Hello Jayaprakash Polamarasetty ,

In testing for the ultimate strength of a material, you simply keep increasing the tension, for example, until the sample ruptures; no cycling is involved. By the time the sample has ruptured it has gone through the elastic region and then the plastic region. In LCF (Low Cycle Fatigue) testing, a cyclic (alternating) application of tension and then compression is applied to the test sample in an attempt to make it fail (rupture) while it is still in the elastic region. In other words, in LCF testing, the alternating stresses are kept low enough that the material stays in the elastic region of the stress-strain diagram. Of course, in LCF testing, you want to apply alternating stress levels that are not too low otherwise it may require unrealistically high number of cycles to induce failure. Thus, there is a certain lower range to the capacity of the testing machine.

[1] Bela I. Sandor; Fundamentals of Cyclic Stresses and Strain; The University of Wisconsin Press; 1972.

Regards,

Thomas Cuff