Yield Stress less than Proof Stress

[Lee.Conti]

Hi All,

I am working on a plastic material with YS = 30MPa,Yield Strain is 2.5%. Modulus is 3400 MPa. I got a stress-strain data (engineering stress-strain) and I want to calculate plastic strain.

The modulus given in the spec is initial modulus and I measured 0.2% from the stress-strain curve, the yield stress/ strain is actually less than YS (30MPa) and Yield Strain (2.5%).

I am not sure if I should assume the plastic deformation starts at 0.2% or 30MPa?

Thanks!

 

[karachun]

But you already have 2.5% Yield Strain, and this strain should correspond to yield stress, but that is not true. Look like you have inconsistent data or you mixed up yield strain and ultimate strain, when test piece broke.

 

[Lee.Conti]

Hi Karachun,

The yield stress and yield strain are given in the material spec. Stress strain data also provided by supplier later on.

The yield stress/ strain listed in the spec is not at 0.2% offset.

 

[karachun]

0.2% offset used only for steel and some other metals, if you working with polymer or some other material then it is ok to have yield strain more than 0.2%.
Don’t forget that 0.2% is offset, not actual strain value, 0.2% is residual plastic deformation after load is removed. Actual strain is bigger.

 

[Lee.Conti]

karachun,

I agreed that plastic has more plastic deformation. I have following questions for using multilinear for nonlinear analysis...

I use initial modulus to calculate the true stress and strain which means the elastic strain is actually smaller than the yield strain given... I am not sure if this would capture the plastic deformation correctly to avoid over design...