Images Of Brittle & Ductile Failure 5

[gearcutter]

There are many questions around the forums relating to the observable differences between ductile and brittle fatigue failures.
I recently came across a failed internally splined sun gear and have posted these images for comparative purposes.
The failure, for me, is unique as both fatigue mechanisms are easily observed on the one part with the unaided eye.
On the lower sides of the fracture surfaces in images 04 & 05; differences in the appearance and shape of the shear lips can be easily seen. The shear lips extend into the major diameter of the internal spline.
I’m not a metallurgist so please feel free to correct if the terminology I’m using is not appropriate.

Image 01. Ductile failure fracture surface left side. Brittle failure fracture surface right side. Image is showing the internal spline.

Image 02. Ductile failure fracture surface. Both beach and ratchet marks are clearly visible. The surface is curved and smooth.

Image 03. Brittle failure fracture surface. The surface is flat, rough and faceted.

Image 04. Ductile failure fracture surface. Both beach and ratchet marks are clearly visible. The surface is curved and smooth.

Image 05. Brittle failure fracture surface. The surface is flat, rough and faceted.




Ron Volmershausen
Brunkerville Engineering
Newcastle Australia

http://www.aussieweb.com.au/email.aspx?id=1194181

 

[flash3780]

I'm no metallurgist either, but perhaps the brittle behavior is due to a fatigue crack, whereas the ductile behavior is due to a resulting overstrain when the fatigue crack finally let go?

 

[flash3780]

^actually, perhaps it's the other way around. Image 4 clearly shows crack growth marks on the ductile fracture. Also, I think that materials tend to act more brittle at higher strain rates.

Anyhow, great pictures of an interesting failure. Thanks for the post.

 

[satchmo]

Good pictures. Here is a file I found on the internet some time ago called "Surface Features of Fatigue Fractures". Goes well with these pictures.