Lattice structural elements

Lattice structural elements

Lattice2d element

Represents two-node lattice element. Each node has 3 degrees of freedom. The element is based on the Rigid Body Spring Model originally developed by Kawai and later delveloped by Bolander for modelling fracture in concrete. The main idea is to model the elastic and inelastic response of a connection of two nodes by a set of springs located at the contact facet of two rigid bodies, which is the mid-cross-section of the element. Displacement jumps are computed at the mid-cross-section, which are smeared out over the element length in the form of strains. The element is defined in x,y plane (see Figure Lattice2d element. Node numbering, DOF numbering and definition of integration point C.). The element features are summarized in Table Table 71.

Fig. 32 Lattice2d element. Node numbering, DOF numbering and definition of integration point \(C\).

Table 71 lattice2d element summary

Keyword	lattice2d
Description	2d lattice element
Specific parameters	thick (rn) width (rn) gpCoords (ra)
Parameters	thick: defines the out of plane (\(z\)-direction) thickness width: defines the width of the midpoint cross-section in the \(x\)-\(y\) plane with the point \(C\) at its centre gpCoords: array of the coordinates of the integration point \(C\) in the global coordinate system. gpCoords is optional; if it is not given, \(C\) is the midpoint of the element, so the eccentricity is zero (frame elements). For a circular LatticeCS (shape 1) the section properties are taken from the cross section and thick and width do not affect the response.
Unknowns	Three dofs (\(u\)-displacement, \(v\)-displacement, \(w\)-rotation) are required in each node.
Tests/Examples	lm/lattice2drandom.in, lm/latticeplastdamvisco_mps_1.in, lm/latticeplastdamvisco_mps_2.in, lm/latticeplastdamvisco_mps_3.in, lm/latticedamagevisco_mps_1.in, lm/latticeplastdamvisco3drandom.in, lm/latticeviscoelastic_mps_1.in, lm/lattice2 dframecircle.in, lm/lattice2dframesteelplastic.in

The theory of lattice2d is described in [GraJir10]

Used with a circular LatticeCS (shape 1, radius), lattice2d acts as a Timoshenko frame element in the \(x\)-\(y\) plane. The area \(A = \pi r^2\), the second moment of area \(I = \pi r^4/4\) and the shear area \(0.9 A\) are computed from the radius, and each can be overridden by area, iz and shearareay of the cross section. Together with the frame materials latticeframeelastic and latticeframesteelplastic, this allows modelling, for example, yielding reinforcement bars in 2d. The generalised strains are the axial strain, the shear strain and the curvature at point \(C\), and the corresponding generalised stresses are the normal force \(N\), the shear force \(V\) and the bending moment \(M_z\). The element has a single integration point, so the bending moment is constant within each element and several elements per member are needed where yielding is expected.

LatticeBoundary2D element

Represents three-node lattice element for boundary of 2d periodic cells. The first two nodes have 3 degrees of freedom as for the element lattice2d. The third node is used to control the loading of the periodic cell. It has three components which are displacements, which are produces of the macroscopic (average) strain components and length of the peridodic cell as \(aE_{xx}\), \(bE_{xx}\) and \(bG_{xy}\) and the length of the periodic cell. The DOFs of the node that lies outside the periodic are computed from those of the periodic image inside the cell and the DOFs at the third node (Fig. 33). The coordinates \(x\) and \(y\) of the third node are the lengths \(a\) and \(b\) of the periodic cell, respectively. The element is defined in x,y plane. The strain components at the additional node have the meaning of average strains in the periodic cell. The specific input parameters for this element in addition to those used for lattice2d are shown in Table lattice2dboundarysummary.

Fig. 33 latticeboundary2D element. Elements with cross boundary of periodic cell use DOFs of node inside cell and average strain values of periodic cell.

Table 72 latticeboundary2D element summary

Keyword	latticeboundary2D
Description	2d lattice boundary element
Specific parameters	location (in)
Parameters	location: number between 1 and 8 which specifies the location of the node with respect to the periodic cell.
Unknowns	Nine DOFs are required, which are \(u\)-displacement, \(v\)-displacement and \(w\)-rotation at nodes 1 and 2, and \(aE_{xx}\), \(bE_{yy}\) and \(bG_{xy}\) at node 3.
Reference	[GraJir10]
Tests/Examples	lm/lattice2dboundary1.in

Lattice3d element

Lattice3d represents a two-node 3d lattice element. Each node has six degrees of freedom as shown in Fig. 34. The element is based on the Rigid Body Spring Model originally developed by Kawai and later delveloped by Bolander for modelling fracture in concrete. The main idea is to model the elastic and inelastic response of a connection of two nodes by a set of springs located at the contact facet of two rigid bodies, which is the mid-cross-section of the element. The properties of the mid-cross-section are internally computed from its vertices which are given as input in the global coordinate sytem. Displacement jumps are computed at the mid-cross-section, which are smeared out over the element length in the form of strains. The input parameters for this element are shown in Table 73.

