Effects of Properties on Fatigue Corrosion/Fatigue 4

[Carburize]

I have some firmly held views on the most important variables affecting fatigue and corrosion-fatigue performance but would appreciate any comments others may have on the following:
How significant is toughness on the fatigue, corrosion-fatigue perfomance of a material?

 

[Maui]

Redpicker, I agree that the types of conditions that components are exposed to in the field are often very different from the conditions under which tests are performed on these same materials in the lab. The actual components may have non-ideal surface finishes, and may be exposed to triaxial loading conditions in corrosive environments. For fatigue life predicitions that are based upon laboratory test results, such deviations from the laboratory conditions would tend to increase the amount of error in the predictions, and reduce the fatigue life of the actual components. One critical aspect of your initial post was this

Furthermore, since CVN values have been shown to be related to KIc values, the conclusion is that materials with higher CVN values will have slower crack growth rates and, therefore, longer fatigue lifes.

I addressed this issue in a prior post where I stated

There is a clear and experimentally verifiable relationship between the fracture toughness and fatigue behavior for specific classes of metallic alloys, but there is no clear and reproducible relationship between the Charpy impact toughness and the fatigue life that I am aware of. Charpy testing is based upon shock or impact loading, but fatigue testing is almost universally based upon continusously varying applied stress levels, not impact.

If you have any references that demonstrate a definite correlation between Charpy impact toughness values and fracture toughness, please post them. I am not trying to be obnoxious here - I would like to read them.

Maui

 

[Carburize]

maui - there has been some interesting work on the Charpy/Kic correlation published during the 70's by
Barsom and Rolf in the US
and
Marandet and Sanz in France
I will try to find some references.

 

[redpicker]

Maui
To follow-up Carburize's comments, the earliest publication I can find of the Barsom and Rolfe correlation is ASTM STP 466 published in 1970. A literature search for documents citing that reference should return later work. This is the most common correlation I have seen.

While I understand that there are issues that make correlation between CVN results and KIc results difficult and inaccurate, there is a widely held belief that a positive relationship exists (although perhaps not direct) between CVN values and KIc values, particularly when the correlation is limited to upper shelf CVN behavior. That is, for the same material in the same condition, that with a higher upper shelf CVN will also have a higher KIc. It is this belief that, I feel, supports the belief that materials with higher CVN values will give longer fatigue life. Unfortunately, field performance does not seem to support this conclusion (which, I believe, is what started this whole thread to begin with). The problem I see this creates is that design engineers will specify higher and higher impact requirements believing that this will give them longer fatigue lives. This results in more costly materials and reduced availability. It would seem that a minimum toughness level that prevents brittle failure is all that is required (the original purpose of the CVN test). Requiring higher toughness (specifically, arbitrary increases in CVN requirements) results in increased costs without any improvement in performance.

 

[Carburize]

redpicker - have a star - a beautiful example of the misuse of the Charpy test.