Grain Orientation for bends 1

[Rob130]

Standard practice is to bend across the grain for single bends.

My question relates to different bends in a flat sheet.

Say you need to make a triangular gusset that is riveted on 2 flanges in a 90 degree corner. If you layed the sheet out to where one bend was cross grain, the other bend would be with the grain and susceptable to cracking.

Would it be a better practice to lay the bends out offset 45 degrees to the grain direction? If layed out in this manner the grain would cross both bends at the 45 degree angle.

I am not aware of any data addressing this issue in metal forming books.

Any input would be appreciated on how best to lay this out.

TIA, Rob

 

[Sparweb]

I think you've already found the solution to your problem.


Steven Fahey, CET

 

[Rob130]

Thanks Steve, I thought so. Being a run of the mill sheet metal guy this layout makes sense to me, but you always run into the "EXPERT" who think parts need to be layed out parallel and perpendicular to the grain. I can't tell you the number of times I was told the 45 degree orientation was wrong.

 

[CRACKKILZ]

Rob130

Rob,I too believe your solution is the best compromise.

In addition, you bring up an interesting point that I believe warrants further discussion. Many MRB aerospace/aircraft engineers are unaware that aircraft mechanics are taught to bend formed angles at 90 degrees to the grain. This is done to minimize the potential for cracking in the radius. I have seen many reports where the engineer mistakenly uses an ultimate allowable stress based on the "L" grain orientation value instead of the "LT" orientation for a repair angle (or visa-versa).

This is not usually a problem for most materials as the "b" basis allowables are normally very close. However, certain materials have drastically different allowables.... for example 301 1/2 hard Fcy (L) = 69,000 psi vrs Fcy (LT) = 116,000 psi.

BOTTOM LINE: If the part drawing or repair does specify grain direction, then assume worst case for the analysis. Else you could have a serious problem on your hands.

Any other comments?