3.6.7 Axisymmetric shell element allowing asymmetric loading
The ABAQUS/Standard element library includes a family of nonlinear thin shell elements with axisymmetric reference geometry that allow asymmetric loading and deformation (SAXA1N and SAXA2N). This section provides their theoretical formulation. These elements encompass a broad range of practical applications from the bending/ovalization of variable diameter pipes to the bending of circular plates. The theoretical formulation of these elements is similar to the general finite-strain shell element described in Finite-strain shell element formulation, Section 3.6.5. Furthermore, this formulation is the shell counterpart to the continuum axisymmetric bending elements described in Axisymmetric elements allowing nonlinear bending, Section 3.2.9.
As with the continuum axisymmetric bending formulation, the restriction is made that a plane of symmetry exists in the r–z plane at . Hence in-plane bending of the model is permitted, while deformations such as torsion about the axis of symmetry are precluded. The symmetries of the undeformed configuration and of the deformation are exploited through the assumption of particular displacement and rotation interpolations around the circumference of the shell. Specifically, Fourier series expansions are used in the or circumferential direction that preserve the plane of symmetry.
Geometric description
Let be coordinate functions parametrizing the reference surface of the shell and let be the coordinate function in the thickness direction, where h is the shell's initial thickness. (For a detailed account of the geometric description of the finite-strain shell formulation, see Finite-strain shell element formulation, Section 3.6.5.) Then points in the reference or undeformed configuration are identified by the normal coordinates mapping
where
is the three-dimensional position of a material point,
is the shell reference surface mapping, and
is the unit normal to the shell reference surface. The fact that
is a unit vector assumes that the reference configuration is (locally) of constant thickness. Owing to the axisymmetric reference configuration,
can be given relative to a global Cartesian coordinate system as
where
is the radius,
is the axial position, and
are the cylindrical coordinates. (Note that the usual convention for cylindrical coordinates
has been changed, which is consistent with the axisymmetric shell elements and the axisymmetric elements allowing nonlinear bending.) By definition the normal field to the shell reference surface is
, which by direct computation yields
where
and
. Relative to the cylindrical coordinate system,
.
The basic kinematic assumption is that for any deformed configuration, the position of a point in the body can be identified by
where
is the deformed position of the material point,
is the deformed shell reference surface mapping,
is the deformed unit director field, and
is the thickness change parameter. Of critical importance for any shell formulation is the treatment of the rotation field; that is, the treatment of the director field
. The geometric description and the incremental update procedure for the director field are given in detail below.
Under the kinematic assumption above, the deformed configuration of the shell is completely determined by the reference surface mapping , the deformed director field and the thickness parameter .
We define the following displacement quantities. Since is an element of a (linear) vector space, we can define the reference surface displacement vector by the difference between the deformed reference surface and the undeformed reference surface; i.e.,
The director field, however, is a unit vector field that is not a member of a linear vector space. The orientation of the director field is defined in terms of a rotation vector
as
Here
is the skew-symmetric matrix with axial vector
, defined by the properties
and
is an orthogonal transformation given by the closed-form expression
Alternatively, quaternion algebra can be used to specify the orientation of the deformed director field
. In this case the orthogonal matrix
is replaced by the quaternion parameter
, where
The orientation of the unit director field then follows as
Similarly, the orthogonal transformation
can be extracted from the quaternion parameter as
Interpolations
Displacement and rotation components are given relative to the cylindrical coordinate system with orthonormal basis vectors that are fixed in the reference or undeformed configuration. A general interpolation scheme for and using a Fourier expansion in the variable is
Here
are the polynomial interpolation functions along the generator lines of the axisymmetric reference configuration;
,
,
,
,
,
are the solution amplitude values (Fourier coefficients);
M is the number of terms used in the interpolation along the generator lines; and
P is the number of Fourier interpolation terms used around the circumference of the reference shell. Note that an axisymmetric deformation is obtained for the choice
.
The symmetry requirement in the r–z plane at , , eliminates many of the above Fourier coefficients. For the displacement vector the only admissible terms are
For the rotation components, symmetry requirements switch the role of the
r and
z components with the
components:
For practical reasons the values of
,
, and
are often required at specific locations around the circumference of the shell. Therefore, displacement and rotation components
,
, and
are used instead of the Fourier coefficients
,
, and
. Furthermore, a negative sign is introduced in the interpolation for
for the following reason: The ABAQUS convention for axisymmetric shell elements is that the axial tangent direction is drawn between nodes in ascending node number (the shell local 1-direction). The normal to the shell is then obtained by a 90° counter-clockwise rotation of the tangent (the shell local 3-direction). However, a positive rotation of the normal field (about the shell local 2-direction) is counterclockwise. This convention implies a left-handed shell local coordinate system. For the axisymmetric shell bending elements, a right-handed shell local coordinate system is required at the integration points; thus, the direction of positive rotation is reversed there.
