## General Mechanical

#### Ansys non linear analysis – bilinear hardening for plasticity

• N0834237
Subscriber

Hello I wanted to simulate  non linear analysis of a compression test of a cylinder rod compressing onto a thick auxetic structure, the materials have unknown properties so I carried out uniaxial tensile test to gather some data, I am trying to simulate the real experiment of the compression test to achieve a similar force threshold with a compression displacement of 20mm, on this particular structure at 20mm displacement along the y axis produced of a force of 300n approx , in ansys with applied bilinear hardening with positive integers I achieved the force to become 178N, which only consists of tensile parameters. The problem is I want to use negative integers of strain and stress to approximate the compressive behaviour but ansys does not allow this and shows yellow error bar over the data table. Is there a way to let ansys accept negative integers for hardening?

• jj77
Subscriber

The plasticity you refer to is what is called rate independent plasticity that uses Von Mises yield criterion (that is if VM stress > yield, then it has yielded).

As you know VM is strictly positive so hence why you only need to put in positive numbers.

Since you do not know the material properties you probably need to set the yield stress a bit higher, assuming that it has yielded at that disp. If it has not and it is still linear elastic then Young's modulus needs to be tweaked.

• N0834237
Subscriber

It is not for Von Mises yield as material is not ductile, It's a special type of PLA in which we want to do elastic, plastic analysis. The pictures shows an example of the bilinear hardening graph but it only accounts for tensile, we want to input compressive data into the graph but displays yellow error.

As you can see the negative axis would represent the compressive behaviour of the material. (only negative of the proportional part)

• jj77
Subscriber

Read the theory and the help manual section about rate independent plasticity and bi linear isotropic hardening that uses VM yield criteria. Again VM is strictly positive so you can not have negative numbers of course.

I suggest to read theory carefully before doing FEA. Go back and study solid mechanics and VM stress for different type of stress scenarios (pure axial, tension, compression, shear, 3D state,..).

• peteroznewman
Subscriber

@VJ0085,

Please start a New Discussion and take your content from the post above and paste it into the new discussion. Then delete the above post.

The reason is you will own the new discussion, which means you will get automatically notified whenever anyone posts a reply.

Right now, N0834237 is getting notified of these replies to this discussion.

• VJ0085
Subscriber

Hi Peter,

I have created a new post now, hopefully, will get guidance. I am actually stuck in this step since days with no idea on how should I proceed. I would be glad if experts could guide me on the same.

Thanks.