Dear Sir, Last day I had an interview at IIT Jammu, Professor asked me why non-ferrous materials don't show a real endurance limit(Horizontal S-N Curve). I'd no answer, later I searched in Dieter but they had no reason for it. May I get some help?
True endure limit can be observed for high strength materials not only for steels. Crystal structures plays an important role in determinning this. For the case of non ferrous metals like Aluminium or copper (which are soft ) true fatigue life is not observed. Presence of more than 5 independent slip systems allows easy gliding. That's why continuous decrease in life with stable crack propogation can be observed.
the larger the better, for example single crystals are used in a jet engine where both the temperature is very high and they undergo high cycle fatigue as they are often going from turned off to rotating at a high speed. fatigue mainly happens by defects moving to grain boundaries, so if there are less grain boundaries fatigue is reduced
@@patrickcurran5536 thanks for your answer. I just want to ask one more thing . Are these defects that you are referring to, induce crack initation at grain boundaries ?? And is thermal fatigue always cause intergranular failure ?
@@ayay9835 defects are things such as dislocations, stacking faults, twin grain boundaries, but the defects such as dislocations coming together is what induces crack initiations at grain boundaries. Dislocations are small holes in the crystal if you put many small holes together you get a big hole or a crack. thermal fatigue i'm not sure, could possibly be related to dislocations moving and somewhat related to creep, but i'm not too sure what thermal fatigue is
Fatigue inherently involves the cycling of stress, therefore the stress amplitude is important. what S-N curve shows is that the higher the stress amplitude (S) lower is the fatigue life (N). But the variation of stress amplitude is done at constant mean stress, usually zero. But, the mean stress does affect fatigue life. Increasing the mean stress decreases the fatigue life. But this aspect is not captured in the S-N curve which is drawn at constant mean stress.