Fig. 34 Lattice3d element. Node numbering, DOF numbering, cross-section vertices and local coordinate system at integration point \(C\).

Table 73 lattice3d element summary

Keyword	lattice3d
Description	3d lattice element
Specific parameters	polycoords (ra) couplingflag (in) couplingnumbers (ra) pressures (ra) mlength (rn)
Parameters	polycooords: array of the coordinates of the vertices of the mid-cross-section of the lattice element in the global coordinate system. couplingflag: flag (optional parameter. Default is 0) which activates coupling with a transport lattice element. couplingnumbers: array of numbers of transport lattice elements (optional parameter), which are coupled with the 3d lattice element. pressures: array of pressure values (optional parameter), which are used to consider influence of fluid pressure on mechanical response. mlength: minimum length (optional parameter) is used to check if the cross- section of the element is not too small. Default value is 1.e-20.
Unknowns	Six dofs (\(u\)-displacement, \(v\)-displacement, \(w\)-displacement, \(u\)-rotation, \(v\)-rotation and \(w\)-rotation) are required in each node.
Reference	[GraBol16]
Tests/Examples	lm/lattice3dshell.in, lm/lattice3dshell_lle.in, lm/lattice3dshell_lle_twist_n4.in, lm/lattice3d3.in, lm/lattice3d4.in, lm/lattice3drandom.in, lm/lattice3ddamplast3.in, lm/lattice3dshell_dmg_bend_n7.in, lm/lattice3dbondplast1.in, lm/lattice3delastic.in (and 17 more)

Lattice3dBoundary element

This element represents a three-noded 3d lattice element for boundaries of 3d periodic cells. The first two nodes have 6 degrees of freedom as for the element lattice3d. The third node is used to control the loading of the periodic cell using three normal (\(aE_{xx}\), \(bE_{yy}\) and \(zE_{zz}\)) and three shear strain (\(cG_{yz}\), \(cG_{xz}\), \(bG_{xy}\)) components. Here, \(a\), \(b\) and \(c\) are the three dimensions of the periodic cell. The DOFs of the node that lies outside the periodic are computed from those of the periodic image inside the cell and the DOFs at the third node (Fig. 35). The connection between periodic nodes is defined as

\[\mathbf{x}^{'} = \mathbf{M} \mathbf{x}\]

Here, \(\mathbf{x}^{'}\) and \(\mathbf{x}\) are the nodes inside and outside, respectively, and \(\mathbf{M}\) is the translation matrix, for which the input is provided in the form of a location parameter as shown in Table Table 74.

Fig. 35 Lattice3dboundary element. Elements with cross boundary of periodic cell use DOFs of node inside cell and average strain values of periodic cell.

Table 74 lattice3dboundary element summary

Keyword	lattice3dboundary
Description	3d lattice boundary element
Specific parameters	location (in)
Parameters	location: array of two numbers between number between 1 and 26, which specifies the location of the two nodes with respect to the 3d periodic cell.
Unknowns	Six dofs (\(u\)-displacement, \(v\)-displacement, \(w\)-displacement, \(u\)-rotation, \(v\)-rotation and \(w\)-rotation) are required in each of the first two node. Node 3 requires the 6 quantities to control the periodic cell \(aE_{xx}\), \(bE_{yy}\), \(zE_{zz}\), \(cG_{yz}\), \(cG_{xz}\) and \(bG_{xy}\)
Reference	[AthWheGra18]
Tests/Examples	lm/lattice3d1.in

LatticeLink2d element

This element is the 2d counterpart of latticelink3d. It is a two-node link in the \(x\)-\(y\) plane with three degrees of freedom per node (\(u\), \(v\) and the rotation about \(z\)), for example between a frame node (lattice2d) and a node embedded in a 2d continuum. The jump of the second node relative to the first, including a rigid arm from the first node to the second, is expressed in a local system with the first axis along dirvector: slip along the bar, lateral jump across it and relative rotation. The material is evaluated in 2d lattice mode, i.e. as the components 1, 2 and 6 of the 3d lattice material, so the same bond materials (e.g. latticeslip, with a1 scaling the lateral and a2 the rotational stiffness) can be used in 2d and 3d. The bond area is \(\pi\) diameter length. The input parameters are shown in Table 75.

Table 75 latticelink2d element summary

Keyword	latticelink2d
Description	2d lattice link element
Specific parameters	length (rn) diameter (rn) dirvector (ra)
Parameters	length: bond length diameter: diameter dirvector: direction vector in which bond-slip occurs (2 components; a third component is ignored).
Unknowns	Three dofs (\(u\)-displacement, \(v\)-displacement and \(w\)-rotation) are required in each node.
Tests/Examples	lm/latticelink2d01.in

LatticeLink3d element

This element represents a two-node 3d link element connecting 3d beam and 3d lattice elements. Each node has six degrees of freedom. The node ordering is important: the first node carries the rigid arm and must therefore have rotational degrees of freedom, while the slip (displacement jump) is evaluated at the second node. Which physical entity is placed first is the user’s modelling choice; it only has to be the node from which the rigid arm is offset. For a lattice model both matrix and reinforcement nodes usually have rotational degrees of freedom, so the matrix node is typically given first and the reinforcement node second, evaluating the slip at the reinforcement. The input parameters for this element are shown in Table 76.

