Fatigue stress concentration factor (Kf)

In summary, the fatigue stress concentration factor (Kf) is reduced compared to the geometric stress concentration factor (Kt) due to the presence of ductility in real-world materials. This reduces the stress at the point of peak stress. While Kt assumes a perfectly elastic and brittle material, most materials have at least some level of ductility, resulting in a lower Kf. Additionally, for static loading of ductile materials, Kt is not computed, but for fatigue loading it is important as failure can be anticipated before reaching the yield point. The reason for Kf being lower than Kt is due to the role of ductility in reducing stress, which can be further researched using the terms "fatigue notch sensitivity" on Google
  • #1
kajalschopra
40
0
Hi,

I had been reading about the fatigue stress concentration factor (Kf) which is reduced compared to the geometric stress concentration factor (Kt). The reduction in turn dpends upon the material's sensitivity to notches.

https://moodlearn.ariel.ac.il/pluginfile.php/456050/mod_resource/content/0/Stress-consentration.pdf

I'm not able to sense why the fatigue stress concentration factor (Kf) is reduced compared to the geometric stress concentration factor (Kt).

What is the reason?
 
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  • #2
Simplified explanation: Kt assumes a perfectly elastic perfectly brittle material. In the real world, most materials have at least a little ductility, if only on a microscopic scale. That ductility reduces the stress at the point of peak stress.
 
  • #3
Simplified explanation: Kt assumes a perfectly elastic perfectly brittle material. In the real world, most materials have at least a little ductility, if only on a microscopic scale. That ductility reduces the stress at the point of peak stress.

I know. Also, for static loading for ductile materials we do not compute kt. The question is for fatigue we do compute kt because failure in fatigue can be anticipated before material reaches elastic limit / yield point.

Why kf is lesser than kt ? Ductility has no role to play in the reduction of kf than kt.
 
  • #4
Google is your friend. Use search terms fatigue notch sensitivity.
 

What is fatigue stress concentration factor (Kf)?

Fatigue stress concentration factor (Kf) is a dimensionless factor used to quantify the effect of stress concentration on a material's fatigue strength. It takes into account the geometric features of a component that can cause stress concentrations, such as sharp corners or changes in cross-sectional area.

Why is fatigue stress concentration factor important?

Fatigue stress concentration factor is important because it helps engineers and designers predict the effect of stress concentrations on the fatigue life of a component. By considering Kf, they can make design modifications to reduce stress concentrations and improve the overall fatigue strength of a material.

How is fatigue stress concentration factor calculated?

Fatigue stress concentration factor is calculated by dividing the maximum stress at the location of the stress concentration by the applied nominal stress. This gives a ratio which is then used to determine the effect of the stress concentration on the material's fatigue strength.

What are some factors that affect fatigue stress concentration factor?

There are several factors that can affect fatigue stress concentration factor, including the material properties of the component, the geometry of the component, and the loading conditions. Other factors such as surface finish, temperature, and residual stresses can also have an impact on Kf.

How can fatigue stress concentration factor be reduced?

Fatigue stress concentration factor can be reduced by making design modifications to eliminate or minimize stress concentrations. This can include changes in geometry, such as adding fillets or blending sharp corners, or using materials with higher fatigue strength. Proper surface treatments and finishing processes can also help reduce Kf.

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