Table of Contents
The Goodman and Soderberg criteria are widely used methods for assessing fatigue failure in materials subjected to cyclic loading. These criteria help engineers predict the lifespan of components and ensure safety in various applications.
Understanding Fatigue Failure
Fatigue failure occurs when a material is subjected to repeated stress cycles, leading to the initiation and growth of cracks over time. This process can result in sudden failure even if the applied stresses are below the material’s ultimate strength.
The Goodman Criterion
The Goodman criterion provides a graphical method to evaluate the safety of a component under combined static and cyclic stresses. It relates the mean stress and alternating stress to the material’s endurance limit.
The Goodman line is defined by the equation:
σm / σult + σa / σe = 1
where σm is the mean stress, σa is the alternating stress, σult is the ultimate tensile strength, and σe is the endurance limit.
The Soderberg Criterion
The Soderberg criterion offers a more conservative approach by considering the yield strength instead of the ultimate strength. It is often used for safety-critical components.
The Soderberg line is described by:
σm / σy + σa / σe = 1
Application in Design
Engineers use these criteria to determine safe stress levels for components under cyclic loading. By plotting the stresses on the Goodman or Soderberg diagram, they can assess whether a design is within safe limits.
- Identify the maximum and minimum stresses.
- Calculate the mean and alternating stresses.
- Plot the stresses on the appropriate diagram.
- Compare with the Goodman or Soderberg line.