Learning Forum

[Andrei Ancuta]

Hello,

I am playing with a parametrical optimisation. The objective function is reducing the mass, and all the contraints are regarding the lifetime. The min lifetime that i want is 25.000. This optimisation is about a structural steel component.

The model have geometrical singularities (witch are also making a contact), so, from this point of view i decided that i should estimate the lifetime using strain-life parameters ( and i think is not very bad because 25.000 is aproximative in the limits of low cycle fatigue).

I am using the default strain-life parameters of structural steel. 

It is clear that i need to define the plasticity of the material. I had choose the multilinear isotropic hardening approach, but it does not have default values, so i needed to chose them from literature.

So, the problem is that i do not have any ideea if the strain stress curve should be the engineering one or the real one. (or even a mixture, the engineering strain and the real stress).

I think the FEM approach should have the real curves and characteristics of the material...but i am not sure if the default values for strain-life parameters are real or engineering ones. 

It is also for the first time when i see how is defined in ansys the low cycle fatigue behaviour.

I tried a lot of combinations on a cube model but i did not saw anything realistic.

I think the strain-life parameters are not defined in accordance with the real curve that i found in literature, maybe this is the problem and it can be for choosing a wrong material to read about(i searched for S275)

[Lydia Lymperopoulou]

Hello Andrei,

If I understand correctly your problem is how to define the strain stress curve for the plasticity model. You can have a look at this course Defining a Multilinear Hardening Plasticity Model | Ansys Courses which explains all the steps that need to be followed.   

[danielshaw]

Andrei:

If you are using the Mechanical Fatigue Tool (FT) you should not use an elastic-plastic material model.  The FT performs StrainLife (EN) fatigue using elastically calculated stresses and the Neuber elastic-plastic correction method.  It does not use FE calculated plastic strains.

[Lydia Lymperopoulou]

You can also have a look at this thread /forum/forums/topic/elasto-plastic-curve-and-multilinear-isotropic-hardening/

[Andrei Ancuta]

Thank you very much for all this informations, is more than enought!
Thank you very much again!

[Lydia Lymperopoulou]

Yes, this is the definition of plasticity