control failure

Stuart & Kathryn Fields wrote:
Don: Be sure to share what you find. I think that I'm only scratching the
surface of some stuff. One thing I found and had never seen before was an
equation relating the increase in stress due to a crack. It scares the hell
out of me. I'm afraid to use a metal fork in my salad. The equation
basically says that the max stress is 2 times the load divided by the area
times the square root of the crack length divided by the radius of curvature
of the end of the crack!!! If the radius of curvature was equal to the
crack length, the max stress is already twice what you would calculate using
the applied load and the element cross section. Now put a reasonably sharp
crack and see what happens....as the radius approaches 0.001 times the
length of the crack......????

That is because the load is not equally spread
across the part, but is concentrated at the end of
the crack.

Think about a piece of metal bar in tension with a
crack halfway across it. The cross sectional area
that has already seperated cannot bear any load at
all as it has already failed. The end of the
crack is taking a lot of load because the crack
pulls apart when it is under tension.

Solution: Don't use cracked parts!! (Well duh)

Don W.