Fatigue failure and it's prediction techniques.
The components that undergo varying load with respect to time and can fail well below the Ultimate tensile stress is called Fatigue failure. If you don't know , this failure contributes up to 90% of the total failure worldwide. So it is certainly very important to predict this type of failure and find a solution to avoid this failure.
To understand fatigue failure we will list out the components undergoing fatigue loading.
1. Drive shaft of the car
2. Wind Turbines
Cause of Fatigue failure
Fatigue failure occurs in 3 stages
1. Crack formation
Due to the constant to and fro motion (straining) crack is formed in the high stress regions ( below Ultimate tensile strength)
2. Crack Propagation
Since the crack can propagate. The crack which started initiating first is now extended .
3. Fracture.
The extended crack will lead to fracture at one point directly or lead to high stress that is over Ultimate tensile strength.
Fatigue Testing
The common method to test and know the fatigue life of a material is to subject the test component to varying load of different stress amplitude and plotting it down. This is also know as SN Curve which is basically a graph of stress amplitude plotted against number of cycles
The above graph has an SN curve, where Y-axis determine the Stress amplitude while X-axis determine the number of cycle. The area under the LCF is a zone where material can be subjected to high stress amplitude (above yield stress) but low life. While high cycle fatigue refers to low stress amplitude (below yield stress) but high fatigue life. There is an area in the graph which leads to infinite fatigue life of the component. Which means any stress that is developed under that stress limit ( endurance limit [Sf] ) will not cause any failure through out the life. It is very important to keep structures like bridges and towers to keep within this limit.
Fatigue failures are usually estimated for stress which is completely reversed , which means the sum of both the stress is always 0 (Mean Stress, Sm). But in practical life which is not always true. At the same time it is not easy and also very time consuming to find SN curve for different Mean stresses. So comes different approaches to determine the fatigue life different material.
Good Man Approach.
In Good Man approach a graph is plotted with stress amplitude on vertical axis and mean stress on horizontal axis. a straight line is plotted between endurance stress on vertical axis and ultimate tensile stress on horizontal axis. Any stress points within falling within this area is said to have infinite fatigue life. However this method cannot be used to determine the fatigue life, but our ideal concern is that the material should have infinite life, which makes this method useful
I coded in python to check if the component has infinite fatigue life.
Goodman Plot using Matplotlib for condition component will not fail
Conditions for which material will fail.
It is not always desired to keep the stress above Ultimate tensile stress because plastic deformation is not preferred. So comes Soderberg Equation where a line is plotted between endurance limit and yield stress and any point under the area is said to have infinite fatigue life.
It is not always a component will undergo constant stress amplitude, it differs over the time and change rapidly. So different techniques are adopted to find the fatigue life of the material in such cases. we will see about that in next blog. Signing of this week .
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