2.10.1 Piezoelectric analysis
The piezoelectrical effect is the coupling of stress and electrical field in a material: an electrical field causes the material to strain, and vice versa. ABAQUS/Standard has the capability to perform fully coupled piezoelectric analysis. The elements that are used in this case contain both displacement degrees of freedom and the electric potential as nodal variables.
Equilibrium and flux conservation
The piezoelectric effect is governed by coupled mechanical equilibrium and electric flux conservation equations.
The mechanical equilibrium equation is
where
is the “true” (Cauchy) stress at a point currently at
;
is the traction across a point of the surface of the body;
is the body force per unit volume in the body (such as the d'Alembert force
, in which
is the density of the body); and
, where
is an arbitrary, continuous vector field (the virtual velocity field).
The electrical flux conservation equation is
where
is the electric flux vector;
is the electric flux per unit area entering the body at a point on its surface;
is the electric flux entering the body per unit volume; and
, where
is an arbitrary, continuous, scalar field (the virtual potential). These quantities are also known by other terms that are frequently used within electrical engineering references. The electric flux vector
q is known as the electrical displacement, and the potential gradient
E is known as the electrical field.
Constitutive behavior: material coupling
Currently the assumption of linear materials is utilized. The basic equations for a piezoelectric linear medium are defined in the following. Following Ikeda (1990), three alternative forms of the constitutive equations are presented:
e-form:
d-form:
g-form:
where
are the material compliances;
are the material stiffnesses;
and
are piezoelectric constants; and
are the dielectric constants. In these equations the superscript
, or
above a particular property indicates that the property is defined at zero electrical gradient, at zero electrical displacement, at zero strain, and at zero stress, respectively. Since all these forms describe the same constitutive relationships, the different mechanical and piezoelectrical constants can be expressed in terms of one another. The following relationships exist among the properties (
Ikeda, 1990):
In ABAQUS the constitutive equations in the
e-form are used:
These are expressed in terms of the piezoelectric stress coefficient matrix
. However, ABAQUS also allows the input of piezoelectric constants in terms of the piezoelectric strain coefficient matrix
.
The constitutive equations in the g-form can also be expressed as
and
These equations can be convenient in interpreting and verifying the results of piezoelectrical analyses.
Kinematics
For the piezoelectric elements both displacements and electric potentials exist at the nodal locations. The displacements and electrical potentials are approximated within the element as
and
where
is the array of interpolating functions and
and
are nodal quantities. The body forces and charges as well as the surface forces and charges are interpolated in a similar manner.
The strains and electrical potential gradients are given as
and
where
and
are the spatial derivatives of
. In geometrically nonlinear analyses these spatial derivatives are defined in the current configuration.
System equations
With these approximate fields and the constitutive properties given above, in conjunction with the equilibrium and conservation equations, the following system of equations is derived in terms of nodal quantities:
and
where
is the mass matrix (no inertia terms exist for the electrical flux conservation equation),
is the mass density,
is the displacement stiffness matrix,
is the dielectric “stiffness” matrix,
is the piezoelectric coupling matrix,
is the mechanical force vector, and
is the electrical charge vector. In these expressions the constitutive properties are specified in a matrix form where
is the mechanical relationship,
is the electrical relationship, and
is the piezoelectrical relationship. The “load” vectors include the body, surface, and concentrated quantities, as shown. The unknowns are the nodal displacements and potentials. Once these are determined, the strains and potential gradients can be computed using the expressions given above. The stresses and electrical flux densities are computed by means of constitutive relationships.