Introduction to Finite Element Analysis
Finite element analysis (FEA) is a computational simulation technique used to analyze the behavior and performance of engineering components and assemblies under various conditions. It is an essential tool in modern engineering design and is used across industries from aerospace and automotive to biomedical and electronics.
The basic premise of FEA is to divide a large system into smaller pieces or "elements" and then reconstruct the full behavior by mathematically solving a model of the individual elements. This technique allows engineers to simulate strength, vibration, heat transfer, fluid flow, and many other behaviors without costly physical prototypes and testing. The power of modern computers allows extremely complex systems with millions of elements to be analyzed.
FEA first emerged in the 1950s as matrix methods of structural analysis and was adopted widely in the aerospace industry. Since then, rapid advances in computing power combined with improved algorithms and modeling techniques have greatly expanded FEA capabilities. Today, FEA software packages integrate seamlessly with CAD programs, allowing engineers to test virtual prototypes and optimize designs early in the development process.
The benefits of FEA include reduced product development costs and lead times, improved performance and reliability through design optimization, and insights into behaviors difficult or impossible to measure through physical testing. FEA allows "what-if" scenarios to be simulated quickly and cost-effectively. However, the accuracy of FEA depends greatly on the analyst's skills and understanding of underlying physics and proper modeling techniques.
The FEA process generally involves these key steps:
1. Defining project goals and scope
2. Creating the CAD model geometry
3. Meshing the geometry into elements
4. Specifying material properties and behaviors
5. Applying appropriate boundary conditions and loads
6. Running the FEA simulation to solve the mathematical model
7. Analyzing and validating the results
8. Optimizing and improving the design based on insights
9. Documenting the simulation parameters and outcomes
Properly applied, FEA has become an indispensable tool for competitive, optimized mechanical design across virtually all engineering disciplines. This step-by-step guide will explore the FEA process in greater detail.
Step 1: Understand the Physics of the Problem
The first step in any finite element analysis is gaining a deep understanding of the physical system you are analyzing. You need to comprehensively review the components, physics, constraints, and behaviors that are involved. Gather as much information as possible at this stage.
Carefully study the geometry and specifications of the CAD model or physical prototype. Review every dimension, surface, feature, assembly constraint, and material property in detail.
Research the physics and mechanical engineering principles that apply to the system. Make sure you thoroughly understand concepts like stress, strain, deformation, dynamics, fluid flow, or thermal behavior depending on your analysis type.
Identify all sources of loads, forces, pressures, motions, temperatures, or boundary conditions that will act on the system. Quantify their magnitudes and directions as accurately as possible.
Determine any fixed constraints that anchor or restrict motion in certain areas of the model. Capture all the forces, moments and physics that will drive behavior.
If analyzing a complex system, break it down into fundamental components, joints, connectors, sub-assemblies and interactions. Review how each part influences the whole.
Consult with designers, engineers and experts involved with the product's development. Leverage their specialized knowledge to fill any gaps in your understanding.
If faced with unfamiliar physics or components, seek additional learning resources and education outside your domain expertise. Never make assumptions - go beyond your comfort zone to fully understand the problem.
Gaining a robust conceptual understanding of the physical system sets up the entire FEA process for success. All subsequent steps rely on the knowledge built in this initial stage, so invest significant time upfront and leave no stone unturned in your pursuit of mastery over the engineering physics at play.
Step 2: Define Goals and Scope
Once you understand the physics and components involved in your mechanical design, the next step is to clearly define the goals and scope of your FEA simulation. This involves determining the type of analysis you want to run, the timeline for completion, and assessing the resources you have available.
Determine Analysis Type
First, decide what specific analysis you need to perform. Some common FEA analysis types include:
Static analysis - used to determine displacements, stresses, strains, and forces under steady loading conditions. This is the most common and economical analysis.
Modal analysis - calculates the natural frequencies and mode shapes of a structure. Used to determine dynamic response characteristics.
Thermal analysis - used to calculate thermal distribution and temperature-induced stresses.
Dynamic analysis - determines the time-varying displacements, strains, and stresses under dynamic or time-varying loads.
Nonlinear analysis - accommodates nonlinear material properties or large deformations.
Optimization analysis - iterates the design to achieve the optimal performance objective within set constraints.
Choose the analysis type that will yield the data you need to evaluate your design.
Determine Timeframe
Next, realistically estimate the timeframe you have available to set up, run, and analyze the FEA simulation. Some analyses like static and thermal run fairly quickly, while dynamic and nonlinear often require more compute time. If time is limited, you may need to compromise by starting with a simpler analysis before moving to more complex simulations.
Factor in the time required for pre and post processing steps, not just solving time. This includes time for importing CAD, meshing, applying loads, and evaluating results. Leaving ample time ensures you can properly set up the simulation and analyze the outcomes.
Assess Available Resources
Finally, take stock of the compute power, software, budget, and expertise available for the FEA work. This determines the scope of what is feasible to simulate within the allotted timeframe.
If software licenses, hardware, or budget is limited, you may need to simulate smaller components rather than the full assembly. Lack of in-house FEA experience may necessitate outsourcing the work or opting for simpler simulations.
