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

Calculation Preview

26 Jan 2010
File Size: 255.71 Kb
Downloads: 189
File Version: 1.1
File Author: John Doyle
File Rating (4/34)

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Comments: 6
johndoyle[admin] 15 years ago
I agree that the curve does not look the way one would expect.
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.
johndoyle[admin] 15 years ago
I have had some time to work through this now MajorMagee. It took me a little while to see what was going on but I used XLC to generate some equations to revealed this clever method for fatigue estimation (and its even cleverer encoding into spreadsheet form). I have added my revised worksheet which includes the XLC equations.
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](/404/). I wondered if you had any comments on this? Filename [Fatigue_Estimator_with_XLC.xls](/images/fbfiles/files/Fatigue_Estimator_with_XLC.xls), size 108032
johndoyle[admin] 15 years ago
I still need to get my head around it but it looks like a very useful method. I have some friends coming round for band practise so I'll have to get back to you later - it will make a worthy repository item I think. Thanks again. John.
johndoyle[admin] 15 years ago
I'm not sure it's really worthy of the repository...
Here's the paper I was referring to. Filename [Fatigue_CUrve_Estimates_1980004.pdf](/images/fbfiles/files/Fatigue_CUrve_Estimates_1980004.pdf), size 1123540
johndoyle[admin] 15 years ago
This looks like a very interesting calculation probably deserving its own place in the repository. Would you like to upload it to the repository MajorMagee? I am not familiar with Peter R. Weihsmann work and I have been studying the calculation to understand the method. I probably would benefit for a little more explanation. I am going to play around with it to see how it compares with the "Stress Life Fatigue.xls" calculation. Great contribution again. Thank you.
johndoyle[admin] 15 years ago
I ran across a paper from 1980 by Peter R. Weihsmann, "Fatigue Curves Without Testing" that uses four-point correlation to generate fatigue estimates from just Tensile, Yield, Elongation and Modulus data. Filename [Fatigue_Estimator.xls](/images/fbfiles/files/Fatigue_Estimator.xls), size 94208
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