Dear Om,

The stress is independent of material only in Elastic regime. Once you cross the yield limit, you enter the plastic regime. In plasticity the stress is governed by the hardening behavior of the material. A high strength steel will definitely give a higher stress in the plastic regime compared to a low strength steel.

Kind regards 

Dear Mr. Prakash,

As far as I see, there are several elements in the assemble except the bracket. That means that if the stress in the bracket is higher than the Yield strength of the steel, then the load on the bracket is smaller. The strength is to be divided among other elements, because the bracket can not stand the load and deforms. If you use stronger steel, the part of load, which takes the bracket, is higher. But if it is still exceeds the Yield strength, you need either to use another steel with higher Yield strength, or change the area of the section. It is commonly considered that the design is efficient if there no elements for which the stress exceeds the Yield strength.

Yours sincerely,

Aleksandr Perestoronin

Dear Om,

The stress is independent of material only in Elastic regime. Once you cross the yield limit, you enter the plastic regime. In plasticity the stress is governed by the hardening behavior of the material. A high strength steel will definitely give a higher stress in the plastic regime compared to a low strength steel.

Kind regards 

Could it be the case that you apply a given Deformation or displacement. In this case the answer would be very simple - maybe you know by yourself.

       Hi Om Prakash,

I totally agree with Muhammad Niazi, and certainly, the applied load must exceed the elastic limit.

This is why the stress increases when the yield increases.

Physically this is explained by the low slope of the plastic zone, which once reached the curve takes an exponential behavior as given by the Ramberg Osgood.relation.

Kind regards 

@All

Thank you for your answers. Now everything is clear to me :)

Stress is infact force by area, dimensionally. Going by physics, you can understand stress as the restriction to deformation when a force is applied.

To my understanding, what you are observing can be attributed to the material behaviour. By looking into the stress-strain curve we know that, for metals, the stress-strain relationship is linear with the Young's modulus as the slope, in the elastic region. If you are comparing two materials with similar Young's Modulus (~ similar slope in the elastic region) and if the load applied is same, the displacement or strain is going to be same for the two cases. In such a case, the material with higher yield strength will definitely have higher stress. You need to concern with whether it is within the yield limit or not.

Also, the stress you observed in the first case was already in the plastic region. The growth of stress with respect to strain in plastic region is slower compared to in elastic region, due to energy dissipation in plastic deformation. However, with a material having higher Yield, the part remains in elastic region longer. Hence, for similar amount of effective strain, stress will be higher in elastic region compared to plastic behaviour. Hence, even though the stress is higher compared to your first case, you need to see if it is within yield and if it is, then the part should be Okay.

I hope this clears your dilemma.

Plastic strain is not responsible for any stress! The stress is only related to the elastic strains. Once you cross the yield limit, the total strain is actually only plastic strain and thus the stress will remain at the yield limit (i.e. perfect plastic materials). But in real materials, due to dislocation pile up, the yield stress increases with increase in plastic strain (this is called hardening). So your total strain increment is partly plastic and partly elastic. The small elastic strain increment causes the stress to grow (according to Hooke's law), When you plot the stress vs the total strain you see a sudden drop in the slope of the stress strain curve as soon as you cross the yield limit because the total strain also includes a large plastic strain which is not contributing to the stress increase.

Most of the steels have almost the same Young's modulus so up to yield both materials (steels) shall have approx. the same stress(at the same strain). But the hardening behavior of steels can vary a lot,. So even if the yield limit is same for both materials, the difference in hardening can give a difference in stress once you cross the yield limit.

Totally agree with Muhammad Niazi, as the stress is independent of the material in elastic regime, and strongly depends on the material behavior in plastic regime. So a high strength steel in plastic regime will give high stresses than low strength steel.