goodman accounts for

6.Fully reversed bending means

7.A larger part tends to have

8.High-cycle fatigue is typically N

9.Shot peening improves fatigue by

10.If amplitude is below Se (steel, polished, infinite life design)