Freeze Plugs and e/D 1

[shootingiron5]

An interesting question I was posed recently was how does freeze plug installation play into fastener edge distance. The only information I have comes out of Michael Niu's book "Stress Analysis and Sizing" that gives the statement that the plug does not restore material to the basic section. Given this information I have been of the opinion that if a plug is installed anywhere between the fastener and edge then theoretically the plug diameter should be subtracted when taking into account the 2D margin. Has anyone ran across this information or a situation like this?

 

[jetmaker]

Not too sure about your situation... are you saying that you hve an edge, then a freeze plug installed at a distance from the edge (say 0.5"), and then a fastener location some greater distance from the edge and freeze plug (say 1" from edge and thus 0.5" from freeze plug)?

If this is the case, I would be more concerned about the short edge distance of the freeze plug causing a fatigue problem. However, for static strength of the fastener, I would conservatively use the distance to the freeze plug. To use the distance less the freeze plug diameter is also conservative as the shearout capablitiy of the sheet is greater than that for the fastener due to the larger diameter of the freeze plug. Also, the bearing capability is greater since the plugs are often made of a 7XXX series alloy which should be as strong or stronger than then consitutent joint materials.

jetmaker

 

[shootingiron5]

Yes, it would be as you describe.

Just for plugs in general - it may be that I have not looked enough but so far I do not find much more information than I stated regarding the installation of plugs. Other engineering personnel I have asked ascribe to the 2D for fastener spacing when using a plug. I am not sure this is really applicable since the plug itself would essentially be applying no shear load.

 

[jetmaker]

My concern would be this: as the e/D of the freeze plug approaches 0.50, then the plug has little to no bearing capability in the direction normal to the edge. If your fastener has a low e/D relative to the freeze plug hole edge, then it will follow the strength criteria of that e/D value.

Now... if your situation is that the freeze plug and hole already exist, and you need to show it good for static, then I would calculate the bearing strength of the freeze plug using the Fbu associated with its e/D. Calculate the bearing load, and then use that as your allowable for your other fastener. This is reasonable when the load is normal to the edge. When the load vector is parallel to the edge, then use the e/D to the freeze plug since the boundary conditions provided by the freeze plug are questionable, and it is thus conservative.

For fatigue, I would use the e/D to the freeze plug, and then account for the freeze plug press fit to be equilalent to shot peening.

The main thing to remember is that if the load goes thru the freeze plug, then you can use its ability to resist load.

jetmaker