Failure Criterion for Ductile Materials under Static Loading

When a component made of ductile materials like steel is subjected to static loading, the strength used as a failure criterion is the yield strength. Let's understand why this is so:

Yield Strength:

- Yield strength is the point at which a material shows a significant increase in strain without an increase in stress. It is the stress level at which a material starts to deform plastically, i.e., it undergoes permanent deformation.
- In a ductile material like steel, the yield point is well-defined, and the material deforms plastically before it fails.
- Therefore, when a ductile material is subjected to a static load, it will fail when the stress exceeds the yield strength.

Ultimate Strength:

- Ultimate strength is the maximum stress a material can withstand before it fails.
- In a ductile material, the ultimate strength is higher than the yield strength. However, it is not used as a failure criterion under static loading because the material undergoes significant deformation before it fails, and the deformation can be observed and measured.
- Therefore, in the design of components made of ductile materials under static loading, the yield strength is used as the failure criterion.

Endurance Limit:

- Endurance limit is the maximum stress a material can withstand without undergoing fatigue failure.
- It is used as a failure criterion only under cyclic loading conditions, where the material undergoes repeated loading and unloading cycles.
- Therefore, it is not relevant to the failure criterion for components made of ductile materials under static loading.

Conclusion:

In conclusion, for components made of ductile materials like steel, subjected to static loading, the strength used as a failure criterion is the yield strength. It is the stress level at which the material undergoes permanent deformation and fails.