FEA Periodic Boundaries

NairnFEA can solve problems with truly periodic boundary conditions in one or both directions.

Introduction

Periodic boundary conditions means to run calculations that are periodic in strain in just the x direction, just the y direction, or in both the x and y directions. For axisymmetric problems the calculations can only be periodic in the z direction. Many researchers use symmetry-based displacement conditions for periodic calculations (thinking they are periodic), but that approach cannot handle problems where plane sections do not remain plane even though the structure and strains are periodic. The periodic conditions in NairnFEA are more advanced than simple symmetry conditions and they are essential for some problems such as shear strain applied to a periodic composite material.

Theory

The figure on the right shows a mesh of width Δx and height Δy in which the left and right edges are parallel to the y axis and each node on the left edge corresponds to a node on the right edge at the same y coordinate. If this structure is a piece on a large object that is periodic in the x direction, it means that the strains are periodic or that:

      [math]\displaystyle{ \varepsilon_{xx}(x+\Delta x,y) = \varepsilon_{xx}(x,y) }[/math]

      [math]\displaystyle{ \varepsilon_{yy}(x+\Delta x,y) = \varepsilon_{yy}(x,y) }[/math]

      [math]\displaystyle{ \gamma_{yx}(x+\Delta x,y) = \gamma_{xy}(x,y) }[/math]

These conditions are satisfied, in the most general sense, if the x and y displacements (u(x,y) and v(x,y)) have the form:

      [math]\displaystyle{ u(x+\Delta x,y) = u(x,y) + k_1 + k_2 y }[/math]

      [math]\displaystyle{ v(x+\Delta x,y) = v(x,y) + k_3 - k_2 x }[/math]

If there are n nodes on each edge, these conditions are satisfied if the problem is solved subjected to the following constraints:

      [math]\displaystyle{ u_i(\Delta x,y_i) = u_i(0,y+i) + k_1 + k_2 y_i \qquad {\rm for}\ i=1,\ {\rm to}\ n }[/math]

      [math]\displaystyle{ v_i(\Delta x,y_i) = v_i(0,y+i) + v_1(\Delta x,y_1) - v_1(0,y_1) \qquad {\rm for}\ i=2,\ {\rm to}\ n }[/math]

These conditions are solved by using FEA method of Lagrange multipliers. All x displacements on the right are replaced by constraints as are all y (except v_1(Δx,y_1), which remains a degree of freedom). In addition, k₁ and k₂ are introduced as two new degrees of freedom. It is better to rewrite these degrees of freedom using

      [math]\displaystyle{ k_1 + k_2 y_i = \Delta u + {du\over dy}(y_i - \left\langle y\right\rangle) }[/math]

where <y> is the mean y on the edges. Physically, Δu the mean displacement jump across the modeled object, which results in an average x direction strain:

      [math]\displaystyle{ \left\langle \varepsilon_{xx}\right\rangle = {\Delta u\over \Delta x} }[/math]

Physically du/dy is the slope (or rotation) of the right edge relative to the left edge. Like nodal displacements, these new terms resulting from periodic boundary conditions can be degrees of freedom (i.e., output of the analysis) or can be inputs (e.g., boundary conditions that impose an average strain or relative rotation to the edges)

Using the same analysis on problems that are periodic only in the y (or the Z of axisymmetric analysis) give a series of constraints to displacements on the nodes at the top of the object relative to the displacements on the bottom of the object. The new constraints lead to two new degrees of freedom - Δv and dvdx. Physically, Δv is the mean displacement jump across the modeled object, which results in an average y direction strain:

      [math]\displaystyle{ \left\langle \varepsilon_{yy}\right\rangle = {\Delta v\over \Delta y} }[/math]

Physically dv/dx is the slope (or rotation) of the top edge relative to the bottom edge.

Finally, if the problem is periodic in both direction, the four new degrees of freedom relate to average axial strains (as defined above for each direction) and to average shear strain by:

      [math]\displaystyle{ \left\langle \gamma_{xy}\right\rangle = {1\over \Delta y}{du\over dy} + {1\over \Delta x}{dv\over dx} }[/math]

For axisymmetric problems, the model cannot be periodic in the R direction. It can be periodic in the Z direction, but to maintain periodicity in the hoop stress (ε_θθ) to top and bottom edges must remain aligned. In other words, the rotational degree of freedom (dw/dr) must be zero (i.e., it is not a degree of freedom). The average axial strain is related to Δw or the displacement drop in the vertical direction:

      [math]\displaystyle{ \left\langle \varepsilon_{ZZ}\right\rangle = {\Delta w\over \Delta Z} }[/math]

Input Commands

In scripted files, you set up periodic boundary conditions with one or two Periodic commands:

Periodic (dir),<(dof1),(value1)>,<(dof2),(value2)>

In XML, the one or two commands, which must be in the <GridBCs> block, are:

<Periodic dof='(dir)' delta='(value1)' slope='(value2)'/>

where

• (dir) defines the periodic direction. NairnFEA allows periodicity in the x direction only, in the y direction only, or in both the x and y directions. For axisymmetric problems, periodicity is allowed in the Z direction only.
• The optional (dof1),(value1) and (dof2),(value2) in scripted files can fix one or two periodic boundary degrees of freedom. In each pair, the dof is the degree or freedom and value is the setting. The options for dof are:
  1. Delta to set the displacement jump in that direction (in mm). When applied to the x direction, the global average strain is <ε_xx> = Delta/Δx. When applied to the y direction, the global average strain is <ε_yy> = Delta/Δy. When applied to the Z</tt< direction in axisymmetric analyses, the global average strain is <εzz> = Delta/Δz. Here Δx, Δy, and Δz are the specimen lengths in those directions.
  2. Slope defines the rotation or relative slopes between the two sides of the periodic direction (dimensionless). When periodic in both x and y directions, the average shear strain is <γ_xy> = dudy/Δy + dvdx/Δx. Here Δx and Δy are the specimen lengths in the two directions, dudy is the slope in the x direction, and dvdx is the slope in the y direction. When only one direction is periodic, the Slope is just the relative rotation of the two edges.
• In XML files, optional (value1) and (value2) set the displacement jump and rotation using delta and slope attributes.

For periodicity to work, the edges of the specimen on the two ends of the periodic direction(s) must be orthognal to that direction, they must each have the same number of nodes, and the orthogonal coordinate (e.g., the y coordinate for x periodicity, the x coordinate for y periodicity, or the R coordinate for Z periodicity) of each node on one side must match the corresponding node on the other side.

Setting the periodic degrees of freedom for Delta and Slope are optional. If they are not set, they are degrees of freedom in the problem (and can be found from output results). If the are set as boundary conditions, they can determine the average normal strains, edge rotations, and average shear strains. Finally, scripted commands can use Shear as a synonym from Slope and XML command can use a shear attribute as a synonym for the slope attribute.

Notes

1. Adding periodicity will greatly increase the bandwidth of the problem. A Resequence Command can be used, but is unlikely to help much. Instead, only use periodic conditions if they are needed to solve the problem.