ive read in the manual that shell elements(after assigning thickness and offset) are getting expanded to solids elements(page 101).
nevertheless iam asking how such elements are getting treated numerically, because there seems to be a lower amount of calculation time to get the fe-solution for geometrys generated by shell over geometrys generated by 3d-solids(same amount of quadratic nodes/elements).
are there some kind of shell or plate equations that are being used to get the solution in thickening direction?
You can find some short information about the formulation of CalculiX’s shell elements in “The Finite Element Method for Three-Dimensional Thermomechanical Applications” book written by G. Dhondt which is basically a theory guide for CalculiX.
Recently, a true shell element (US3) was added to CalculiX. Its formulation is discussed in the article “A three-node shell element based on the discrete shear gap and assumed natural deviatoric
strain approaches” by G. Rama et al. reference in the documentation. You may want to carry out some tests, comparing its performance with CalculiX’s expanded shell elements.
Knots created when implementing shell expansion reduce the actual number of equation to process when an item is modelled using ccx shells compared to a pure 3d solid, however the method is less efficient than the softwares that use actual shell formulations. Look for information about knots in the manual.
“To assure that the resulting 3D elements are connected, the newly generated nodes are considered as a knot” (Manual Reff.)
shell element is expanded to solid element, knot only generated for a set with the normal greater than 20deg to reference or cases with different thickness/offset direction, shell and beam, constrained nodes, rotational forces.
classical shell element with improved (US3) is available in the last versions, however it has many limitation. developer still recommended using continuum ones for uniformity of the solver…