Goodman and Safety Factor

[Drej]

Dear group,

I have been dealing with fatigue assessments the past few months, specifically to do with components in the automotive industry. I am interested to know your thoughts/opinions on how to obtain the fatigue factor of safety based on the data from the Goodman diagram. The reason I ask is because I have been told conflicting ways of doing it, thus:

Method 1)
Draw the Goodman diagram. Plot a point 'x' which is the mean/alternating stress (this point will lie inside the Goodman line). Measure the distance from the origin to 'x' - call this distance 'L1'. Then measure the distance from the origin through this point to the intersection of the Goodman line - call this 'L2'. Factor of safety (FOS) = L2/L1. This is how I have always been told to obtain the FOS.

Method 2)
FOS = 1/((Sa/Se)+(Sm/Su))

The two methods differ in value by approx 50%.

Any thoughts most appreciated, cheers.

 

[metengr]

The factor of safety equation used in fatigue analysis is the method below;


Sa/Se + Sm/Sut = 1 or Sa/Se + Sm/Sut = 1/Nf

where Sa = stress amplitude
Se = endurance limit
Sm = mean stress
Sut = ultimate tensile strength
Nf = cycles to failure

Any combination of mean stress and stress amplitude that lies on or below the Goodman line will have infinite life. In some cases, I have seen a concentration factor that is applied to alternating stress Ka*Sa in the above equation.