Rearranging the Fourier series expansions and making the substitution , the interpolations for the displacement components are
Similarly, replacing
and
with
and
respectively, the interpolation for the rotation components becomes
In the above interpolations,
,
, and
are physical displacement and rotation components at
and
are trigonometric interpolation functions with the property that
defined by:
: , ,
: , , ,
: , , , ,
: , , , , ,
As with the continuum axisymmetric bending element,
is the highest-order interpolation offered with respect to
. The element becomes significantly more expensive as higher-order interpolations are used, and it is assumed that the general-purpose finite-strain shell is less expensive than using this element with
.
Virtual work
The virtual work expression from the three-dimensional theory is
where
V is the current volume of the deformed body,
are the curvilinear components of the Cauchy stress tensor,
are the components of the Lagrange strain tensor, and
are the variational or linearized strain measure components. By definition the Lagrange strain tensor components are given by
Note that in the statement of virtual work, no choice has thus far been made regarding the curvilinear coordinate functions
. Furthermore, the current volume measure
is given by the parametric relationship
We now introduce the kinematic assumption Equation 3.6.7–1 into the definition of to find
where differentiation is now with respect to the parametric coordinates, so that
and
. Define the following shell strain measure components and kinematic relationships:
In the above,
are the components of the second fundamental form of the undeformed reference surface.
Substituting the above definitions into the virtual work expression, we find (after some manipulation) that the volume integral reduces to the following integral over the deformed reference surface
where
and the current reference surface Jacobian determinant is
. In the above virtual work expression, the
term in
has been neglected. This term is
—where
h is the thickness and
R is some characteristic radius of curvature—and is negligible in light of the kinematic assumption
Equation 3.6.7–1. The shell stress resultant components are defined by the following integrals through the thickness of the shell:
For thin shells the Kirchhoff-Love approximation, which states that the deformed director field is (approximately) the normal field to the deformed reference surface, is introduced along with the plane stress assumption . Consistent with these approximations, we neglect all terms and terms proportional to the gradient of the thickness parameter. Accordingly, we set
We can now summarize the virtual work expression for thin (Kirchhoff-Love) shells:
where the shell resultant components are defined in terms of the Cauchy stress tensor components by the integrals
In the expression for
,
is interpreted as a constraint stress that enforces that the director field remain normal to the reference surface. Two other contributions to the virtual work expression
and
, where
are
and, thus, neglected.
Orthonormal surface coordinate system and coordinate transformation
It is desirable to define stress resultant quantities relative to an orthonormal basis in the deformed configuration. To do this, we define a normal coordinate system , where and are tangent to the deformed reference surface and is the unit normal field.
Define the following notation. Let be the components of relative to the basis ; that is
Furthermore, let the inverse of the matrix of components
be given by
, such that
Note that the basis vectors
and
induce distance measuring coordinates
and
such that
It follows from
Equation 3.6.7–5 that the orthonormal tangent vectors are given by
For the material calculations, it is important to express both the strain and stress quantities relative to the local orthonormal frame . Accordingly, let and be the membrane and bending stress resultant components relative to this local orthonormal basis. Thus, we can write
where we recall that
. Then the stress resultant contributions to the virtual work expression can be transformed as follows. First, the membrane contribution:
Recall, however, that by the definition of the coordinates
,
. Thus,
Similarly, the bending contribution is:
Let
be the two-dimensional alternator, such that
and
. Then
and
Since the second term in the brackets is proportional to the bending curvature, we neglect this term relative to the first, yielding
Strain displacement operators
We now write the virtual work expression Equation 3.6.7–4 in matrix operator notation. Define the following stress resultant component vectors:
Define the matrix strain displacement operators as follows:
where
is the column of zeros
. Then the virtual work expression
Equation 3.6.7–4 is equivalently stated
Corotational coordinate system
Thus far and are any two orthonormal vectors in the tangent plane to the reference surface. We can uniquely choose these two vectors by requiring the matrix components of the incremental reference surface deformation gradient , where
to be symmetric; i.e.,
. Note that by definition
. This symmetry condition defines a corotational orthonormal basis in the deformed configuration. This orthonormal basis is calculated as follows.
