4. Materials & Boundary Conditions

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Materials

The materials definitions for CMUTs are very similar to that in conventional OnScale models. There are only one or two extra considerations that need be included.

  1. Use of BB hourglassing for Isotropic materials - see the matr hrgl command. To help with absorbing boundaries it can be useful to add some additional values to the simple elastic material properties:
    •  Reducing the hourglass suppression from the default 0.01 to 0.005, or commonly 0.0
    • Add some Artificial Viscosity to reduce high frequency 'chatter' from the contact elements
  2. Materials for Shell elements have to be defined independently in the shell command
  3. If using Shells, then 'Fake' materials with zero mass & zero stiffness must be used to represent the dielectric properties.
  4. Anisotropic materials can be used for continuum elements but only Orthotropic Anisotropy is available for Shells via prop orth
  5. Shell materials can have damping applied through Artificial Viscosity using shell avis
  6. Shell can be assigned Thermal properties if Thermo-mechanical analyses is required (for pre-stress calculation as an example)

Note: The properties of Silicon and Silicon Nitride (the common materials used in CMUTs) can vary considerably. The material database in OnScale has some standard properties included but the user must ensure material properties for their models are accurate.

Boundary Conditions

Boundary conditions for CMUT models are largely unchanged, although for models that are operating into an air load i.e. the user wishes to extract beam profiles for in-air CMUTs, or similar metrics, then the use of the Large Deformation options updt and updg in the boun side command can sometimes help stability for longer, low-frequency models.

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