# Should one use multiple elements through the thickness with multiple materials?

To get realistic results from a FEM calculation, it is common (or so I've been taught by someone who may not be up to date) to use at least four or five elements through the thickness of a beam or plate. I am now modelling a composite plate bending problem, with two or three materials.

Should I use four or five elements through the thickness of every material, or can I reduce my calculation time by using one or two elements per material, as long as the total adds up to four or five?

• A better method is (1) use a FE package with elements that don't suffer from shear locking (the reasons for that problem were understood and fixed 30 years ago!) and (2) use a FE package that has proper "composite shell" elements. Sep 27, 2018 at 12:15
• @alephzero Thanks. Perhaps (1) is true (I'm using NX NASTRAN 12); this is just a rule I've been taught by someone who is probably not up to date anymore. Regarding (2), I'm specifically interested in some of the behaviour caused by the nonzero thickness near the edge, so shell elements are not an option. Sep 27, 2018 at 12:33
• @alephzero Yup, a quick look into the docs shows that whoever warned me about (1) was probably mistaken. Are there any other reasons to use multiple elements through the thickness? Sep 27, 2018 at 12:36
• I would advise to run a convergence test, gradually increasing the number of elements through the thickness until your results dont change. Sep 27, 2018 at 13:42
• I don't know what is in NX Nastran 12 - I've only ever used MSC Nastran. In MSC Nastran, shear locking only affects a minority of elements (with old mathematical formulations that nobody wants to change, for whatever reason). Just avoid those element types for new models, and the problem doesn't exist. Sep 27, 2018 at 17:44