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Author: deajohn5556
Time: 09/06/12 10:27 PST
This is a reply to message no. 21804 by donahldo
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Current Topic:
stress concetration factor (SCF)

The relation is

 

K = 1 + q(Kt-1)     where Kt is the theoretical stress concentration factor that is dependednt on geometry only.  For example wherever there is a sudden change in cross section, as on a stepped or filleted shaft, there is a geometry dependent stress concentration factor that can vary  between 1 and 3  or even 4 and higher in certain cases. The SCF is multiplied by the nominal stress to get the actual stress.  This new stress is not much problem for  ductile materials with well defined yield and elongation, as stresses will locally redistribute and not cause failure.

On the other hand, q is the index of sensitivity and is related not to the geometry but to the material itself.   Tis is utilized for fatigue calculations. Some materials are less sucsceptible to failure in fatigue for rather sharp radius stress concentration.      The value of q ranges from 0 to 1.

 

For example, let's say Kt = 2.5 for a stepped shaft and for the material q = 0.5

Then the value of K is

K = 1 + 0.5(2.5 -1 ) = 1.75

 

For another example, let's say Kt = 2.5 for a stepped shaft and for the material q = 1.0

Then the value of K is

K = 1 + 1.0(2.5 -1 ) = 2.5

 

Notice that K is never greater than Kt

The IOS needs to be obtained from charts for the material as a function of notch radius and are not always readily available, in which case use q= 1 for conservatism.

 

If a stress concentration factor exceeds 4, it probably needs to be reduced with q less than unity.

 

 

 

 

 

rrion

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