What Will Happen Beyond Allowable Stress?

Elastic deformation, plastic deformation and failure.

Yes, sort of.

Hello!

It depends on the allowable stress value. Here is what comes to my mind:

In most structural cases, the ultimate safe load is decided based on a factor of safety. That is, dividing the yield stress (stress which begins yielding) by a number (factor of safety) resulting in ultimate safe load. The allowable stress would be due to a load which is about 50-60% of this ultimate safe load. That means, in its normal operation, the stress (known as working stress) in the member would be about 60% of the ultimate safe value - hence, keeping it far below the yield point (or the ultimate failure point in case of brittle materials (0.2% proof stress)). Hence it all depends on the value of safety factor chosen for the application.

If stress in the metal exceeds this designed value, due to various reasons: improper usage, excessive loading, corrosion, flaws in the microstructure, fatigue or due to other external sources, then it leads to yielding, if the metal is ductile (Otherwise, failure is imminent in case of brittle metals since they fail without any warning). Also, during this phase, if the load is constant over a long period of time special cases such as "creep" can also occur, which may speed-up the path to failure.

Yes, different metals behave differently on different loading conditions, metallurgy variations, etc.

Hope this suffices.

Regards.

Go to Wikipedia......look for "Stress Analysis"

Try to understand the section describing Factor of Safety