Practical Finite Element Analysis
for Mechanical Engineers


Practical Finite Element Analysis
for Mechanical Engineers


Practical Finite Element Analysis
for Mechanical Engineers


Practical Finite Element Analysis
for Mechanical Engineers


15 Common Mistakes in FEA

15 Common Mistakes in FEA

Nowadays, Finite Element Analysis (FEA) is implemented early in the design cycle of many products. Thanks to FEA, the development process is quickly guided in the right direction, avoiding expensive mistakes.

With FEA, it is now possible to deliver higher-quality products in a shorter design cycle. However, the question of the ability of FEA analysts to use this method arises. Engineers cannot employ the method without an advanced education in FEA.

Indeed, engineers must follow specific methods and guidelines to make sure that their simulations properly predict the reality. Here are the most common errors encountered during structural analysis using FEA when solving solid mechanics problems.

Mistake #1: Doing FEA Without Clearly Knowing the Objectives of the Analysis

Before to start modeling, it is important to take time identifying the FEA goals. I recommend you answer the question: “What should be captured by the FEA?”

A precise definition of all objectives is important for determining the modeling techniques as well as the solution you will use and will influence many of the assumptions you will make during the building process. For example, different modeling techniques are used to model linear and nonlinear problems. There are also different modeling techniques for capturing peak stress in a particular region of the component vs stiffness of the whole component.

As an FEA analyst, you may not be the only user, or end user, of the FEA. Many other engineers may use it (the clients). Therefore, you must communicate with your clients to clearly define the goals of the FEA. This step will permit you to establish that all stakeholders understand its purpose as well as its capabilities and limitations.

The objective is to ensure that the right assumptions will be made during the development of the FEA and that everyone understands all the effects the FEA will capture.

Below are few examples of goals:

  • Stiffness under a given loading?
  • Peak stress?
  • Interface loads?
  • Stress concentration effect?
  • Ultimate strength?
  • Load distribution (load model)?
  • Thermal stress?
  • Fatigue life?
  • Design optimization?
  • Instability?
  • Simulation of nonlinear effects (geometrical, material, or contact)
  • Simulation of dynamic effects (vibration, frequency response, transient)

Mistake #2: Doing FEA Without Understanding the Physics behind the Analyzed Phenomenon

FEA software can solve a variety of single and multiphysics problems, including structural problems and issues of heat transfer, fluid dynamics, acoustics, magnetism, and electric fields.

So, the knowledge of the physics behind the analyzed phenomenon is fundamental.

Structural engineers use FEA to ensure that a structure has the necessary strength, stiffness, or life to prevent failure (excess stress, resonance, buckling, or detrimental deformation that may compromise structural integrity). So, the most important qualities you need before modeling a mechanical system using FEA are your engineering judgment and a robust general physics background (statics, dynamics, and strength of materials).

Physics

Mistake #3: Doing FEA Without Understanding How the Structure is Likely to Behave

To produce a reliable FEM that will enable you to make useful predictions, you must make a deep dive into the mechanical system in question. You need to understand the environment of the system and how it reacts to external stimulation. Do not use FEA to predict how your mechanical system will behave, do the opposite, understand how the system behaves in the real-life in order to produce a reliable simulation.

Use your engineering knowledge to create an FEM that will represent the mechanical system and external stimulation. This will transform the actual mechanical system into a model that can provide useful predictions such as displacements, stresses, strains, and internal forces.

You must understand the real behavior of the mechanical system you wish to model and be able to predict its behavior before beginning your modeling work.

Mistake #4: Using the Wrong Type of Elements

As an FEA analyst, you must perform the discretization of the geometry for the parts you intend to model. During the process of discretization, you must choose your elements from a library. FEA software libraries include a large variety of elements for modeling arbitrary geometrical structures.

Selecting the element type is one of the most basic decisions you will make as an FEA analyst. Each element available in the library is associated with a family, and each family contains specific building blocks having the 1D, 2D, or 3D topologies. So, precise knowledge concerning the various available families is necessary before beginning the modeling work. Depending on the structural behavior of the modeled parts, the elements capabilities, the computing time, and required accuracy, you must select the elements from one or several families to model the proper mechanical effects.

element

Mistake #5: Defining Unrealistic Boundary Conditions

During the process of creating an FEM, defining the boundary conditions is the stage at which many engineers make incorrect assumptions.

