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Is taking Average stress, where there is stress concentration, is safe?

 
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Dr. N. Subramanian
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PostPosted: Thu Jun 03, 2021 6:00 pm    Post subject: Is taking Average stress, where there is stress concentration, is safe? Reply with quote

An engineer asked a question recently:
I have a doubt. In office we have decided to average stresses in case it gets concentrated at certain location. So for averaging we take the value of an area within 1m square area. This is done for moment, shear and tension. But I would like to know your opinion about it. In some cases I feel it's too risky to go with it . When the value of average and highest value is too much. So I am confused about it.
My reply:
If the stress concentration is due to a small hole then it can be ignored and average stresses can be taken. But if the average stress in the member is high, you can't ignore it.


The effects of stress concentration at holes is normally neglected in design. In reality the stresses at holes can be as high as 3 times the average stress on the 'net' section. Such localized overstress is neglected in normal design practice, due to the ductility of steel. Thus, when the steel yields due to overstress at points of stress concentration, additional stress is transferred to the adjacent areas of cross-section, where the stress is lower than yield stress. This stress distribution allows the designer to take the average stress in the design. This property of ductility permits the initially yielded zone to perform without rupture as the stress in the reminder of the cross-section continues to increase. However, one has to be careful, as in certain situations, this ductile property of steel should not be relied upon. These situations include fatigue loading and extremely low temperature. In These situations, such stress concentrations will result in brittle fracture.
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vikram.jeet
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PostPosted: Fri Jun 04, 2021 4:46 am    Post subject: Reply with quote

Hol in plate

For small holes for bolts, as in gusset plates , the dia of holes is small compared to overall width /Length of plate and stress concentration may not affect , and whatever concentration of localized nature is taken care of vicinity steel thru   ductility as explained by Dr NS sir.
But if a plate carries hole for Lifting hook, it can not be left as stress concentration will be heavy due to comparable size of hole ,  circular stiffening strips around.  hole along with other stiffners to be provided .
lN present situation  ifhole dia is  small , average stress can be taken , but    otherwise jprovide stiffening g elements around hole , if functioning do not hinder.  Stiffening eelements  -circular stripsi
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