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Old 06-24-2011, 07:32 PM   #88
Benesesso
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Quote:
Originally Posted by WayneC1 View Post
A second metallurgist has looked at the pic's and his response is :-


"It appears to be a one time overload fracture"
Agree with the "one time" part, but we must be careful with the word "overload". From the obvious lack of distortion/stretching, etc. at the fracture, it is a brittle fracture for sure.

But an overload has to be related to something, e.g., design loads or an overload for the condition of the metal/part. If a given part has a design limit of X, there is/should be a factor of safety that is applied. For steel lifting cables, the factor of safety is 5. A cable rated for a safe working load of 1,000 lb. must be able to withstand a load of 5,000 lb.

A metallurgical overload is different from a design one. A part may be manufactured way too thin by error, such that a full design load would be an overload. But a metallurgical overload condition does not consider what the design load/proof load is.

There are brittle fractures, ductile fractures, fatigue fractures, stress corrosion fractures, corrosion fatigue fractures, etc. Overload fractures, also called gross overload fractures, are either ductile or brittle, or some combination of each. So far the visual evidence of these latest forks indicates a brittle overload fracture. It means that for whatever reason, the applied load/stress exceeded the material's ability to withstand it.

Some of the factors which can cause an overload fracture include things which concentrate stresses, such as threads, holes, casting porosity, excessive surface roughness, and other casting problems.

In a tough material, such as nuke submarine hulls and nuke power reactor pressure vessels, an overload fracture usually involves a lot of physical distortion, bending, stretching, etc., and the actual fracture mode is known as shear. In a brittle material, the mode is not shear, but is cleavage right thru the individual grains of metal, along certain atomic planes. There is little or no physical distortion involved.

Both of these fracture modes are known as transgranular (thru the grain), while fractures involving hydrogen are usually intergranular (between the grains)--almost certainly not the case for these forks.

So, what we appear to have is a one-time, brittle overload fracture, but of course that does not explain why it occurred. Did a small bump in the road raise the local stress at some casting defect beyond a critical crack-initiation level? A tough material would likely be able to handle high local stresses because it plastically yields a bit, whereas a brittle material could not yield. When a critical stress level is reached in a brittle material, sudden fracture occurs. The speed of such fractures has been estimated at a mile per second in long steel pipelines when the temperature decreased below the tough/brittle transition temp.--aka ductile/brittle trans. temp.

Alum. doesn't have steel-like transition temps., so the riding temp. isn't an issue here.

BTW, finished the camping trip--too many bugs, including biting horse flies. Tried OFF!, a little help but not enough. Guess I'm getting soft--but NOT brittle!
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