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Mechanical Engineer - Interview Questions
How do you calculate the fatigue life of a mechanical component?
The calculation of the fatigue life of a mechanical component involves estimating how many cycles of loading the component can endure before it fails due to fatigue. Fatigue failure occurs when a material undergoes repeated cyclic loading and unloading, leading to the accumulation of microscopic cracks and eventual failure, even if the loads are below the material's ultimate strength. Here are the general steps involved in calculating the fatigue life of a mechanical component:

* Stress Analysis : Determine the applied loads, both static and cyclic, that the component will experience during its service life. These loads can be axial, bending, torsional, or a combination.

* Stress History : Create a stress-versus-time history for the component based on the applied loads. This involves identifying the maximum and minimum stress levels experienced during each cycle of loading.

* Material Properties : Gather material properties, especially those related to fatigue, such as the material's fatigue strength (S-N curve), endurance limit (if applicable), and other relevant properties like Young's Modulus and Poisson's ratio.

* Stress-Life (S-N) Curve : Plot or obtain the stress-life (S-N) curve for the material. This curve represents the relationship between stress amplitude (S) and the number of cycles to failure (N) under cyclic loading conditions. It provides the basis for estimating fatigue life.

* Stress Calculations : Use the stress history and material properties to calculate the stress amplitudes experienced by the component during each loading cycle. This may involve calculating alternating stresses (σa) and mean stresses (σm) if applicable.

* Cycle Counting : Determine the number of cycles (N) at each stress amplitude level using the stress-life curve. This may involve extrapolating the curve if data is limited.

* Miner's Rule : If the component experiences multiple stress levels during its lifetime, apply Miner's rule or other cumulative damage models to combine the damage caused by each stress cycle. The rule assumes that damage accumulates linearly with the number of cycles.

* Calculate Total Fatigue Life : Sum up the fatigue life contributions from all stress levels to estimate the total fatigue life of the component.

* Factor of Safety : Apply a factor of safety to the calculated fatigue life to account for uncertainties in loading conditions, material properties, and manufacturing variability.

* Check Against Design Life : Compare the calculated fatigue life with the required or expected design life of the component. Ensure that the calculated life meets the design criteria.
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