The topic of boundary conditions is less understood by FEA beginners, but it is not uncommon for experienced engineers to also have difficulty properly defining them. Boundary conditions have a major impact on the results of an analysis, and a small mistake when defining them can make the difference between a correct and incorrect simulation.

To properly define your model’s boundary conditions, you will need to follow a strategy, which will permit you to test and validate them.

The role of boundary conditions in the simulation process includes:

  • Fixing the value of displacements in a specific region of the model
  • Applying representative loads in a specific region of the model
  • Replacing a part not modeled in the model to simplify it
BCs

Mistake #6: Ignoring the Mesh Convergence Study

As an FEA analyst, your objective is to predict the real-world behavior of a structure with a very high level of accuracy. Accuracy is directly related to the quality and density of the mesh used to discretize the components: as the elements are made smaller (mesh refinement), the computed solution approaches the true solution.

This process of mesh refinement is a fundamental step in developing an FEM that lends confidence to the model and the results.

Mesh refinement is required to represent a curved surface or an edge using straight-sided elements. Clearly, the geometry of the boundaries will be better represented if the mesh is refined.

By definition, a converged mesh, from a numerical accuracy standpoint, is one that produces no significant differences in the result when mesh refinement is introduced. Mesh convergence is a concern when producing a model to capture peak stress or strain. If test results such as strain gauge records exist for the part you wish to model, it is easy to determine the mesh density needed to capture the correct stress. Unfortunately, you will usually not have such records prior to beginning the meshing process. Therefore, a convergence study must be conducted for the regions of peak stress to ensure that the mesh size is sufficiently fine to correctly capture the phenomena of interest and critical stress.

Mesh

Mistake #7: Post-Processing Mistakes

Today, finite element simulations are used to simulate very complex structures and mechanical systems. Thanks to the immense calculation capacity of modern computers, it is possible to solve extremely refined FEMs. A consequence is the vast amount of output data for these simulations, which is where post-processing comes into play. Postprocessing is often defined as the “art of results interpretation.” The quantity of results generated by the solver is so large that the FEA analyst needs to use a dedicated software, called a post-processing software, to organize and interpret the results.

There are many rules that the FE analyst has to follow in order to properly extract the results from the simulation. Another important topic when post-processing stresses or strains is the management of the singularities.

Post-Processing Mistakes

Mistake #8: Avoiding to Model Contact

Contacts define how parts interact and are typically used to understand behavior and load transfer within an assembly. Contacts are very useful in FEA simulations because they allow multiple bodies to interact, without the need to add elements to the model.

There are advantages to contact conditions in a model, since it is possible to model more realistic behaviors, but their addition to a simulation can be costly. Indeed, contact conditions create significant computational complexity in a model, and numerous parameters must be managed by the FEA analyst.

By default, FEA software do not assume any contact between the bodies of a model. It is the user who must specify where contact occurs. The problem with contact conditions is that small parameter changes can cause large changes in the system response. Therefore, when contact conditions are used, it is important to conduct robustness studies to check the sensitivity of the numerical parameters.

Using contact conditions in a model can lead to convergence problems and will require a longer computation time. It may require that you have advanced experience as an FEA analyst. Therefore, a question arises concerning the utility of modeling contact conditions in a model: How does the model respond, if you do not consider the contacts in the analysis?

Keep in mind that there are situations in which the fact that contacts are not used does not change the results. However, in other cases, their use will completely change the model’s response and internal load distribution. The answer to the above question depends on your ability to judge whether or not contact conditions can be avoided.

Mistake #9: Running the Wrong Solution

In order to properly capture the right behaviors with your simulation, you will have to run the right solution. It is an important choice since the modern FEA packages offer a wide range of solutions in order to solve different types of behaviors.

To select the right solution, you will have to define the following:

  • Is your problem a static or a dynamic one?
  • Is it a linear or a nonlinear problem?
  • If it is a nonlinear problem, which type of nonlinearity is it?
  • If it is a contact problem, what type of contact?
FEA

Mistake #10: Neglecting Verification and Validation

In FEA, both careless users and experienced engineers who are not careful to systematically validate their models can easily make significant mistakes. Expensive decisions in terms of both time and money can sometimes turn out be based on incorrect answers. FEA is demanding, in the sense that the FEA analyst must be proficient not only in mechanics but also in mathematics, computer science, and especially FEA itself.

Therefore, it is important to employ validation and correlation procedures whenever FEA is used to solve a problem. When there are no test data you can use to validate an FEA during the first design stages, you must employ methods for verifying its quality and ensuring there are no errors. Ultimately, when test data are available, correlation with the FEA must be done to ensure that the modeling abstractions do not hide real physical problems that will occur after the structure has been manufactured.

