Choosing an element#

All the elements have six dofs per node and can be combined in one model, e.g. shells stiffened by beams, or beams connected by springs, since they share the dofs of the nodes, see Conventions.

Shells#

The shell elements are flat, with first-order shear deformation theory (FSDT) kinematics, the constitutive matrices of a ShellProp, see Shell and beam properties, and the von Karman membrane strains in the geometrically non-linear analyses.

Element	Nodes	Use
`Quad4`	4	The recommended quadrilateral. Mixed quadrature of the transverse shear terms against shear locking, and the drilling enrichment of the in-plane displacement field, see `quad4`. A strip one element wide is too flexible in torsion, see the zero-energy modes in the Notes of `quad4`.
`Quad4R`	4	One integration point, cheaper than `Quad4` for the internal forces and the stiffness matrices, with an hourglass control that adds an artificial stiffness, scaled by the `hgfactor_*` arguments of `Quad4R.update_KC0()`; no drilling enrichment, see `quad4r`. The transverse hourglass stiffness also stiffens the twist, by an amount that depends on the problem, e.g. 6 to 7 % on the linear deflection of the hinged roof of Sze et al. (2004). The hourglass control is derived for isotropic plates and applied to laminates with equivalent moduli of the membrane compliance for \(u\) and \(v\) and of the bending compliance for \(w\), \(r_x\) and \(r_y\).
`Tria3DSG`	3	The recommended triangle. Discrete shear gap transverse shear field, free of shear locking by construction and without a tuning parameter, see `tria3dsg`. No drilling enrichment: for problems dominated by in-plane bending, `Quad4` and `Tria3R` have one.
`Tria3R`	3	Kept for continuity with the results obtained before `Tria3DSG` existed. Its transverse shear, taken at the centroid, locks and is reduced by the factor of `alpha_shear_locking`, `0.7` by default, a value that must be verified per problem class, see `tria3r`.

Practical rules:

Prefer quadrilaterals in bending-dominated regions. A mesh of linear triangles is stiffer than the quadrilateral mesh it is split from, and its response depends on the direction of the diagonals; where triangles are needed, alternating the diagonals reduces this bias.
The four nodes of a quadrilateral in a curved surface are, in general, not coplanar, and the element is flat: keep the warping, the out-of-plane deviation of the nodes over the in-plane size, small. The triangles are always flat.
The distorted-mesh variants of the benchmarks of MacNeal and Harder (1985), doi:10.1016/0168-874X(85)90003-4, show how much distortion an element tolerates.
The drilling rotation \(r_z\) of all the shells has, by default, a physics-based stiffness, drilling_model = 0, without user parameter. drilling_model = 1 selects the fictitious penalty of Nastran, controlled by K6ROT, which reproduces the results of versions older than 0.10.0.
The aerodynamic matrices of the piston theory, update_KA_beta, update_KA_gamma and update_CA, exist for Quad4 and Quad4R only.

Beams, truss and spring#

Element	Use
`BeamC`	The recommended beam. Timoshenko kinematics with the consistent shape functions of Luo (2008), which solve the static equilibrium equations of the beam, such that one element gives the exact tip deflection of a cantilever with shear deformation, and the nodal displacements of a frame under nodal loads, see Linear static analysis of beams and frames.
`BeamLR`	Timoshenko kinematics with linear interpolation and reduced integration; it converges to the same solutions with mesh refinement, quadratically, and needs many more elements, see `tests/test_timoshenko_beam_literature.py`.
`Truss`	Axial and torsional stiffness only, linear analyses and mass matrix, without geometric stiffness matrix, hence no linear buckling. The documentation of `truss` recommends `BeamC` instead.
`Spring`	Six constant stiffnesses, `kxe, kye, kze, krxe, krye, krze`, between the same dofs of two nodes, in the element coordinate system, e.g. for elastic supports and connections; no mass and no geometric stiffness matrix.

The cross section of the beams and of the truss is given by a BeamProp, whose integrals are taken in the element coordinate system, see Shell and beam properties. Its orientation is defined by the vector vxyi, vxyj, vxyk of update_rotation_matrix, see Conventions. The beams have a transverse shear modulus for each plane of bending, G12 for the element \(xy\) plane and G13 for the \(xz\) plane, and the beams and the truss take the torsional stiffness GJ; the three follow G unless they are set.

Methods of each element#

Method	Quad4, Quad4R	Tria3DSG, Tria3R	BeamC, BeamLR	Truss	Spring
`update_KC0`	yes	yes	yes	yes	yes
`update_fint`, `nonlinear=1`	yes	yes	yes	linear only	linear only
`update_KCNL`	yes	yes	yes
`update_KG`	yes	yes	yes
`update_KG_given_stress`	yes	yes
`update_M`	yes	yes	yes	yes
`update_KA_beta`, `update_KA_gamma`, `update_CA`	yes

The arguments of each method are documented in the API pages of the elements, Finite elements.

Mass matrices#

The argument mtype of update_M selects how the kinetic energy of the element is integrated, with the same values for every element:

`mtype`	Mass matrix	Elements
`0`	consistent	Quad4, Quad4R, Tria3R, Tria3DSG, BeamC, BeamLR, Truss
`1`	lumped	Quad4, Quad4R, Tria3R, Tria3DSG, BeamC, BeamLR, Truss
`2`	reduced integration, one point	Quad4, Quad4R, Tria3R

Any other value, and 2 for an element without a reduced-integration mass matrix, raises a ValueError. Each matrix is the integral over the element of the inertia of the section between the interpolated velocities: for the shells, \(I_\rho\), \(I_{\rho z}\) and \(I_{\rho z^2}\), the intrho, intrhoz and intrhoz2 of the ShellProp, which include the offset, see Shell and beam properties; for the beams, the integrals of the BeamProp. The consistent matrices integrate it exactly. The lumped matrices integrate it with the points at the nodes, where each shape function is 1 at its own node and 0 at the others, such that, for the shells, node \(i\) carries the weight \(w_i\) times

\[\begin{split}\begin{bmatrix} I_\rho & 0 & 0 & 0 & I_{\rho z} & 0 \\ 0 & I_\rho & 0 & -I_{\rho z} & 0 & 0 \\ 0 & 0 & I_\rho & 0 & 0 & 0 \\ 0 & -I_{\rho z} & 0 & I_{\rho z^2} & 0 & 0 \\ I_{\rho z} & 0 & 0 & 0 & I_{\rho z^2} & 0 \\ 0 & 0 & 0 & 0 & 0 & 0 \end{bmatrix}\end{split}\]

in element coordinates, for \(u, v, w, r_x, r_y, r_z\), with \(w_i = A/3\) for the triangles and the Jacobian determinant at the node for the quadrilaterals, and no coupling between nodes. A lumped shell matrix is diagonal only without offset, when \(I_{\rho z} = 0\); with an offset, the coupling keeps the centre of mass of the element at the offset. The mass, first moment and rotary inertia given by each scheme are discussed in the Notes of update_M, e.g. Tria3R.update_M() and Quad4R.update_M(). The lumped beam matrices are diagonal, without the first moments intrhoy, intrhoz and the product intrhoyz of the section, see BeamC.update_M().