Biomedical Engineering Reference
In-Depth Information
It can be seen that the isotropic Coulomb friction model is used in this analysis.
In this model, two contacting surfaces can carry shear stresses up to a certain magni-
tude across their interface before they start sliding relative to each other. This state is
known as sticking. The developed shear stress is the function of friction coefficient and
contact normal pressure
P
,sothat
τ
=
μ
P
. In addition, there could be an initial cohesion
sliding resistance
b
, which is referred to as COHE in ANSYS and can be added to the
equivalent shear stress. Therefore, the Coulomb friction model can be defined as
τ
=
μ
P
+b
. Although contact cohesion provides sliding resistance, even with zero normal pres-
sure, there is, conversely, a maximum contact friction stress,
τ
lim
, at which, regardless of
the magnitude of normal contact pressure, sliding will occur.
The geometry of the modeled problem is shown in Figure 3.13. The computational time
for this model can be reduced by taking advantage of the symmetry of the problem and
only modeling half of the geometry. For the structural part, that is, the top and bottom
plates, the PLANE42 element, was used. This element is defined by four nodes having
two DOFs,
U
x
and
U
y
, at each node. In order to model the piezoelectric PVDF film, the
PLANE223, which is an eight-node coupled-field solid element, was used.
For the piezoelectric analyses, this element has three DOFs at each node, including
U
x
,
U
y
, and Volt. The contacts between the PVDF film and the top and bottom surfaces
were modeled using TARGE169 which was used to represent various target surfaces
associated with the contact elements, CONTA171. The latter element is a 2D, two-node
surface-to-surface contact element that represents the deformable surface of a contact
pair and is applicable to 2D structural and coupled-field contact analysis. This element is
associated with the 2D target segment element (TARGE169) defined with a set of shared
real constants. Using the real constants, various controlling parameters such as COHE
(
b
), the cohesion sliding resistance, TAUMAX (
τ
lim
), and the maximum contact friction
F
n
Top plate
PVDF film
metalized
on both sides
Bottom plate
Figure 3.13
The geometry of the problem in which a PVDF film is sandwiched between two
solid plates and a load is applied in the normal direction
