What is the Goodman Diagram?

The Goodman Diagram, also known as the Modified Goodman Criterion, is a fundamental tool used in mechanical engineering to estimate the fatigue life of materials subjected to cyclic loading. It establishes a linear relationship between mean stress and alternating stress to determine the safe operating limits of a component. Unlike simple static analysis, the Goodman criterion accounts for the fact that components fail at much lower stress levels when the load is repetitive.

How to Use the Goodman Calculator

To use this tool, you need four primary inputs: the alternating stress (the amplitude of the stress cycle), the mean stress (the average stress over the cycle), the endurance limit of the material (the stress level below which fatigue failure is unlikely), and the ultimate tensile strength (the maximum stress the material can withstand before breaking). Once entered, the calculator applies the formula:

(σa / Se) + (σm / Sut) = 1 / n

Where 'n' represents the Factor of Safety. If n is greater than 1, the design is theoretically safe from fatigue failure for infinite cycles.

FAQs on Fatigue Analysis

What is the difference between Goodman and Soderberg?

While the Goodman criterion uses the Ultimate Tensile Strength (Sut) as its limit on the mean stress axis, the Soderberg criterion uses the Yield Strength (Sy). This makes Soderberg more conservative and safer for applications where any plastic deformation must be avoided.

When should I use the Goodman Diagram?

It is most effective for brittle materials and for estimating the fatigue life of ductile materials where the mean stress is tensile. It is widely preferred in industrial design because it provides a realistic middle ground between the conservative Soderberg line and the more aggressive Gerber parabola.