Stress Life Fatigue.xls

Purpose of calculation:
Fatigue design of cast or wrought iron and steel.
Note another calculation procedure is required for welded steel and iron.
Approach: Stress-Life Analysis - the stress-life method is typically used for long life situations (millions of cycles) where the stresses are elastic. It is based on the fatigue limit or endurance limit of the material. Material properties from polished specimens are modified for surface conditions and loading conditions being analysed. Stress concentration factors are used to account for locally high stresses. An effective stress concentration in fatigue loading is computed. An estimate of the fatigue life is determined from the Goodman diagram.

Calculation Validation
This calculation has been checked against three published calculations.

1) Input Applied Stress
2) Input Material Data
3) Include Cast Factor (reduced fatigue performance due to casting defects).
4) Determine Size Factor, Ko
5) Determine Surface Finish Factor, Cs
6) Determine Allowable Unnotched Zero-Mean Alternating Stress
7) Calculate New SN limits to account for mean stress
Modify SN diagram for stress concentration
8) Calculate Strength Reduction factor kf6 for 106 cycles
9) Calculate Strength Reduction factor kf6 for 103 cycles
10) Introduce SN reductions to account for stress concentration
11) Draw SN Diagram
12) Fatigue Damage Assessment
Note: Use Miner's Rule applicable for cumulative damage (n1/N1 + n2/N2 +…<1)

Calculation Reference

Gene Mathers from TWI has written an overview of fatigue.

Juvinall Machine Design

Fatigue Design

Stress Concentration