[olddog]

I'm no expert, but I will share some experiance. I have worked with some top engineers and metalergests (sp?) in industry. In particular we were working on gear pumps used to pump molten polymers (plastic) at pressures over 4000 psi. We had shafts that were breaking.

The bolt that you pictured, show signs of a classic failure. The outer ring of the bolt has what they call beach marks. Then the center is where a brittle failure occurred. What causes the beach marks in the outer ring is a slow process. A crack initiates, usually from a scratch or some type of a stress riser such as a sharp machined corner without a radious. The crack slowly grows. As the bolt flexes under load, the two sides of the crack rub on each other, causing the beach marks. As the crack grows, the bolt has less and less cross sectional area, weakening the bolt. At some point the strength of the bolt becomes less than the load put on it and it fails.

If I were to guess - as I am not an expert - the crack has been growing for a long time. Obviously a heavier load on the bolt will make it fail sooner rather than later. Once the crack was started, even if you had never reved it above an idle, the bolt would have eventually failed. It would just have taken longer to continue to crack, until the meat left in the bolt was too small to hold the load at an idle. It usually does take enough load to cause some flexing of the bolt for the crack to start, but not necessarily a higher load than it was designed for.

For a price their are labs that can analize that bolt, and they can tell how many cracks started at exactly what points. I don't think it would be worth the price tag to analize it that far.

Where it broke is just above the joint line of the cap. Perhaps it was nicked when pulling the cap on a bind or something along those lines. The imperfection could have been when the bolt was manufactured.