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.
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)
Gene Mathers from TWI has written an overview of fatigue.
Juvinall Machine Design
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The estimate breaks down as the strains become relatively small since the Elastic Slope "AB" does not include an option to flatten it completely out at the fatigue limit.
The "wrong way" kink that is visible reflects the transition from small changes in stress resulting in large changes in strain above the yield point to the opposite in the elastic range. Setting a lower cut-off of 10^3 hides most of this in the typical fatigue plot.
Remember - Reasons that sound good, are not always good sound reasons.
Now I understand the method better the only difficulty I am having is understanding the general shape of the stress vs. cycles produced. As shown in the image below The curve seems to kick down at 1E4 cycles whereas I am used to seeing steel fatigue curves with an endurance limit at 1E6 as shown in the lower plot which has been taken from Stress Life Fatigue.xls. I wondered if you had any comments on this? Filename Fatigue_Estimator_with_XLC.xls, size 108032
Here's the paper I was referring to. Filename Fatigue_CUrve_Estimates_1980004.pdf, size 1123540