Obtain the ABAQUS convention pair of orthonormal surface vectors . We then rotate these vectors in the tangent plane to the reference surface about the normal vector by the angle , where is determined by the symmetry condition . Thus, we define
It follows by definition that
Therefore, define the quantity
The symmetry condition requires that
From this it can be determined that
and
are then determined by
Equation 3.6.7–6.
Having determined the updated vectors , we can calculate the quantities required for coordinate transformation. First the incremental deformation gradient components and the Jacobian matrix components:
Note that due to the symmetry of the incremental deformation gradient components, there is no ambiguity in writing
. We can now calculate the inverse components
Consistent linearization
The iterative solution procedure requires the calculation of the consistent tangent stiffness. This stiffness has two parts, one resulting from the material model and the other resulting from the changing geometry. We denote the second variational quantities with . Returning to Equation 3.6.7–4, the virtual work expression can be written as
where
,
is the Jacobian determinant of the reference configuration parametrization given by
, and components are relative to the corotated frame with
. We assume constitutive behavior such that
Thus, the variation of the virtual work expression yields
The first term in the integrand forms the material stiffness, while the second two terms form the geometric stiffness. The second variation of the strain measure components are calculated in
Finite-strain shell element formulation, Section 3.6.5. The second variation of the membrane strain is
The second variation of the bending strain is zero; i.e.,
Incremental degrees of freedom: interpolations and configuration updates
At the beginning of an increment we have the configuration at iteration k, denoted . In the incremental solution procedure we solve for the incremental displacement field and the incremental rotation field in components relative to the reference cylindrical coordinate system:
We use these incremental fields to update the configuration to iteration
.
1. Reference surface update: The displacement increments are interpolated using the same interpolation scheme as the total displacement vector in Equation 3.6.7–2:
The reference surface position map is updated from the displacement increment by
2. Rotation field update: The incremental rotation field is updated with the same interpolation scheme as the total rotation field in Equation 3.6.7–3:
This incremental rotation vector corresponds to a finite rotation, characterized by quaternion parameters as
The total rotation quaternion parameter can be updated by the update formula
Similarly, the deformed unit director field can be updated from the incremental rotation field as
Here we have used the notation
to denote the rotation of the vector
by the quaternion
. The curvatures are calculated from the gradient of the director field, which is updated by
where
For completeness, we record the values of
. First, along the generator lines we have
For the circumferential derivative we must account for the derivative of the basis vectors in the
-direction:
and
. Hence,
. Introducing the interpolation function, we have
where
are computed from the definitions of the interpolation functions
as given above and
and
are given by the interpolation for the incremental rotation field as given in the Rotation Field Update.
Strain increment and stress resultant update
Following the formulation of the finite-strain shell element formulation in Finite-strain shell element formulation, Section 3.6.5, the three-dimensional (finite) strain increment is calculated as
Here the increment in the membrane strain components
is given by the Hughes-Winget second-order approximation to the logarithmic membrane strain increment
and the increment in the bending strain components
is given by the expression
where
As an example of the stress update procedure, consider the simple case of a Saint Venant-Kirchhoff material model. In this case,
where
are the plane stress elastic coefficients given by
Note that
, and the components
are relative to the current orthonormal basis
.
Pressure loads and load stiffness
For geometrically linear problems, equivalent nodal loads due to applied surface pressure are readily calculated since the geometry is axisymmetric. For geometrically nonlinear problems, however, asymmetric deformations must be taken into consideration.
The equivalent nodal loads associated with surface pressure p can be obtained by considering the virtual work contribution to the external loading
where
S is the parametric surface coordinate in the
R–
Z plane and the reference surface position is
with
and
. Recall that the current position of a point can be expressed in terms of the axial interpolator
and the circumferential interpolators
by
The terms in
Equation 3.6.7–7 can be worked out as follows:
and, hence,
The variations
are written
, where the components have interpolations similar to those in
Equation 3.6.7–7. Therefore, the virtual work contribution becomes
With the introduction of the interpolation functions, we obtain the equivalent nodal forces:
For geometrically linear analysis, the equivalent nodal forces reduce to the standard axisymmetric expressions
The linearization of the pressure loading term leads to the following pressure load stiffness matrix:
with
In the case of hydrostatic pressure (p dependent on z), additional terms must be included in the pressure load stiffness. These terms appear due to the variation of the pressure magnitude and are readily obtained from the expression
With use of the interpolation functions and denoting the additional contributions with an over-bar, we obtain the additional load stiffness contributions:
Penalty constraints: transverse shear and drill rotation
It is necessary to enforce rotation constraints at selected points on the element surface to prevent singular modes of deformation. One axial transverse shear constraint is enforced on the rotation field between each pair of nodes within each nodal plane. One circumferential transverse shear constraint and one drill rotation constraint on the rotation fields and are enforced between each pair of nodes on a circumferential line of nodes. In each instance the rotation field is constrained to follow the nodal displacements. To summarize, for element SAXAMN:
constraints are enforced, where
is the order of integration in the axial direction and
is the number of Fourier modes.
