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How to Calculate Fatigue Life?

Until now, you would have studied stress-based algorithms. Ideally, the strain-based algorithms or low cycle fatigue come from severe loading, including temperature and bolt loading. Several factors cause the material to drift in the plastic zone.  The number of times you would turn on or park the vehicle would not be as great as the vibrational cycle number.

Hence, the material will last for 5000 cycles instead of an eternity in the plastic zone. Therefore you need to focus on plastic strain instead of considering the stress fatigue calculations. The formula for fatigue calculation is given here.

A means the geometrical constant:

K means the stress intensity factor. This factor controls the fatigue life of the material instead of the stress and strain absolute.

Random vibration

Random vibrations mainly have different amplitudes and diversifying patterns. The higher frequencies will lead to higher energy infusion in the element that gradually plummets as we go ahead.

When considering the random vibration, you consider the statistical parameter barriers. For example, if you have to walk over the frequency range, the stress graph will profusely vary as it is a white frequency range. As you cannot consider the individual value of the stress, you can extract the RMS value of the stress. RMS value would help you understand the equivalent stress value. You can compare the RMS value to the endurance limit of the material. You have to take the value and plug it into the equation to ultimately understand the number of cycles which is a conservative approach undoubtedly.

As the profile for the random variations is uncertain, the normal distribution is important. The uncertainty implies that stress might not be pretty high all the time.

When do you need to perform the fatigue analysis?

Generally, you need to perform a fatigue analysis for your existing plants to evaluate the actual failure cause. The analysis could be performed only if the project specification permits the new plants. Before starting the analysis, you have to be Ready with the following information, like the fatigue curve of the piping material and enough process information for finding the complete number of cycles throughout the design of the piping system.

The allowable stresses for the fatigue analysis would be required to be interpolated logarithmically from the fatigue curve, which is based on the cycle numbers designated in the fatigue load cases. The calculated stress would be assumed to be a peak-to-peak cycle value. On the flip side, harmonic and dynamic load cases would be calculated stress, so the extracted allowable need to be divided by two prior to the comparison.


It would be best to learn about fatigue calculation when you expose the product to real-time situations, and the environmental factor plays a crucial role. Hence, the calculation of fatigue life is simple; hence only point is to understand the core basics.

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