Yellow Saddle
Guru
- Location
- Loch side.
I found a wheel nut in the street today. When you pick up a wheelnut, you can be 99% sure the stud had broken off, since the conical design prevents wheelnuts from loosening by themselves (actually by precession but that's a long story). I thought the nut demonstrates the discussion here nicely. Have a look.
The broken off stud is still inside. The break is clean as if a machine had cut it. The little brown spots on the stud is rust, which gives us an interesting perspective on how the bolt broke. It broke in one single event, otherwise we would have seen half the stud discoloured as the growing crack rusts behind the crack.
Have a look at the sharp piece of metal at the five-to position in the bottom photo. That's a piece of thread from the stud that broke off. The nut broke just after the start of the second thread coil and thus ripped the first thread coil right off.
What happened? the bolt was over tightened and thus put the bolt under critical strain where the cyclical stresses of cornering went beyond the metal's fatigue limit. The sharp stress riser of the thread gave the crack a nice place to start but thereafter it was effectively a travelling stress crack. Had this bolt not been overtightened it would still be on the truck.
It demonstrates nicely how only the first two or three thread coils support the attachment and how - assuming the right steel was used in the first place - overtightening can compromise a well-designed structure.
Think of this next time you sprint on the drops attached to the stem with some hardware store bolts.
The broken off stud is still inside. The break is clean as if a machine had cut it. The little brown spots on the stud is rust, which gives us an interesting perspective on how the bolt broke. It broke in one single event, otherwise we would have seen half the stud discoloured as the growing crack rusts behind the crack.
Have a look at the sharp piece of metal at the five-to position in the bottom photo. That's a piece of thread from the stud that broke off. The nut broke just after the start of the second thread coil and thus ripped the first thread coil right off.
What happened? the bolt was over tightened and thus put the bolt under critical strain where the cyclical stresses of cornering went beyond the metal's fatigue limit. The sharp stress riser of the thread gave the crack a nice place to start but thereafter it was effectively a travelling stress crack. Had this bolt not been overtightened it would still be on the truck.
It demonstrates nicely how only the first two or three thread coils support the attachment and how - assuming the right steel was used in the first place - overtightening can compromise a well-designed structure.
Think of this next time you sprint on the drops attached to the stem with some hardware store bolts.