Transverse shear
The transverse shear strain is the measure of the amount the director field has rotated relative to the normal to the shell surface. We define the transverse shear strain as
with no sum on
c and force this quantity to be zero with a penalty constraint. Note that
is a unit vector tangent to the shell surface defined by the displacement field along a parametric coordinate line. Recall that
and
. Hence,
is the axial transverse shear strain and
is the circumferential transverse shear strain.
For convenience, record the variation of the unit vector :
with no sum on
c. Equivalently, the definition of
can be used to write
The linearized transverse shear strain is calculated as
Since by definition
, it follows that
with no sum on
c.
For completeness, the second variation of the transverse shear strain is
where terms proportional to
have been neglected and the coupling between
and
has been symmetrized.
Drill rotation
Mathematically, the equations governing the deformation of the shell are invariant with respect to drill rotations; that is, a rotation of the director field with axis of rotation parallel to the director. It is necessary then to assign a kinematic definition to this rotation. We define the drill strain as the difference between the rotation of the circumferential tangent vector as measured by the displacement field and that measured by the rotation field. Accordingly, we define the drill strain by
Here,
is the rotated reference axial tangent vector and
is the deformed (unit) circumferential tangent vector, defined by
In the above
is a linear approximation to the axial tangent vector in the reference configuration given by
, where
and
are the position vectors to two adjacent nodes in an axial plane. The drill rotation constraint then requires that the component of rotation about the normal to the surface match the in-plane rotation of the surface as measured by the displacement field.
The linearized drill strain calculation is similar to the transverse shear linearized strain calculation. Without repeating the calculation,
Similarly, the second variation of the drill strain
where terms proportional to
have been neglected and the coupling between
and
has been symmetrized.
Zero radius: collapsed edge
For the case when the reference radius of any point on the shell surface goes to zero, all of the offset nodes collapse to the same point and the edge constraints along that circumferential edge become redundant. It is, therefore, necessary to treat the zero radius case separately.
For the zero radius case all redundant degrees of freedom are constrained to follow the average motion of the nodes at 0° and 180°. The edge constraints are broken into two parts: First, a circumferential transverse shear strain is defined that requires that the rotation in the radial direction follow the circumferential rotations at the first and last nodal plane:
Introducing the interpolations, the linearized strain is
Note that
. Second, a drill rotation strain is defined that requires that the rotation about the
Z-axis be zero:
This leads to a linearized strain given by
Note that
.
Mass matrix
At each material point the displacement components in the three directions (radial, axial, circumferential) are dependent only on the corresponding nodal displacement components. Hence, the mass matrix does not involve any coupling between the radial, axial, and circumferential degrees of freedom, and we can write the mass matrix in the form of three separate expressions:
Similarly, we can write the terms associated with the rotational degrees of freedom as:
Here the superscripts
m and
n refer to a particular node in the
r–
z plane, and the superscripts
p and
q refer to a particular position along the circumference. The interpolation functions
,
, and
are the product of interpolation functions
in the
r–
z plane and interpolation functions in the
-direction:
The area integral used to form the mass matrix can be split then into an integral along the length of the element in the
r–
z plane and an integral around the circumference. For the
r–
r component of the mass matrix this yields
and for the
component of the mass matrix this yields:
The matrix can be written in a convenient form by defining the primitive mass matrix as
or for the rotational components as
These primitive mass matrices are the same mass matrices that are used for the regular axisymmetric shell elements. We can also define the circumferential distribution matrices
The various components of the mass matrix then take the form
The circumferential distribution matrices can be evaluated for various values of the number of terms P in the Fourier series. After some calculations the following results are obtained:
:
:
:
: