Bolts snapped during removal of fender... why ??? how to remove remaining ??

2002 Monte Carlo LS
I was installing a hitch on said vehicle... what should be an easy function. Anyway... there are 6 bolts holding rear rupport to the frame. 8mm (about 5/16 thick)
not very thick at all.
Anyway.... not rusted, I applied normal torque, the head snapped off on one. Removed a couple.... head snapped off on another.... removed another and the head snapped off of the third.
Okay.. I'm used to car manufacturers building their products so they can't be disassembled. However the bolts were not rusted, absolutely no reason why they should snapped... (except for some paint overspray).
My speculation is that these bolts have a low shear strenth as they seemed to have a gray powdered appearance. Any reason why...
Second, even though they have a low shear strength, the remaining stud is difficult to remove. I've tried the usual vise grips., easy outs, stud removers and other devices. studs will not turn. I'm planning on drilling them out, but the material seems to be too hard for regular steel twist drills.
So, anyone know what the story is on the type of bolts used to hold the rear bumper assembly in place.... why so weak ?? Is this to make it difficult to remove and replace the rear bumper in case of an accident... thus isnuring replacement. (Between the auto manufacturers and the government, I'm beginning to sound like a conspiracy nut !!)
Second, what's a good type of drill bit to use to drill these puppies out... what type of tip material would be hard enough ??
Thanks !!
Peter
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In great generalities, very high strength steel can be brittle, or can fail due to what is called brittle failures. Steels can become more brittle than normal if they are exposed to corrosive environments, if they have work hardening, and if they have stress raisers (areas like the 90 degree transition between a bolt head and the bolt body) where mechanical stress can concentrate.
Overtightening can lead to stress fatigue, which is not uncommon in this sort of situation.
You already say that these bolt stubs are very hard, so it is not unbelievable that some of the reasons for failure listed above might be at work here.
This is just my guess, mind you.
Now, how do you get them out... - Get better drill bits and drill through them. Rethread and fit a slightly larger bolt, or put a nut and washer on the backside if possible.
- If you can get to them with a torch and heat them cherry red (without burning the car up, of course) , you can sometimes remove the temper (thereby annealing, or softening, them). This may make them easier to drill through.
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Whatever the reason... (nothing is obvious), they were "brittle".. As you mentioned it's possible they were overtorqued to keep the bumper attached and couldn't handle the reverse torque necessary for loosening. It wasn't as if the heads twisted off. Seems that when reverse torque was applied they just popped off.
I'm planning on drilling through the remaining studs tomorrow.... bumper is off the car.... access is pretty easy..... I'll just hacksaw the stud off short and drill through the 1/2 " remaining, then try to turn them out. if worst comes to worse the nuts are spot welded in place (pretty obvious) and will be easy to pop out and replace with a larger diameter nut and bolt set.
Thanks again for pointing out the possible causes...
Peter
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