Analyzing Fatigue Failure in a Shaft Under Bending and Bearing Loads

In summary, the conversation is about performing a fatigue analysis on a shaft made of Al-6061 with a divided force of 700N and a linear speed of 1m/s or 382 rpm. The individual is trying to find out why the shaft is failing and how much weight needs to be subtracted for it to work. They also discuss the stress concentration factor and how it may not have been properly taken into account in the calculation. The individual provides a link to a website for calculating stress concentration factors and mentions that the nominal bending stress should be at least 35MPa at the fillet between 17mm and 25mm diameters.
  • #1
guy_alon
2
0
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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  • #2
What stress concentration factor are you using for the corner where it's breaking?
 
  • #3
do you mean Kf from the soderberg equation?
i calculated it and i got
Kf = 1.72
 
  • #4
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
 
  • #5
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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1. What is a fatigue analysis problem?

A fatigue analysis problem is a type of engineering problem that involves predicting the failure of a material or structure under repeated loading and unloading cycles. It is an important aspect of design and maintenance in industries such as aerospace, automotive, and civil engineering.

2. How is a fatigue analysis problem solved?

A fatigue analysis problem is typically solved using specialized software that takes into account factors such as material properties, loading conditions, and geometry of the structure. These tools use mathematical models and algorithms to simulate the stress and strain experienced by the material, and predict its fatigue life or the number of cycles it can withstand before failure.

3. What are some common causes of fatigue failure in materials?

Some common causes of fatigue failure in materials include cyclic loading, which can be caused by vibrations, thermal cycling, or mechanical loading. Other factors such as material defects, stress concentrations, and environmental conditions can also contribute to fatigue failure.

4. How is fatigue analysis used in industry?

Fatigue analysis is an important tool in industry for predicting the lifespan of materials and structures, and ensuring their safe and reliable operation. It is used in the design and testing of products to optimize their performance and durability, as well as in maintenance and inspection to identify potential fatigue issues before they result in failure.

5. What are some limitations of fatigue analysis?

While fatigue analysis is a valuable tool, it also has some limitations. It relies on mathematical models and assumptions, which may not always accurately reflect real-world conditions. Additionally, the vast number of variables involved in a fatigue analysis problem can make it challenging to accurately predict fatigue life. Therefore, it is important to use caution and verify results with physical testing when possible.

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