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What are the advantages and disadvantages of using quads (shells) vs using hexas (solid) in my mesh for a fatigue analysis using a finite element software. Thanks.

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It really depends a lot on your model and types of loads but here are some generalities:

  1. Shell elements usually require a lot less computational work than solid elements. This means that you can achieve reliable results with much fewer elements and within a fraction of the time. The rule-of-thumb is to use 1-D (Beams) or 2-D (shells) elements as much as possible before resorting to 3-D solid elements as a way to reduce overall model complexity.
  2. Shell elements are recommended for specific geometries and load types: Sheet metal parts or more generally, parts with large (>>10) length-to-thickness ratios are perfect candidates for using shells as they will portray the bending behavior quite accurately with much fewer elements than their solid-mesh counterparts.
  3. Solid elements are preferred when analyzing bulky feature-rich components such as hubs, shafts, etc.
  4. Solid element meshes are usually faster to set up. To correctly define shells you must pay attention to the directional nomenclature as they have a top and bottom direction. Most of the times you will need to convert your model to surfaces by extracting mid-surfaces which is not always straight-forward.
  5. Unlike solid elements, shells have more degrees of freedom which correspond to rotations. You must make sure sure that all necessary restraints are fulfilled in order to avoid undesired deformations and stresses

Ultimately it boils down to this:

  • Shells are preferred due to their lower computational requirements
  • Shells are best suited for large thin parts
  • Solid meshes are usually faster to set up
  • Solid elements are preferred for chunky parts

In complex real-life models you will hardly ever use a single element type. Most often your model will require using 1-D, 2-D and 3-D elements at once. The choice of element types is highly dependent on the nature of your loads and their application. When dealing with buckling and stability of vessels you will most likely pick shell elements. If you are interested in fatigue analysis, your mesh must be very fine around any potential stress concentrations as they are usually the places of crack initiations, so a solid mesh will usually require a large number of elements. Finally i would like to mention that in small models, usually the computational effort between either shells or solids is barely noticeable specially with a powerful CPU/GPU, so you can simply pick the one that is quicker to set up and the most appropriate for your geometry.

Cheers!

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Bottom Line is that either can be used successfully if properly applied.

The most appropriate has to do with:

  • the application and (e.g. symmetries in geometry vs. irregular shapes and loads)
  • what you are aiming to get out of the analysis. (e.g. just a single value for the entire structure, or looking at more detail at locations with problems).

The main difference will be in performance benefits as described in more detail by @JCME. The main benefit of the solid elements is that they might be able to give you a more detailed spatial characterisation of the fatigue factor.

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