Analyzing Fatigue Failure in a Shaft Under Bending and Bearing Loads

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Discussion Overview

The discussion revolves around the fatigue analysis of a shaft made of Al-6061 under bending and bearing loads. Participants explore factors contributing to unexpected fatigue failure, including stress concentration and loading conditions, while seeking to understand the conditions under which the shaft may fail.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant reports that their fatigue analysis indicates the shaft will never break, which contradicts observed failures after approximately 3.5 years of use.
  • Another participant inquires about the stress concentration factor being used at the failure point.
  • A participant confirms using Kf from the Soderberg equation, calculating it to be 1.72.
  • It is suggested that fatigue failure may arise from not properly accounting for factors such as stress concentration, surface factor, and gradient factor in the calculations.
  • One participant references Peterson's Stress Concentration Factors, estimating Kt between 2.51 and 2.58 for specific dimensions of the shaft and suggests that the nominal bending stress at a critical fillet could be around 35MPa without applying Kt.
  • There is a question about the positioning of the bearings and whether the dimensions used in calculations are correct.

Areas of Agreement / Disagreement

Participants express differing views on the factors contributing to fatigue failure, with no consensus reached on the exact causes or the validity of the calculations presented.

Contextual Notes

Limitations include potential missing assumptions regarding loading conditions, the accuracy of stress concentration factors, and the specific geometry of the shaft. The discussion does not resolve these uncertainties.

Who May Find This Useful

Individuals interested in fatigue analysis, mechanical engineering, materials science, and those dealing with shaft design and failure analysis may find this discussion relevant.

guy_alon
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hello,

im trying to do a fatigue analysis on a shaft but keep getting that it will never brake,
which is impossible , because of various cases that it did after something like 3.5 years
the shaft is made of Al-6061 and a divided force of 700N is working on him.
the linear speed at the edge is 1m/s or 382 rpm and it has 1 bearing on each side.
i attached a file with an illustration with more detailes and a picture
i want to find out why it fails and how much weight i need to substruct in order for it to work.

thank you
Guy Alon
 

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What stress concentration factor are you using for the corner where it's breaking?
 
do you mean Kf from the soderberg equation?
i calculated it and i got
Kf = 1.72
 
What I've generally noticed is that when something fails in fatigue (high cycle fatigue) when it was otherwise not expected to fail, is that the calculation doesn't take certain factors such as stress concentration, surface factor, gradient factor, etc... into account properly. Check these factors; here's one for bending of a stepped shaft for example.
http://www.tribology-abc.com/calculators/e1_1b.htm
 
Based on Peterson's Stress Concentration Factors 2nd Ed. I'm getting Kt between 2.51 and 2.58 under bending only, for D=25mm, d=17mm and r=0.5mm:

https://efatigue.com/ (click on stress concentration)

According to the drawing, it seems that the bearings are positioned where d=17mm, is this correct ? If so and considering a distributed load, I'm expecting the nominal bending stress to be at least 35MPa at the fillet between 17mm and 25mm diameters, without applying the estimated Kt. How much are you obtaining ?
 
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