# Fatigue Testing: Calculating ΔK vs da/dN from R & Max Stress

• arehman1289
In summary, the person is asking for help with plotting ΔK vs da/dN for their lab report on fatigue testing. They have been given an equation and are using ASTM E647 for assistance. They are looking for step by step help and have been directed to a handbook for information.
arehman1289
Hello Everybody,

Im doing my lab report on Fatigue testing for a Single Edge Notched Tension (SENT) sample and need to plot ΔK vs da/dN for a series of a vs N readings at different R and maximum stress values (attached in the picture). I've just been given one equation for Ki and told that I need to use ASTM E647 for assistance (attached equation). However after hours of going through it I was unable to figure out how to get ΔK vs da/dN from the data and information. Any step by step help would be highly appreciated. Please please.

Thanks

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There is a discussion of this starting on page 9-147 in MIL-HDBK-5H. That handbook has been superseded, but it is full of good information AND is available online. Just search MIL-HDBK-5H to find a copy. This figure summarizes two pages of step by step discussion that should help you.

## 1. What is fatigue testing?

Fatigue testing is a type of material testing that measures the ability of a material to withstand repeated or cyclic loading without failure. It is commonly used to assess the durability and reliability of materials under real-world conditions.

## 2. How is ΔK calculated in fatigue testing?

ΔK, or the change in stress intensity factor, is calculated by taking the difference between the maximum stress intensity factor (Kmax) and the minimum stress intensity factor (Kmin) during a fatigue test. This value is then used to plot the fatigue crack growth rate (da/dN) against the stress intensity factor range.

## 3. What is da/dN in fatigue testing?

Da/dN, or the fatigue crack growth rate, is a measure of how quickly a crack grows in a material under cyclic loading. It is typically plotted against the stress intensity factor range (ΔK) to determine the fatigue life of a material.

## 4. How is R calculated in fatigue testing?

R, or the stress ratio, is calculated by dividing the minimum stress (Smin) by the maximum stress (Smax) during a fatigue test. This value is used to determine the loading conditions and can affect the fatigue crack growth rate and overall fatigue life of a material.

## 5. What is the importance of calculating ΔK vs da/dN in fatigue testing?

The relationship between ΔK and da/dN is crucial in determining the fatigue life of a material. By plotting these values, engineers and scientists can understand how a material will behave under different loading conditions and make informed decisions about its use in real-world applications.

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