A preform is modeled as a porous medium, and the mathematical modeling uses Darcy's
model. Hence, the preform is characterized using permeability tensor.
Draping computations play a crucial role in determining the tensor material
properties. This data has to be transformed at each solution integration point using
the local coordinate orientation. This is essential for the RTM process, and it is
seen in the image below. The preform, which is characterized in a flat orientation,
is draped to take the shape of the mold before the RTM process begins. To simulate
the process, the material data is required in local orientation based on the mold
shape.
Lay Flat and Draped Preform
Preforms are characterized in a flat orientation. That is, its material properties
such as permeability tensor, conductivity tensor, specific heat, etc. are measured
in this orientation. The process of shaping the preform to its final product shape
is called the draping operation. Often what is measured as an orthotopic tensor in a
flat orientation will be a full tensor in its draped position. This transformation
has to be computed to perform the simulation.
Draping Computations
The draping computation has many internal steps, and HyperMesh is used for this
analysis. The details of the process, shown in the figure below, are completely
automated and hidden in Inspire. It is computed in batch mode. RTM Solver uses
this exported ply orientation to compute material data in each integration point.
Known Limitations in Draping Computation
For complex models, the automatic midsurface extraction may fail. The
workaround is to create the midsurface manually and Inspire will use that interface and not try to create
one automatically.
Inspire does not support post-processing of draping results.
Density
Units: kg/m3
Keyword: Density
Description: Density of the preform. This the density of the fiber (glass,
carbon. kevlar, etc.)
Description: Defined in Darcy's law for porous media. The oil mining
industry uses a unit called Darcy which is physically more intuitive.
A medium with a permeability of 1 Darcy
permits a flow of 1cm3/s of a fluid
with viscosity 1 cP (0.001Pa⋅s) under a pressure
gradient of 1atm/cm acting across an area of
1cm2. The permeability of sand
is approximately 1 Darcy.
1 Darcy
= 9.8692327×10−13 m2=
98.692327μm2
This is derived as The GUI can show Permeability in Darcys, m2, or
μm2.
Porosity
Units: None
Keyword: Porosity
Description: (1.0 - Fiber Volume Fraction). Often the industry is more
familiar with the term Fiber Volume Fraction, and it is easy to
determine.
Compressibility
Units: 1/Pa
Keyword: Compressibility
Description: Used in VARTM analysis. This term has to be non-zero.
Pore Size (Diameter)
Units: m
Keyword: PoreSize
Description: This is the average pore size of the preform. Needs a special
flag to use this experimental feature, and it is not exposed in the GUI.