Dear Gupta

Thank you for your answer, but still the problem is there such a result for metals and what is its interpretation?

Depended on the material , if it composite or not

Can you do the fit properly? Did you fit your equation to compressive stress-strain curve?

Yes, I used true stress strain curve and then took the ln of both of them and draw it for the plastic region and do the fitting process.

You start the curve from the yield point right? What is the accucary of your fit? and what is the material?

I used Excell fit so it should be acceptable, and I got two regions both of them above 1. 

The material is Aluminum but it contains some oxidation about 5 percent

I remember getting n - values greater than one as well during my final year research.  The material used was Aluminum Alloy AA6082 - O sheet, and the obtained  n - values were greater than 1.4. At first I could not understand it as well, until I found a document with n - values way much greater than mine, in a range of 2. An n - value greater than 1 could be due to the threshold stress intensity, which is an important characteristic of the cyclic fracture toughness of the material, affected by the heat treatment methods the tested material has gone through. 

Dear Eliaser 

Thank you so much.

Sadeem

An idea is to derive strain hardening exponent separately for each phase in MMC and then ROM as in this article:

Article A dislocation-based, strain–gradient–plasticity strengthenin...

Dear Dr. Tian

Thank you for your answer, however, even this is hardly yielded a result for n larger than one. Does your new model bring a new way for finding n (the strain hardening exponent)?

Thank you