What occures when bolt tension is higher than material yield? 2

[Aride]

I am attaching an Aluminum_ADC12 bracket with a typical M10 bolt to rigid surface. My bolt tension values are higher than the aluminum brackets yield for the M10 bolt head. What kind of issues will I be running into? Current testing shows that I am not destroying (i.e. fracturing) the bracket or impregnating the bolt into its surface for that matter. This has yet to occur due to the fact that my tension values are just above bracket yield. My concern is about fatigue failures on the bracket due to stress concentrations/risers, but am I missing something else? I typically would stay under bracket yielding with bolt tension but in this case my torque and bolts are constrains at this is a request from higher up. Any thoughts?

Thanks

 

[CoryPad]

How are you comparing bolt tension (a force) with the bracket yielding (a stress or pressure = force divided by area)?

The calculation of interest in this case is surface pressure (sometimes called bearing stress). You take the bolt tension and divide by the contact area between the bolt head and the bracket. You then compare this value to the allowable surface pressure for the bracket material. A rule of thumb is that the allowable surface pressure is approximately equal to the materials ultimate tensile strength (due to elastic and plastic constraint from the surrounding material). Even if you reach the pressure limit, that just means you begin indentation of the bracket, which does not necessarily mean part failure. You will need to decide what the part limits are with respect to static and cyclic loading, temperature exposure, etc.

This type of analysis is contained in VDI 2230 Systematic calculation of high duty bolted joints.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[desertfox]

Hi Aride

You may not be embedding the fastner head into the bracket material at this point for 3 reasons

1/ the aluminium may have a higher yield stress than the
figure you are using because usually yield stresses
quoted for any material are a minimum although I would
not advise working above this figure generally.

2/ If your using a washer which you haven't mentioned then
that might be spreading the load sufficiently enough
for the bracket not to reach yield.

3/ If your using a torque wrench your preload from bolt to
bolt may vary by +/- 30% possibly more

However if you are yielding the alumium bracket then I would expect embedding of the head or washer into the material, which will reduce your bolt pre-load for your given torque, which defeats the object of your set tension
force and torque as a design criteria.

Can you provide more information like Torque/Tension figures
your working too and whether or not you are using washers,is this joint seeing static or dynamic loads?
Finally how critical is the joint tension, how are you measuring torque/tension and how have you estabilished that
your bolt tension is sufficient to yield the bracket.

regards

desertfox

 

[adi1]

I would agree with Desertfox and Cory.

Generally the material specs would be "min" specs.If you are right exactly at the min spec then u "might" not see any joint failure.Based on the accuracy of the tool, if you over torque, then there might be possible damage to the bracket and eventually joint fails.