Hot Rods Does drilling holes in sheet metal cracks stop it from spreading or is that a waste of time?

Does drilling holes in sheet metal cracks stop it from spreading or is that a waste of time? What size should be the drill hole? The crack is a stress crack. Poor design.

 

As long as you find the true end of the crack and drill a bit beyond the end and then cut the little bit to reach the hole it does in fact stop it from spreading. That should also be done before welding said crack. If you look on older utility trailers and older farm implements you'll not only see spot where they've been welded but also fresh cracks through welds on top of welds. It's because they didn't relieve the tension before welding. Usually a 1/4" bit is big enough to do it

 

Works in structural steel…

 

Yes, in aviation (skins) it's known as 'stop-drilling' and has been practiced for a good 100 years. It was a common practice in WWII aircraft. The drilled holes would allow the craft to fight on, w/o further damage until the plane was in for scheduled mechanical maintenance. Simultaneously, the S&S (structural and surfaces) guys would make more rigid repairs.
Generally speaking, if the repaired cracks return, there's an underlying substructure issue such as damage or pre-loading.

 

Would centering the drill bit on the end of the crack by ok?

 

Stop drilling in aviation is never permanent , aircraft are given flight cycles (pressurisation) or hours before the stop drilled cracks are fixed permanently as said above,.the best way to explain how stop drilling works is the stress travels along the crack around the radius of the stop drill and back up the original crack .......... until the original stress cracks out the edge of the stop drill hole . which is why theyre temp repairs and not done on primary structures

 

If you plan on welding the crack immediately, there’s no reason to drill the crack on automotive sheetmetal. It’s not stressed the same way that thicker structural steel or cast iron is. The heat of the weld (unless you are doing something stupid like cooling the weld immediately) will normalize the stresses in the area.

 

Just drill it and then weld it with as little additional heat input as you can. No matter what remains in the area near the hole, new stresses will be imparted into the metal when you weld it. Just do the best you can and then hammer weld it some to relieve the stresses.

 

Drilling a hole may help, short term. The act of welding the crack will add more stress to the already stressed sheet metal. Often the only way to fully relieve the stress is to replace the sheet metal at the structure support points or add seams on each side of the crack in the sheet metal.

 

After welding the crack I've thought about using a support brace on the back side of the panel. I would drill two holes adjacent to the crack and then spot weld the support brace behind. The support brace would just be another sheet of metal. Is this a good ideal?

 

This explanation from an EAA article shows the procedure we would use to repair cracks in aircraft sheet metal. You could use a similar procedure using spot welds instead of rivets.