The common steps for carrying out a general purpose FEA is summarised below:
- Model Idealisation
- Spatial Domain Identification
- Element Selection
- Mesh Discretisation
- Material and Geometric Definition
- Boundary Conditions
- Pre-analysis Checks
- Job Submission
- Results Verification
The first step involves defeaturing small details of the model. Defeaturing is necessary but must always be done carefully as it is often the major source of error of an analysis. Assumptions made in this step will affect the results strongly and must be coherent throughout the analysis. For example, fillets are often omitted as they are difficult to mesh. However, defeaturing fillets in the region of interest for stress can be a hazardous decision due to stress singularity, which is common in structures with sharp re-entrant corners.
2. Identify Spatial Domain
Second is selecting the spatial domain. It is not always necessary to perform an analysis in 3D. Using 2D plane stress, plane strain or axisymmetric elements can not only reduce the computational time by an order of magnitude, sometimes it can also lead to more accurate results if used wisely. This is because the extent of approximation, as well as the possibility of making human errors both increase with the size and complexity of the model. Even in a 3D analysis, it is possible simplify the problem by making use of line and area elements instead of brick elements. Therefore when identifying the spatial domain for a problem, always remember the KISS principle - Keep it simple, stupid!
3. Select Element Type
Element selection involves making decisions on element topologies, formulation and accuracy. In simple words element topologies considers for example triangles vs. quadrilaterals, element formulation considers for example full vs. reduced integration, and element accuracy considers for example the use of linear vs. quadratic shape functions. As a rule of thumb quadrilaterals perform better than triangles, quadratic shape functions perform better than linear. Reduced integration formulation must be used carefully as they can produce meaningless results due to hourglassing.
4. Discretise Mesh
In the early days of the development of FEA, FE analysts often spend most of their time specifying node numbers and nodal connectivity from scratch. Modern pre-processors now eliminate the need for this by providing highly automated meshing procedures. This has no doubt improved our efficiency towards building fully workable geometric models, but critical decisions that require analyst's attentions remain. This include obtaining the best balance between computational time and approximation errors, and avoiding bad quality mesh.
5. Assign Material and Geometric Properties
Material properties for linear analysis are usually straightforward. Complexity rises significantly with the presence of nonlinear materials. For elastoplastic material as an example, the analyst must fully understand the input requirements for the hardening rule and the system's conventions to be able to define a correct material behaviour beyond yielding.
Geometric properties are usually required for line and area elements. These may be the cross-sectional area of a truss, second moment of area of a beam, and the thickness of a shell element. Again, understanding the input requirements is important and if in doubt, it is best to run a simple test which can be verified analytically.
6. Apply Load, Supports and Constraints
Boundary conditions can be defined in different forms, this can sometimes involve confusing terms such as "edge load", "face load", "global load", and more... No matter which form the user defines the boundary conditions in, the program will convert all loads to nodal forces, and all displacements to nodal displacements. Making use of automated facilities is always preferred as they can avoid silly mistakes to be made, as well as ensure distributed loads/ pressure to be applied properly across complex surfaces.
7. Perform Pre-analysis Checks
Follow our checklist for carrying out pre-analysis checks:
This is usually a Go/No Go process and leaves little opportunity for user intervention. If the job cannot be processed, the user will have to troubleshoot the problem according to the generated error message. If the job runs successfully, results can be manipulated using a wide range of options available. This usually include printing displacement and stress contours, vectors of reaction force, and printing results in a table format etc.
9. Verify Results
The ease of performing complex analysis have led many to perform engineering simulations even before having an initial guess about what to expect. This makes it difficult to judge whether the generated results are any good. Verification is therefore an important task as part of the good modelling practice.
Second is selecting the spatial domain. It is not always necessary to perform an analysis in 3D. Using 2D plane stress, plane strain or axisymmetric elements can not only reduce the computational time by an order of magnitude, sometimes it can also lead to more accurate results if used wisely. This is because the extent of approximation, as well as the possibility of making human errors both increase with the size and complexity of the model. Even in a 3D analysis, it is possible simplify the problem by making use of line and area elements instead of brick elements. Therefore when identifying the spatial domain for a problem, always remember the KISS principle - Keep it simple, stupid!
3. Select Element Type
Element selection involves making decisions on element topologies, formulation and accuracy. In simple words element topologies considers for example triangles vs. quadrilaterals, element formulation considers for example full vs. reduced integration, and element accuracy considers for example the use of linear vs. quadratic shape functions. As a rule of thumb quadrilaterals perform better than triangles, quadratic shape functions perform better than linear. Reduced integration formulation must be used carefully as they can produce meaningless results due to hourglassing.
4. Discretise Mesh
In the early days of the development of FEA, FE analysts often spend most of their time specifying node numbers and nodal connectivity from scratch. Modern pre-processors now eliminate the need for this by providing highly automated meshing procedures. This has no doubt improved our efficiency towards building fully workable geometric models, but critical decisions that require analyst's attentions remain. This include obtaining the best balance between computational time and approximation errors, and avoiding bad quality mesh.
5. Assign Material and Geometric Properties
Material properties for linear analysis are usually straightforward. Complexity rises significantly with the presence of nonlinear materials. For elastoplastic material as an example, the analyst must fully understand the input requirements for the hardening rule and the system's conventions to be able to define a correct material behaviour beyond yielding.
Geometric properties are usually required for line and area elements. These may be the cross-sectional area of a truss, second moment of area of a beam, and the thickness of a shell element. Again, understanding the input requirements is important and if in doubt, it is best to run a simple test which can be verified analytically.
6. Apply Load, Supports and Constraints
Boundary conditions can be defined in different forms, this can sometimes involve confusing terms such as "edge load", "face load", "global load", and more... No matter which form the user defines the boundary conditions in, the program will convert all loads to nodal forces, and all displacements to nodal displacements. Making use of automated facilities is always preferred as they can avoid silly mistakes to be made, as well as ensure distributed loads/ pressure to be applied properly across complex surfaces.
7. Perform Pre-analysis Checks
Follow our checklist for carrying out pre-analysis checks:
- Visually check mesh quality
- Check types of elements, accuracy, and their formulation
- Ensure consistent units across input parameters. Including material properties, load, support, and geometric dimension
- Check that all boundary conditions have been applied correctly. Pay particular attention to make sure that no nodes, edges or faces have been left out
- Check the specified dimension of the analysis. A structure that looks 2D may also be analysed in a 3D environment so be very careful
- Check that contact relationships between contactable bodies are correct
- Check that you only request the results that you need from the solver. This can help reducing the processing time for generating unnecessary results
This is usually a Go/No Go process and leaves little opportunity for user intervention. If the job cannot be processed, the user will have to troubleshoot the problem according to the generated error message. If the job runs successfully, results can be manipulated using a wide range of options available. This usually include printing displacement and stress contours, vectors of reaction force, and printing results in a table format etc.
9. Verify Results
The ease of performing complex analysis have led many to perform engineering simulations even before having an initial guess about what to expect. This makes it difficult to judge whether the generated results are any good. Verification is therefore an important task as part of the good modelling practice.
- Check if the deformed shape looks reasonable
- Use of finer mesh and look for convergence of results
- Compare displacements and stresses with back-of-the-envelope calculations
- Verify if reactions are equal to the sum of applied loads in each of the x, y, z coordinate directions
- Check the weight of structure by summing all vertical reaction solutions without applying external loads
- Ask your colleagues to perform a quick check. Often it is just a simple problem that you are not aware of.
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