Fatigue has a direct reflection on body .It has also a relation with the mind .In certain cases fatigue becomes the outcome of physical excretion very often when we involve & also endorse some of our mental study & program for our longer hrs than also our body demands  a relaxation in order to avoid fatigue.

In a fatigue is a nature process of our working environment under a hard working involvement of mental time,the fatigue appears as a regular habit with the change of the mind we may come out with our regular life.

Fatigue creates tension as it is attitude of mind when our mind reacts with the very disturbing problems of our life it creates Tension in our mind disturbing the working action of our life creating the mental tension,worries,& frustration .

This is my personal opinion 

Harshvardhan thanks for the literature..I'll go through it. Mr Parikh I had asked question regarding metals and alloys and not related to body fatigue..

Hi Randhir,

There is no direct answer to your question, but if you talk about experimental results, there is no compulsory reason it should be the same since material's behavior can be different in compression and tension. Example is through the Baushinger effect showing different plastic limit in tension and compression. This is also true for fatigue. However, if you talk about numerical analysis and suppose your material is isotropic homogeneous, in this case you should have identical behavior, else there is a mistake somewhere ...

Regards.

Hi Randhir,

I don't know the exact reason. But in my opinion, tensile load open up the cracks in the materials (even micro sized) and compressive load close the cracks. If you start with tensile load, some cracks (surface crack or bulk crack) increase in size and can't be healed during compressive cycle. But if you start with compressive load, some cracks get close and open up upto their initial size at the time of tensile cycle. Thus fatigue life is bit affected due to few initial cycles.

Parshant the reason you suggested may be correct but the cracks are developed generally after the stable region of the deformation curve. I don't think there is any crack formation in the first quarter cycle. So the reason for more life in compression start may be related Bauschinger effect or back stress developed during the first quarter cycle.

Compression forces tend to close up cracks that are the main cause for fatigue fracture, whereas tensile forces tend to enhance crack growth.