Patran-nastran: hertz contact- user fatal message 25004 (ta1m)

[Luca96]

hello everyone. i’m an aerospace engineering student. for the thesis work, i am performing a nonlinear implicit analysis (sol 400) and i have a problem. in particular i am realizing a hertzian contact between a steel cylindrical indenter and a plate with the following equivalent mechanical properties: e=1.3086e+09 pa and poisson ratio =-0.5166.
i realized the contact conditions and set up the analysis. the analysis setting can confirm that it is correct because previously set up the same analysis but with the plate made of aluminum , nastran did not give problems.
if i assign to the plate a material (isotropic) with the mechanical characteristics described above, nastran in f.06 reports that " user fatal message 25004 (ta1m)- poissons ratio of mat1 id 2 is less than zero or greater than 0.5".
i cannot understand how to solve this problem, since my aim is to analyze how a plate with a negative poisson ratio behaves.
if anyone can help me i would be very pleased.

 

[F_Ingrasciotta]

the plate on which you are simulating contact has a negative polka dot coeff, -0.5166, what material is it?
from a short research I found that auxetic materials have coeff. poisson negative. . .

 

[Luca96]

In theory there would be no material that possesses such mechanical properties. Originally it is a honeycomb structure with a geometry (hexagonal chiral) which leads it to contract or expand laterally if subject to compression or traction, in a completely contrary to a material that we are accustomed to seeing. since geometries are not quite immediate to be realized (due to their complexity), a homogenization method is used that allows to replace the honeycomb structure with a continuous material and with equivalent mechanical properties. the result of this method, through analytical formulas, brought as a result e=1.3086e+09 pa and poisson ratio =-0.5166. In this way I can create a simple plate (in my case a 2d solid so that the field of deformations is slow, since such formulas bring me as equivalent mechanical property information only in the plane).
That said, the main objective is to make a contact of hertz between a cylindrical indenter and such plate. Of course it is right to emphasize how a cross section has been chosen to the axis of the cylinder and imposed boundary conditions of symmetry to be able to reduce the problem from 3d to 2d.
This analysis was tested when you have a plate made of aluminum, the data is very close to the analytical results resulting from the hertz theory. if the plate (always made of aluminium) possesses the mechanical properties described above using homogenization, by the fatal error.
to this I cannot find a solution because I cannot change the value of mechanical properties.
the final purpose is to demonstrate how the "auxethic" plate has a better resistance to indentation.
If you can recommend a solution I would be very grateful.
Thank you in advance.

 

[biz]

interesting problem. we know that some experimental materials, such as polymer foams, expand laterally if subjected to traction. see "foam structures whit a negative poisson's ratio" roderic lakes.
I've never been able to solve these problems. I'd like someone to answer.