Table 76 latticelink3d element summary

Keyword	latticelink3d
Description	3d lattice link element
Specific parameters	length (rn) diameter (rn) dirvector (ra) l_end (rn)
Parameters	length: bond length diameter: diameter dirvector: direction vector in which bond-slip occurs. l_end: array of pressure values (optional parameter), which are used to consider influence of fluid pressure on mechanical response. mlength: minimum length (optional parameter) is used to check if the cross- section of the element is not too small. Default value is 1.e-20.
Unknowns	Six dofs (\(u\)-displacement, \(v\)-displacement, \(w\)-displacement, \(u\)-rotation, \(v\)-rotation and \(w\)-rotation) are required in each node.
Reference	[GraAnt19]
Tests/Examples	lm/latticelink1.in

Latticelink3dboundary

Represents three-node 3d boundary link element connecting 3d beam and 3d lattice elements. The first two nodes have the same meaning as for latticelink3d. The third node is used to control the loading of the periodic cell using three normal (xx, yy and zz) and three shear strain (yz, xz, xy) components. The specific input parameters for this element in addition of those for latticelink3d are shown in Table Table 77.

Table 77 latticelink3dboundary element summary

Keyword	latticelink3dboundary
Description	3d lattice link boundary element
Specific parameters	location (in) rn
Parameters	location:
Unknowns	Six dofs (\(u\)-displacement, \(v\)-displacement, \(w\)-displacement, \(u\)-rotation, \(v\)-rotation and \(w\)-rotation) are required in each node.
Reference	[GraAnt19]

Lattice3dNl element

Lattice3dNl is the geometrically nonlinear (large-rotation) counterpart of the lattice3d element. It accepts the same input and the same cross-section options (facet/shell via polycoords, or beam via the cross-section properties together with zaxis and s), but the displacement jumps and nodal forces are evaluated in the deformed configuration, so the element is suitable for large rotations and catenary action. The rotational part of the generalised strain is the exact relative rotation of the two nodes, i.e. the axial vector of \(\mathbf{R}_2 \mathbf{R}_1^{\mathrm{T}}\), which reduces to the difference of the nodal rotation vectors for rotation about a single axis.

Table 78 lattice3dnl element summary

Keyword	lattice3dnl
Description	3d geometrically nonlinear lattice element
Specific parameters	polycoords (ra) shellnormal (ra) [zaxis (ra)] [s (rn)]
Parameters	polycoords, shellnormal, zaxis, s: as for the lattice3d element.
Cross-section	As for the lattice3d element.
Unknowns	Six dofs (\(u\)-displacement, \(v\)-displacement, \(w\)-displacement, \(u\)-rotation, \(v\)-rotation and \(w\)-rotation) are required in each node.
Reference	[AbdGra24]
Tests/Examples	lm/lattice3dnlshell.in, lm/lattice3dnl_logrot.in, lm/lattice3dnlshell_dmg_bend_n7.in, lm/latticelink3dnl_assembly.in, lm/latticeframe3dnl.in

LatticeContact3d element

LatticeContact3d is a two-node contact (interface) element. In contrast to lattice3d it couples only the translational degrees of freedom of the two nodes, so it can connect 6-DOF lattice nodes as well as 3-DOF continuum nodes, and it transmits no moment. The translational displacement jump between the nodes is expressed in a local frame (one normal and two shear directions) and passed to the material as a three-component generalised strain. The jump is not divided by the element length, so the material works directly with displacement jumps. Internal forces and stiffness are scaled by the contact area. A typical use is the bond between reinforcement and concrete: the converter emits latticecontact3d for all elements of a given material with #@element material <mat> latticecontact3d, using the Voronoi facet as polycoords. The element is summarised in Table 79.

Table 79 latticecontact3d element summary

Keyword	latticecontact3d
Description	3d node-to-node lattice contact (interface) element
Specific parameters	[polycoords (ra)] [area (rn)] [normal (ra)]
Parameters	polycoords: coordinates of the vertices of the contact facet polygon, given as for the lattice3d element (typically the Voronoi facet). The contact area is the polygon area and the normal is the polygon plane normal (Newell’s method). area: contact area, required if polycoords is not given (fewer than three vertices). normal: contact normal (local axis 1), used together with area; if omitted, it is taken along the line connecting the two nodes.
Cross-section	A latticecs record, as for the lattice3d element.
Material	A lattice material acting on the three translational components, typically latticebondplast; the rotational components 4 to 6 remain zero.
Unknowns	Three dofs (\(u\)-displacement, \(v\)-displacement, \(w\)-displacement) are required in each node. Rotational dofs present at the nodes are not coupled.
Tests/Examples	lm/contact_single.in, lm/contact_cubes_separation.in, lm/contact_cubes_shear.in