As long as the steel is not bent it is un-damaged

[generallobster]

I remember Dr. Bill Watenburg answering a mans question about steel barbells a few years back on KGO (largest west-coast news radio station). The man asked about how olympic power lifters can flex the steel bars signifigantly when perfoming these feats of strength. But the bars always return to true. The man was told that steel has a flex limit to which it can always return to true as long as the limit is not passed. The steel retains 100% of its original strength or toughness as long as it has never passed the point where it takes a permanant bend. Does anyone know if this is true? Does it depend on the alloy? I assume if it is true it only aplies to flexing rather than hitting and shocking.

 

[Snickersnee]

The short answer is yes and no.

If you don't bend the steel passed a certain point, which varies with steel/temper, it wonn't break or take a set bend. It also won't lose any strength(though there might be a weird alloy somewhere), but actualy it might work harden.

Anyway, with time and repeated flexation, the steel will start to take a set. But you're talking about a lot of flexation. Think about steel springs, same thing.

 

[Mike Moore]

All metals deform when they are placed under stress. The steel beams and columns in the buildings you live and work in and in the bridges you drive across all deform. They're designed so the deformation is not generally noticeable.

Over the course of many cycles (millions or more) the steel will start to fatigue and eventually fail.

 

[Jeff Clark]

The question is fundamental to materials science. At the bulk level the answer is mostly that if the metal was not permanently bent it has not been damaged. The trick is that much of metal failure starts from small cracks that later become big cracks and failures. The stresses (pounds-per-square-inch tensions in our case) on a structure are not uniform. They get higher around cracks, holes, and sharp indentions. A large force or sudden impact can start a hidden crack in one of these high stress areas. There are even earlier effects where the stresses can cause metal lattice dislocations in these stress points which makes these areas progressively more brittle (the spots are work-hardened like they were cold hammered).

So you could have a knife point break when you pry with it where you previously got away with the exact same abuse. It is just that over time you work hardened some stress concentration point, then you started a micro-crack, which became a macro-crack, which became a break.

The area where this problem is a big time menace is in air plane structures. Vibration and inflation/deflation cycles flex aluminum components that are not overly rugged (if you make an airplane too heavy it don't fly). The structures fail and parts of the plane can fall off. The problem is not unique to aluminum. They first became real aware of stress concentration and metal fatique when they made iron-hulled ships. The whole ship would break in half starting at the corner of a large square hatch opening.

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