thumbnails in fatigue fracture 1

[dentnbent]

First of all is a thumbnail only on fatigue fractures? and secondly is the thumbnail area where the cracking started or ended? Thanks,, yes I am a newbie..

 

[EdStainless]

The beachmarks are curved. The origin of the fatigue is back, the concave direction. Near the origin they should be small enough that you will need an SEM or TEM to see them. Often you will see marks, but between then there will be hundreds of microscopic ones.
Once the fatigue progresses far enough that the remaining area cant carry the load the rest will fail in tensile overload.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[metengr]

First of all is a thumbnail only on fatigue fractures?

No. Stress corrosion cracking (SCC) can provide a similar appearance on a fracture surface.

and secondly is the thumbnail area where the cracking started or ended?

To add to edstainless's comment above, the thumbnail typically is evidence of where the crack started.