Realistically scoping the work upfront prevents having to cut corners later. Define the goals and constraints to set your FEA project up for success.
Step 3: Import and Check the CAD Model
Importing a CAD model correctly into FEA software is a critical step that can make or break the entire simulation. The analytical results are only as good as the model geometry that's brought in. Here are some best practices when importing CAD into FEA:
Import Model File
Confirm you have the latest CAD model file from the design team. Old revisions can lead to inaccurate FEA results.
Make sure to obtain the model file in a compatible format like STEP, IGES or Parasolid. Native CAD formats often do not translate perfectly.
Pay attention to model orientation when importing. The FEA software may orient parts differently than how they were modeled.
Repair Any Geometric Errors
Scan for gaps, overlaps or misalignments between components after importing. Fix any geometry errors before meshing.
Repair import errors like missing faces, edges or bodies. Consult the CAD team if major fixes are needed.
Use defeaturing tools to simplify complex geometry that is not critical to the analysis.
Simplify the Model
Remove any small features that won't affect the simulation. This improves mesh quality.
Eliminate unnecessary detail and components irrelevant to the physics.
Make liberal use of symmetries to reduce model size. Mirror symmetric parts if applicable.
Create an idealized approximation if the exact CAD geometry is too complex to mesh.
With a clean CAD model successfully imported, you can be confident in creating a high quality mesh for accurate FEA results. Don't cut corners on preparing the geometry prior to meshing.
Step 4: Plan the Meshing Strategy
The meshing process discretizes the CAD model into small elements so the physics and material models can be applied to solve the FEA problem. While a finer mesh leads to more accurate results, it also increases the computational requirements for the simulation. Therefore, planning a suitable meshing strategy involves finding the right balance between accuracy and solve time.
When planning the mesh, focus first on regions that will experience high stress/strains or see critical results. These areas typically need a finer mesh to capture gradients and stresses accurately. Regions seeing lower stresses or simpler physics may only need a coarse mesh.
Pay attention to model interfaces and boundaries between components. The transition between fine and coarse mesh regions needs to be graded properly to avoid irregular elements. Model interfaces are also prone to inaccurate results unless meshed carefully.
The type of analysis should guide mesh sizes as well. For example, dynamic shock analysis requires a finer mesh to capture transient effects compared to a quasi-static analysis. Contact simulations need finer mesh at contact surfaces. Thermal analysis may need finer mesh to resolve thermal gradients.
Mesh metrics such as aspect ratio, skewness, orthogonality should be checked to avoid irregular or sliver elements. The geometry itself may need cleanup to improve mesh quality. Techniques like defeaturing holes/fillets help create meshable models.
Newer adaptive meshing algorithms automatically refine mesh based on a sizing function. This eliminates tedious manual work in assigning mesh sizes. Mesh controls can also refine mesh iteratively based on result errors. This focuses computation on critical regions.
With experience, guidelines will develop for mesh sizes based on analysis type, geometry, materials etc. But it's always prudent to start with a pilot mesh and refine based on initial results. The meshing stage sets the foundation for accurate FEA results.
Step 5: Define Material Properties
The next step is to define the material properties for each component in your CAD model. The material data will determine how the finite elements in the mesh will react under applied loads. Accurate material definitions are critical for achieving realistic FEA results.
The main material properties needed for structural FEA simulations are:
Young's modulus (modulus of elasticity)
Poisson's ratio
Yield strength
Ultimate tensile strength
Density
Young's modulus determines how much a material will deform under stress. It is a measure of the stiffness of a solid material. Metals tend to have high Young's modulus while polymers are lower.
Poisson's ratio characterizes how much a material expands or contracts laterally when loaded axially. It ranges between 0 and 0.5 for most engineering materials.
Yield strength indicates the stress level at which plastic deformation begins to occur in the material. Ultimate tensile strength defines the maximum stress before fracture.
Density of the material is needed to account for gravitational body forces and inertial effects in dynamic analysis.
These key properties can be obtained from material testing data, online databases like Mat Web or CES Edu Pack, CAD libraries, or directly from the material supplier. Always use values appropriate for the analysis temperature.
Ideally you should test representative material samples under expected operating conditions. This will provide the most accurate data for the FEA simulation.
Once you have the required material data, it can be mapped to your CAD model. Make sure to assign the appropriate properties to each part based on its material composition. Double check units are consistent between the source data and your analysis software.
Accurate material definitions allow the FEA solver to predict realistic stress levels, deformations, frequencies and other results. Garbage in, garbage out - so invest time in sourcing high quality material data!
Step 6: Apply Loads and Constraints
Applying appropriate loads and constraints is crucial for getting accurate results from your FEA simulation. Loads represent forces, pressures, accelerations, temperatures, etc. that are applied to the model. Constraints define restrictions on motion and deformation.
