To "stop drilling" a bumper crack, or not?

[rationull]

I'm setting up to epoxy repair a crack in my car's plastic bumper cover this weekend, and reinforce it with some fiberglass screen. When researching the repair, I've read some people suggesting doing a stop drill to keep the crack from spreading. My logic up to this point (i.e. leaving it be) was that the crack had settled and the surrounding area was unstressed, in whatever position it "naturally" ended up in. However, that will change when I epoxy it together, since I'll probably have to move it a bit to align the pieces, and I'm wondering if I should put a small hole at the end to keep it from spreading.

I'll do this if it's necessary but if it's not, I'd just as soon avoid drilling into the bumper. So, is stop drilling a good strategy on plastic like it is on glass and metal?

 

[MaximusD]

I'd be worried about spider webbing the crack. I'd just throw some epoxy on the back side of it and go from there.

 

[Trav]

Drill it! Seriously if you don't your wasting your time.
The holes fill easily and it should be repaired front and back for a proper permanent repair.

 

[Panzerman]

Drilling stops the crack from going. Its a old time trick, it works. I use it all the time in repairing ATV fenders.

 

[46Harry]

Stop drill the crack for sure. If you don't, there is a good chance the crack will spread. A small hole maybe 1/8 inch is good. We do this to plastic or composite aircraft parts regularly to stop cracks from spreading.

 

[Kira]

Hello, I know this is SLIGHTLY OFF-TOPIC but if ever you're in Philadelphia go take a look at the Liberty Bell. You'll see where they tried to "stop-drill" the crack many times. The original pour of the bronze was interrupted and a cooled surface which didn't flow into the pour's continuation was created, I was told. You can see where they increased the size of the drill bit too. Those rascals at the White Chapel Bell Factory (London) sold us shoddy goods. Kira

 

[qwertydude]

I've never had any luck with fiberglass repair of anything but fiberglass items. The problem is fiberglass and epoxy don't flex like the original material and tend to pop off or flake.

With bumper covers and similarly motorcycle body panels, I've had much better and cleaner results by plastic welding. It can even be done simply with a cheap soldering iron and appropriate filler material. Chisel tip soldering irons tend to work better.

 

[rationull]

Sounds like I will be stop drilling it, then!

I'm not going to use the fiberglass screen as a core part of teh repair, just wanted to add a little backing material to hold things together since the spot where the bumper cracked is a common spot. The plan for the "real" repair is to epoxy it from the back.

I will touch up the front with paint if needed, but the car is dark and the crack barely shows up as it is.

 

[dwcopple]

when stop drilling, do you go on the end of the crack itself or a hair away from it?

 

[rationull]

My understanding is that you want to round out the very end of the crack to distribute any stresses over a circle instead of a point.

 

[The Critic]

This video might help:

http://www.youtube.com/watch?v=k3aM4lSgRHY

 

[Shannow]

Originally Posted By: rationull
My understanding is that you want to round out the very end of the crack to distribute any stresses over a circle instead of a point.


Talking metal here, but stop-drilling has a very strong basis in theory.

The crack, at its tip has an almost infinitely tiny point (almost, it's down to molecular level), which makes the stresses high enough to keep the crack running.

The drilling puts a dead flat wall in the way, as the end of the hole is almost as flat as the tip of the crack is sharp. The hole drilled can be tiny, as the crack tip is miniscule, but you want to remove all the overstressed material from the area around the crack tip.

A stop drill may not stop the crack every time, but it is always slower growing after than it would be undrilled...(in boilers and turbines, we often mark them with the sequential date of drilling to keep track of things.

 

[rationull]

Did the repair yesterday, and it went well as far as I'm concerned. Did end up stop drilling, and I'm glad -- gives me a little more piece of mind. I will probably post some pictures sometime this week if anyone is interested -- it's not a perfect, professional looking result but I learned a bit and it's what I was shooting for.

 

[Robenstein]

Stop drilling works....I seen it done on military guns by arsenals to stop slide/frame cracks.