Composite analysis using 3D elements for Honeycomb core

Hi everybody,

 

I want to perform a non-linear structural analysis of a sandwich panel. As stated by the title of this topic, I modelled the sandwich panel by using 2D shell elements for upper and lower skin, and 3D solid elements for honeycomb core (I have created the 3D solid elements by using solidmap/linear solid command and matching elements of upper and lower skin).

The panel has been loaded with a centrifugal force using RFORCE, trying to simulate the centrifugal force of a rotor (the panel I modelled is located at the tip of that rotor). Being the panel subjected to inertial force, I decided to use the Inertia Relief (activating it on PARAM with value -1) and adding Support1 constraints.

The question (the issue) is the sequent:

Upper and lower skin plies have been modelled using 2D elements, MAT8 as material, and PCOMPP property. To model the core, I decided to use 3D elements, MAT9ORT as material card and PSOLID as property.

Looking at the online help, MAT9ORT 'Defines the material properties for linear, temperature-independent, and orthotropic materials for solid elements in terms of engineering constants', so I decided it was suitable for my aim.
To fill in engineering constants, I have used values coming from HEXCEL datasheet (Aramid Fibre/Phenolic Honeycomb), considering 1 directions as the Y direction of the AxysSystem, 2 direction as the X direction of the AxysSystem and 3 as the Z direction. Following the used properties:

 

• E1=E2=0.01 (very small according to HEXCEL guidance material)
• E3=138 MPa (HEXCEL material properties)
• mu12=mu13=mu23=0.01 (very small according to HEXCEL guidance material)
• G12=0.01 (very small according to HEXCEL guidance material)
• G13=45MPa HEXCEL material properties)
• G23=23MPa HEXCEL material properties)

 

As long as I start the analysis, I get the following message:

 

 A fatal error has been detected during input processing:

  *** ERROR # 1551 ***
  for material id = 2 referenced from property id = 4.
  Material coefficients Gij on MAT9 card produce material matrix with negative
  eigenvalues. This violates material stability conditions.
  Eigenvalues of G: 1.010049e-002 9.900507e-003 -3.630604e+002
                    1.000000e-002 2.300000e+001 4.500000e+001

 *** Run terminated because of error(s) in the input data.

 

I don't know exactly what It means..../emoticons/default_unsure.png' title=':unsure:' /> I suppose is something related to the material card...

But the stranger thing is that changing the E3 value from 138 MPa to 0.138 MPa it works.../emoticons/default_unsure.png' title=':unsure:' /> I'm completely confused

 

Does anyone know the reason why It behaves that way??

 

Is correct to use MAT9ORT to simulate 3D honeycomb?

Thanks in advance

 

Attached you can find the model. Thank you

Unable to find an attachment - read this blog