The complete Verification & Validation process includes:

  • Accuracy Checks
  • Mathematical Checks
  • Correlation with Test Data

Mistake #11: Using a Non-Consistent Unit System

Most FEA software are units-free. You must choose your own system of units and use it consistently. You are free to choose the system of units but be aware that there is a link between the units of input data and the obtained results. Most post-processing software are also units-free, so, from the modeling stages through post-processing, you must guarantee the units’ consistency.

Units

Mistake #12: Relying Only on Graphics

I should warn engineers using the finite element method to solve everyday problems that FEA is a powerful tool for your work, but it does not replace your engineering judgment. The finite element method enables you to accurately solve very complex problems but only if you follow good modeling practices. In the end, your sense of logic and mechanics as an engineer is irreplaceable.

Remember that FEA is not just about knowing a software but a question of understanding the physics behind a modeled structure and the real behavior of its parts. The importance of the FEA software itself is often overestimated, compared to the understanding of the modeling process. This lays a trap for the engineer, who may base their FEA judgment more on the graphical aspects than on the engineering data. You can avoid such a trap by knowing good modeling practices and employing your engineering judgment at each stage of the model development process.

The importance of the FEA software itself is very often overestimated, compared with a good understanding of the modeling process. Most FEA beginners focus on graphical aspects over engineering data and modeling techniques, but it is hard to blame them, because critical factors of a simulation are generally hidden behind the software’s interface. Furthermore, it is very common for new engineers to be given an FEA role with little training.

Always remember that an FEA software is just a black box for solving very complex systems of equations that cannot be solved quickly. FEA is simply a tool used to aid in your structural analyses. If you do not master the modeling techniques, you will likely submit an incorrect system of equations to the solver.

Mistake #13: Not Documenting the FEA

From an industrial point of view, an FEA and its associated results must be traceable years after the project is completed. Most FEA documentation today focuses mainly on the results.

However, these results are influenced by many parameters, so all the assumptions of the simulation should be documented as well. FEA documentation has many purposes:

  • To present the assumptions
  • To track the data used to perform the FEA
  • To provide a basis for further analysis
  • To permit FEA analysts to replicate the analysis
  • To permit training of personnel
  • To establish in-house expertise in FEA
  • To provide legal documents when liability is involved

The FEA documentation should provide sufficient details to permit any FEA analyst to reproduce, verify, and validate the FEA. Depending on the nature of the FEA, the documentation may differ, given the specific parameters used in some FEMs. However, some information is common to all FEAs and should be present in all documentation:

  • The model identification
  • The source of geometry
  • The model assumptions
  • The simulation parameters
  • The validation and correlation

Finally, it is important to note that the target audience of FEA documentation does not only include FEA analysts. Therefore, all reporting work should be conducted with the objective of establishing confidence in the model.

Mistake #14: Not Following Good Modeling Practices

The main danger of FEA is not knowing whether an answer given by the FEA software is correct. To address this issue, you must follow the best modeling practices and use validated modeling techniques defined by previous users.

Many FEA analysts make incorrect assumptions and generate flawed analyses, simply because they are unaware of basic FEA rules. Good knowledge of proper modeling techniques makes a vast range of tools available, enabling you to make the right assumptions and choices to accurately model and solve problems using simulation.

Good Modeling Practices

Mistake #15: Assuming that the FE Analysis is Conservative

Never assume that the FEA produces conservative results; there are many situations in which an FE analysis will underestimate the results.

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ABOUT THE AUTHOR
Dominique Madier

Dominique Madier is a senior aerospace consultant with 20 years’ experience and advanced expertise in Finite Element Analysis (FEA) of static and dynamic problems for linear and nonlinear structural behaviors.

He has conducted detailed finite element analyses for aerospace companies in Europe and in North America (e.g., Airbus, Dassault Aviation, Hispano-Suiza [now Safran], Bell Helicopter Textron Canada, Bombardier Aerospace, Pratt & Whitney Canada, and their subcontractors) on metallic and composite structures such as fuselages, wings, nacelles, engine pylons, helicopter airframes, and systems.

He is the author of the book “Practical Finite Element Analysis for Mechanical Engineer”: 650+ pages about the best practical methods and guidelines for the development and validation of finite element models.

He earned a Master’s degree in Mechanical and Aerospace Engineering from Paul Sabatier University, Toulouse, France.
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