Obervable Differences Between Brittle & Ductile Fractures. 1

[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

 

[Metalguy]

Ductile/brittle fracture mode transitions are not only temp. dependent but also strain-rate dependent and the effect of constraint (biaxial and triaxial stress). When the fatigue crack grew to the critical size for the stress, the other side fractured very fast.

"You see, wire telegraph is like a very long cat. You pull his tail in New York and his head is meowing in Los Angeles. Do you understand this? Radio operates the same way: You send signals here, they receive them there. The only difference is there is no cat." A. Einstein

 

[metengr]

gearcutter;
If you have an interest, I would encourage you to search on the internet for topics related to ductile and brittle fracture surface features and you will find university-level PowerPoint presentations for undergraduate courses, files/technical book reviews and technical papers.