There are several types of loads and constraints you may need to apply:
Forces
Point forces - applied at a node
Distributed forces - applied along an edge or surface
Inertial forces - centrifugal, coriolis, angular acceleration
Bearing loads - applied through bearing elements
Pressures
Surface pressures - normal to a face
Fluid pressures - from surrounding fluid
Contact pressures - between components
Fixtures
Fixed geometry - zero displacement and rotation at selected nodes
Symmetry - constrains nodes to move symmetrically about a plane
Contact conditions - friction, bonding between components
Displacement constraints - prescribe node motions
Forces and pressures should be applied in the appropriate directions - don't just apply loads normal to all surfaces. Add bearing loads through properly meshed bearing elements.
Apply fixtures judiciously to prevent rigid body motions without over-constraining the model. Double check your boundary conditions make physical sense.
It's easy to obtain unrealistic results if inappropriate constraints are applied. Review your loads and boundary conditions carefully before running the analysis. Make changes if needed to achieve physics-based conditions.
Validate the Mesh
Before you run the simulation, it's critical to validate the mesh to avoid inaccurate results or failed solutions. A poor quality mesh is one of the most common causes of errors in FEA.
Some key checks to perform on the mesh:
Check for distorted elements: Look for overly stretched or skewed elements in the mesh which can lead to problems. If found, consider refining the mesh in those areas.
Verify appropriate element types: Confirm you have chosen suitable element types for the analysis physics and expected results. Certain elements like shells vs solids may not be appropriate.
Assess element aspect ratios: Elements with high aspect ratios (width/height) should be avoided if possible. Aspect ratios higher than 5 can reduce accuracy.
Look for gaps: Check that node connectivity is intact and there are no gaps between elements. Gaps can prevent load transfer between elements.
Examine boundary layers: Ensure fine mesh gradation near load application and boundaries for accuracy. Abrupt element size changes can cause errors.
Check free edges: Free/unconnected edges may need special treatment. Make sure nodes along edges are not under-constrained.
Confirm material assignments: Verify all parts have the intended material properties assigned before analyzing. Mixed material assignments can invalidate results.
Review warnings: Address any warnings from the meshing tool related to close proximity, tolerances, or mesh quality.
Compare mesh metrics: Review metrics like skewness, aspect ratio, and Jacobian before and after meshing to spot deviations.
Run preliminary solutions: Attempt trial solutions on a coarse mesh to identify problems before the full solution.
Following mesh validation best practices prevents wasted time from inaccurate FEA solutions and gives confidence in your analysis results. Don't rely on software checks alone - diligently inspect the mesh manually.
Step 8: Run the FEA Simulation
Once the finite element model is set up with a mesh, material properties, loads and constraints, it's time to actually run the simulation and solve the finite element equations. This step performs the complex numerical calculations at the heart of finite element analysis.
The FEA software takes the finite element mesh and creates a large system of simultaneous equations that represent the physics of the model. Each node in the mesh generates a set of equations based on the applied boundary conditions. For example, a simple linear static analysis may produce a structural stiffness matrix relating nodal displacements to nodal forces.
The software uses methods like direct sparse matrix solvers to efficiently solve these large sets of equations. The time required to solve the FEA model can range from seconds to hours depending on its size and complexity. Reviewing the estimated compute time before running the simulation is recommended to ensure it fits within project timelines. Monitor solve progress andMemory usage as well.
The end result of the simulation process is determining the behavior of the model under the defined conditions - this typically includes stress/strain, displacements, reaction forces, modal frequencies etc. But the raw output data still needs to be analyzed and presented in the next steps. Always review simulation runtime messages carefully to check for errors or convergence issues before proceeding.
Step 9: Analyze and Present Results
Once the simulation is complete, it's time to analyze the results and present the findings to stakeholders. This is one of the most important steps of the FEA process.
The FEA software will generate vast amounts of numerical data. While the raw data provides valuable insights, it needs to be translated into more consumable formats. There are several techniques to analyze and represent the results:
Contour Plots
Contour plots use colors to visualize the FEA results on the model geometry. For example, von Mises stress contour plots display a color gradient showing the stress distribution. Areas with high stress concentrations become quickly apparent. Contour plots are one of the most effective ways to interpret FEA data.
Animations
Animations bring the FEA model to life by showing deformations, stress propagation, modal shapes, and more. Simple animations can be created directly in the FEA software. For presentations, it may be worth exporting the model and results into 3D animation software to create professional videos.
Comparisons
Compare FEA results against hand calculations, analytical solutions, or physical test data. This validation gives confidence in the accuracy of the simulation. If disparities exist, the model requires refinement. Comparison plots are an excellent way to communicate validation.
Recommendations
Provide design recommendations based on the insights gained from FEA. Highlight areas that pass design criteria as well as problematic locations that require design modifications. Recommend changes to geometry, materials, boundary conditions, and other parameters to improve the performance.
Reports
Compile the key FEA results into a technical report for stakeholders and decision makers. Use relevant plots, animations, comparisons, and recommendations to summarize the critical information. Clearly convey if the design requirements have been met.
Presenting compelling FEA deliverables is crucial for stakeholders to understand the simulations and make sound engineering decisions. Proper analysis and presentation of results completes the FEA process.