|MadSci Network: Engineering|
Shally, This is a interesting question. According to "Strength and Fracture of Engineering Solids 2nd Ed." by Felbeck and Atkins (Fatigue Crack Initiation, pp 430-432) the beginnings of fatigue cracks begin within the material at the atomic level. Naturally occuring 'dislocations'(or imperfections in the atomic structure) begin to rise to the surface even after only 1 load cycle. As these dislocations rise they join with others. By the time they reach the surface the fatigue crack is beginning to form but is still too small to be seen physically. At this point the imperfection now at the surface is given by Felbeck to be on the order of 300 picometers (300x10^-12 m !)- still too small to be seen with the naked eye. This is where fatigue crack growth would continue to occur, eventually becoming visable to the naked eye. An additional reason for fatigue crack initiation which occurs only at the surface is the existance of surface imperfections which cause "local stress concentrations" which can also lead to fatigue cracking. So the short answer is that while the fatigue fracture process can begin within the part, the effects of the process can not be seen until they reach the surface. This is why it appears to nucleate on the surface. Additionally, this is a very short over-simplification of how fatigue acts as a failure mode. There are entire texts devoted to the phenomenon. While I haven't read any that I would recommend, Chapter 18 in the Felbeck book mentioned above is a very good start. Good Luck, Steven Miller email@example.com Undergrad - Mechanical Engineering San Diego State University
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