Impact Strength and Fatigue Relations?

[driveline]

I have been trying to find some good data that relates Charpy impact values to fatigue resistance. I know that the Charpy test is rarely used in a design sense, but the stubborn engineer in me keeps finding hypothetical connections between the two. If two materials have the same hardness and tensile strength, won't the one with higher Charpy values fair better in fatigue?

 

[Metalguy]

In general, no. Impact strength is rate-of-stressing dependent, while fatigue strength is remarkably independent. There are probably some studies which *may* indicate some impact/fatigue relationship, but I haven't seen any.

 

[Carburize]

Since the crack initiation and propagation in the Charpy test and in a fatigue situation are so different I would be surprized if any true relationship exists. It might be argued that in the case of a fatigue crack initiated by metallurgical inclusions then a "cleaner" material would perform better in fatigue and it would also be expected that the "cleaner" material would also demonstrate higher charpy impact energy at least on the Upper Shelf, but again I don't think there is a real connection.

 

[davefitz]

There may be a relationship between charpy impact and crack propagation rate, but this implies fracture mechanics governs and not traditional fatigue of polished specimens.

As an aside, the new european fatigue analysis techniques borrow heavily from BS 5500 in that they basically abandon traditional fatigue methods and favor fracture mechanics prediction of fatigue failures. They postulate that the vast majority of fatigue failures occur in the weld heat affected zone HAZ at pre-existing cracks formed during the welding process. If correct , then the charpy impact values in the HAZ, related to PWHT QC , has a strong affect on fatigue life.

 

[driveline]

davefitz, This new european fatigue analysis technique is similar to what I was assuming. I was trying to make a connection between a material's susceptibilty to quench cracks and inclusions and fatigue life. But now I see that the Charpy sample would have to contain the specific stress riser to reveal any significant data.

Do you happen to know the name of the new european fatigue analysis technique?

 

[davefitz]

the new european codes are :

EN 13445 part 3 clauses 17+18 which are aimed at using the BS5500 method of predicting failure due to weld crack propagation.

a good primer on the new european methods is at :
<

http://www.kuleuven.ac.be/bwk/materials/Education/master/wg12/t1000.htm>