@Aayush

No, ductile materials usually do not have a clear yield point, excepting carbon steels with lower and upper yield limit. Yield sets ion gradually because dislocation movement begins in grains with more favourably oriented slip systems and then spreads. This is the reason why the Rp0.2-criterion was invented - the onset of yield is not a well-defined point of the stress-strain curve.

In Abaqus, you enter a definite yield stress that allows the program to clearly distinguish between elastic and plastic deformation. This is usually o.k. because the exact onset of plastic flow does not affect the results overmuch compared with other uncertainties.

Yes, unless the material is pre-strained.

I agree with Muhammad,

the initial value for plastic strain should be zero

Yes, the first strain value must be zero, but if you want to enter a stress-strain-curve, you need to specify the strains for the subsequent points.

I want to enter a temperature dependent yield stress data. But i am confused as to why the plastic strain value will change. Should'nt it be zero at all values of yield stress?

Defining a von Mises plastic material In ABAQUS, you can specify an isotropic uniaxial hardening law, entering a tabular data in which you will have a yield stress and a plastic deformation. The first point of this tabular data (that might be also the last, if you are considering perfect plasticity) must be always the yield stress associated to zero plastic strain. Entering other points, you can fit a uniaxial curve and define how your material will be subjected to hardening. After the last point you enter, ABAQUS assumes perfect plasticity. 

If you have an yield stress that depends on the temperature in perfect plasticity (I think it's your case), you will have different values of yield stresses (each one with zero plastic strain) as a function of the temperature. 

Good luck!

A

Thank you for your detailed answer Sir. It was very helpful.

But in a total strain table I guess you start the plastic strain from the 0.2% level. If you also have an impact or an explosion before the temperature increase you may use kinematic strain hardening. Note also that the E-modulus in e.g. steel will fall with increasing temperature.

No, the yield stress in abaqus is referred to 0 equivalent plastic strain. You are simply referring to the definition of conventional yield stress (0.2% of residual deformation). The  input convention is different.

The Getting-started manual has a very clear explanation on how exactly to enter plasticity data in ch. 10

For the yield stress the plastic strain begin from 0.

After you must to compute plastic_strain = epsilon_total - epsilon_elastic.

The more rigourously and physically accepted is to compute plastic_strain = Integrale (epsilon_plastic_point) where epsilon_platic_point is epsilon_total_point - epsilon_elastic_point). Epsilon_point is the strain rate.

Dear ayush..i feel u  r confused with the onset of plasticity and yield stress..the experimental yield stress is determined for 0.2 %...which corresponds to some amount of plastic strain..however for modeling purpose, the the onset of plasticity is required i.e. stress at zero plastic strain....hope this will help...

Jalaj Kumar Sir...I think the 0.2% criteria is used when the yield point on the stress-strain curve is not exactly defined , which is generally in the case of brittle materials. Ductile materials generally have a well defined yield point.

Ok. But be careful with your experimental limitations and simulation assumptions..good luck..

@Aayush

No, ductile materials usually do not have a clear yield point, excepting carbon steels with lower and upper yield limit. Yield sets ion gradually because dislocation movement begins in grains with more favourably oriented slip systems and then spreads. This is the reason why the Rp0.2-criterion was invented - the onset of yield is not a well-defined point of the stress-strain curve.

In Abaqus, you enter a definite yield stress that allows the program to clearly distinguish between elastic and plastic deformation. This is usually o.k. because the exact onset of plastic flow does not affect the results overmuch compared with other uncertainties.

@Martin Baeker

Thank you for clearing that Sir. I also had one other doubt. Sir, does strain hardening take place only in materials which are not perfectly plastic and why?

That is basically the definition of strain hardening. Materials that are perfectly plastic by definition have a constant value of the flow stress regardless of plastic deformation.

Hardening models would require onset of plasticity...and not definite yield stress...

In an FEA model, the onset of plasticity is defined by the yield stress in uniaxial direction (i.e. determined from supplied hardening curve) and the yield surface model you specify (for e.g. Von mises or Hill-48 etc). So once the equivalent stress reaches the yield surface, the material starts deforming plastically.

Dear

Hoe can I define material properties using work hardening?

Dear

How can I define material properties using work hardening?

Dear Abuzar,

Point #1 : Yield stress with plastic strain = zero.

Point #2 : Ultimate stress with plastic_strain = epsilon_total - epsilon_elastic.

Regards

Waleed