Question: Write on:Soderberg and Goodman design criteria.

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- Mean stress as well as stress amplitude exist when a component is subjected to fluctuating stresses.
- It has been observed that the mean stress component has an effect on fatigue failure when it is present in combination with an alternating component.
- The fatigue diagram for this general case is shown in Fig. below. In this diagram, the mean stress is plotted on the abscissa and stress amplitude on ordinate.
- The magnitudes of mean stress and stress amplitude depend upon the magnitudes of maximum and minimum force acting on the component.
- When stress amplitude is zero, the load is purely static and the criterion of failure is Sut or Syt. These limits are plotted on the abscissa.
- When the mean stress is zero, the stress is completely reversing and the criterion of failure is the endurance limit Se that is plotted on the ordinate.
- When the component is subjected to both components of stress, the actual failure occurs at different scattered points shown in the figure.
- There exists a border, which divides safe region from unsafe region for various combinations of mean stress and stress amplitude.
- Different criterion are proposed to construct the borderline dividing safe zone and failure zone. They include Gerber line, Soderberg line and Goodman line.

- Gerber Line: A parabolic curve joining Se on the ordinate to Sut on the abscissa is called the Gerber line.
- Soderberg Line A straight line joining Se on the ordinate to Syt on the abscissa is called the Soderberg line.
- Goodman Line A straight line joining Se on the ordinate to Sut on the abscissa is called the Goodman line.

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