Metal fatigue

[cirthix]

I'm talking on the small scale of things, what really is metal fatigue and why does it happen?

 

[Veramocor]

IANAMS (I am not a material scientist) so someone may be better able to explain it.


Anyway as you put a metal in tension/compression it can be either elastically or plastic. When it is in the elastic region of the stress/strain curve, after the stress which caused it to stretch/compress is removed the metal will return to its original state. After a certain stress level is reached the metal will start to plastically deform. Plastic deformation can not be reversed once it happens. Plastic deformation causes defects in the crystal structure called dislocations. Initially these dislocations will actually strengthen the metal. This process is caused work hardening. Eventually however as you continue to plastically deform the metal more and more defects will occur in the metal crystalline structure till the internal metal bonds can no longer sustain the force/stress on them and they fail.

 

[Armitage]

IANAMS either, but fatigue can happen at well below the yield stress (plastic deformation). In fact ... geek trivial pursuit factoid here ... Al fatigues at any stress level. Most metals have a fatigue limit ... stress levels below this limit will never cause fatigue failure no matter how many cycles you give it.

But for the real answer ... I dunno. I always dealt with it at the macro level.

 

[winr]

This is a neat link.


http://technology.open.ac.uk/materials/mem/mem-mf.html

 

[white]

IAAMS (I am a Materials Scientist) and fatigue failure occurs by first, crack formation at some point of high stress concentration, second, crack propagation, where this crack advances little by little with each cycle, and finally failure when this crack has reached the critical size. Critical crack size depends on whether the crack is on the surface or on the interior of the material, the fracture toughness of the material, the direction the crack is oriented relative to the direction of the applied stress and the magnitude of the applied stress.

In order to combat this problem of failure, the shape of the part can be designed in such a way to minimize fatigue crack initiation sites. In general, this will mean making the part smoother with fewer sharp edges. Stress concentration goes up in areas with a small radius of curvature. The parts could made more resistant to fatigue by introducing compressive plastic deformation at the surface by shot-peening. Or it could be carburized to introduce carbon into the outer surface. This also results in a harder surface that makes the piece more resistant to fatigue failure.

 

[oldman420]

heres a link to a great site for this kind of information. I got stuck here for a while
http://www.efunda.com/home.cfm