Doubt on Fatigue Strength S-N diagrams

In summary, when plotting an S-N graph, it is common to start by connecting the value of Sm (for N=1000 cycles) to the Endurance Limit, Se. For steels, Sm is typically taken as 0.9 times the Ultimate Tensile Strength (Sut). However, before the component reaches 1000 cycles at 0.9 Sut, it will yield at approximately 0.5 Sut, since Sy (yield strength) ≈ 0.5 Sut. Therefore, yielding may be the cause of failure for stress values, rather than fatigue. This means that if the allowable tensile stress (Sta) based on yield strength is lower than the fatigue strength (Sn),
  • #1
Abhishekdas
208
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A standard way of plotting an S-N (fatigue strength vs no of cycles) graph is to start off by taking a value of Sm (for N=1000) cycles and joining it to the point depicting Se(Endurance Limit). Now we commonly take Sm= 0.9Sut for steels. Now I was wandering, before you actually get to use the component for 1000 cyces at 0.9 Sut the component will yield at around 0.5Sut(before you reach Sm), as Sy(steel)≈.5Sut. SO should yielding be the cause of failure for values of stress rather than fatigue?
Thank you in anticipation.
 
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  • #2
Abhishekdas: Yes, you are correct. I.e., if the allowable tensile stress (Sta) based on tensile yield strength (Sta = Sy/FSy) is less than the fatigue strength (Sn), then Sta governs over Sn, in which case you would use Sta, where FSy = yield factor of safety.
 

1. What is an S-N diagram?

An S-N diagram, also known as a Wöhler curve, is a graphical representation of the relationship between cyclic stress (S) and the number of cycles to failure (N) for a specific material. It is commonly used to analyze the fatigue strength of a material and determine its endurance limit.

2. What is fatigue strength?

Fatigue strength refers to the maximum stress that a material can withstand for a specific number of cycles without experiencing failure. It is an important factor to consider in engineering applications, as repeated cyclic loading can lead to cracks and ultimately failure of a component.

3. How is an S-N diagram constructed?

To construct an S-N diagram, a material is subjected to a series of cyclic stress tests at various stress levels. The number of cycles to failure is recorded for each stress level, and the data is then plotted on a graph with stress on the vertical axis and number of cycles on the horizontal axis. The resulting curve shows the relationship between stress and fatigue life for the material.

4. What factors affect the fatigue strength of a material?

Several factors can affect the fatigue strength of a material, including its chemical composition, microstructure, surface finish, and environmental conditions. Additionally, factors such as stress concentration, temperature, and loading frequency can also impact a material's fatigue performance.

5. How is an S-N diagram used in engineering?

An S-N diagram is an essential tool in engineering design as it helps determine the safe operating limits for a material under cyclic loading conditions. Engineers use S-N diagrams to select appropriate materials for specific applications and to optimize the design of components to prevent fatigue failure.

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