Endurance limit / fatigue strength at high temperatures

[kajalschopra]

Hello All,

I had been reading a book on Machine Design.

I understand that at high temperatures, yield strength of the material drops. The behaviour of the fatigue strength i.e. the drop in fatigue strength may at times be taken to be of same proportion as that of yield strength.

It is also said that at high temperatures, the fatigue strength gets dependent on time.

I do not understand that why fatigue strength is a function of time?

Thanks
Kajal

 

[Lord Jestocost]

"Material properties are dependent on the temperature. The tensile strength, yield strength and modules of elasticity decrease with increasing temperature. It should be expected that fatigue properties are also affected by the temperature. The effect of a high temperature on mechanical properties can be associated with [time dependent, ed. by LJ] transformations of the material structure due to diffusion processes, aging, dislocation restructuring (softening), and recrystallization. In general, such processes imply that plastic deformation can occur more easily at an elevated temperature. This can lead to the well-known creep phenomenon defined as continued plastic deformation under sustained load. With respect to fatigue, it can imply that more plastic deformation and creep occur in the plastic zone of a fatigue crack which may apply to both microcracks and macrocracks. As a result, fatigue damage accumulation might be enhanced. Furthermore, other failure mechanisms are possible. During creep under sustained load, creep failures occur by grain boundary sliding, void formation (also often at grain boundaries), void growth and coalescence."

from: (2009) High-Temperature and Low-Temperature Fatigue. In: Schijve J. (eds) Fatigue of Structures and Materials. Springer, Dordrecht

With respect to creep: https://en.wikipedia.org/wiki/Creep_(deformation)