Page 286 - Aeronautical Engineer Data Book

232 Aeronautical Engineer’s Data Book
edges of a wireframe to define the surfaces.
They can therefore define structure bound­
aries, but cannot distinguish a hollow object
from a solid one. Surface models can be used
for geometric assembly models etc., but not
analyses which require the recognition of the
solid properties of a body (finite element stress
analysis, heat transfer etc.).

Solid models
Solid models provide a full three-dimensional
geometrical definition of a solid body. They
require large amounts of computer memory for
definition and manipulation but can be used for
finite element applications. Most solid model­
ling systems work by assembling a small
number of ‘building block’ reference shapes.

12.7.3 Finite Element (FE) analysis
FE software is the most widely used type of
engineering analysis package. The basic idea is
that large three-dimensional areas are subdi­
vided into small triangular or quadrilateral
(planar) or hexahedral (three-dimensional)
elements then subject a to solution of multiple
simultaneous equations. The general process is
loosely termed mesh generation. There are four
types which fall into the basic category.
• Boundary Element Modelling (BEM): This
is a simplified technique used for linear or
static analyses where boundary conditions
(often assumed to be at infinity) can be
easily set. It is useful for analysis of cracked
materials and structures.
• Finite Element Modelling (FEM): The
technique involves a large number of
broadly defined (often symmetrical)
elements set between known boundary
conditions. It requires large amounts of
computing power.
• Adaptive Finite Element Modelling
(AFEM): This is a refinement of FEM in
which the element ‘mesh’ is more closely