Goodman Fatigue Safety Calculator For Ductile Materials: Alternating Stresses

Goodman Fatigue Safety Calculator For Ductile Materials: Alternating Stresses

With Safety Factor


Without Safety Factor


Goodman diagram(Robert L. Moth: Machine Elements In Mechanical Design).

What Is Goodman Theory?

Machine Elements in Mechanical Design (What’s New in Trades & Technology)

If you are interested in the guidebook that is used for this article, click on the given link above or the ‘Shop Now’ button to check it from Amazon!

Goodman theory is a very useful theory that you can use for fatigue calculation of your ductile material, that undergoes alternating stresses. Goodman equations are very basic but, definitions of variables in that theory are very important and depend on you.

We have also a Von-Mises stress calculator for ductile materials.

You can calculate your design’s safety situation with Goodman theory in two modes;

  • With Safety Factor: In this case, the calculation of the safety situation is done with the safety factor. Definition of safety factor is very important, and changes according to the application and material.

The above equation gives the Goodman line that is indicated as ‘Safe stress line’ Goodman diagram above.

Kt(Unitless): This is the magnifying factor that magnifies the absolute maximum value of alternating stress that changes with time sinusoidally.

Sigma A(Unit of Pressure: It is the alternating stress that its value changes with time sinusoidally.

SigmaM(Unit of Pressure): Mean stress value of sinusoidal alternating stress.

Sn(Unit of Pressure): Endurance strength value of your material in terms of stress.

Su(Unit of Pressure): Ultimate tensile stress value of your material.

N(Unitless): Factor of safety.

  • Without Safety Factor: In this calculation option of Goodman theory, a factor of safety is not considered.

All the variables are the same with calculation with safety factors.

How To Use Goodman Theory Calculator?

Expression of Goodman Theory calculator.

It has very basic use. Just select the calculation option of Goodman Theory which is explained above; ‘With Safety Factor’ or ‘Without Safety Factor’. Then enter the required variables and values inside the brackets as shown by the red arrow above. Click on ‘Calculate’! button to calculate. The result and the explanation of the safety situation will be given.

Do not forget to take a look at the other engineering calculators in Mechanicalland!

Click on the ‘Reset’ button to calculate a new problem.

You must need to take care of your units in calculations. Your variables must-have in the consistent unit sets. In here, you must use pressure units. To convert pressure units to obtain consistent values, you can use the MB-Unit Converter tool to do it!


As an engineer, every time is sure about your calculations. Mechanicalland does not accept the responsibility for the calculations that you made here.

Do not forget to leave your comments and questions below about Goodman’s theory of fatigue loads at ductile materials. Your precious feedbacks are very important for us!

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