Re: VRD-- I'm doing a experimental report on fatigue fracture of

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
smiller@kahuna.sdsu.edu
Undergrad - Mechanical Engineering
San Diego State University