FEA Procedure
Below is a typical flowchart outlining the process of performing FEA:
- Data gathering involves collecting the geometric model of the structure or component, related drawings, material properties, and load cases.
- Generation and verification of the analysis model is to define the problem, including the purpose of the analysis and the physical dimensions and shapes of the structure or component to be analyzed. An accurate yet simplified model is created to reduce computational load, with appropriate element types chosen. Mesh density is increased in areas expected to have high gradients. Boundary conditions are applied to simulate real-world constraints, and external loads and forces acting on the structure are defined. Verification and validation are essential for ensuring the accuracy of the FEA model and results, which can be achieved by comparing them with known solutions or simple analytical models. If available, results should also be validated against experimental data or real-world measurements.
- Development of solutions includes solving the problem by setting up the analysis. The appropriate type of analysis, whether linear static, non-linear, or dynamic, is selected based on the nature of the problem. The analysis is run while monitoring the solver's progress to ensure convergence, adjusting the model, mesh, or solver settings as necessary. The model may be refined based on initial findings, which could involve adjusting mesh density, improving boundary conditions, or modifying geometry. Insights gained from the analysis should be used to optimize the design for better performance, reduced weight, cost efficiency, and other desired outcomes.
- Assessment of analysis results involves comparing the results against specified standards. Post-processing tools are used to visualize stress distributions, displacement shapes, magnitudes, and temperature distributions in thermal analyses. The results are analyzed to understand the physical behavior of the structure and to identify any areas exceeding the specified standards.
- Optimization of system design and generation of recommendations is to modify the system as necessary. The analysis process is repeated until the results meet the specified standards.
- Documentation of the analysis results detail the problem definition, modeling process, boundary conditions, loads, solver settings, results, and conclusions. It should include all relevant visualizations and interpretations, along with recommendations based on the analysis, such as design modifications, material changes, or further testing requirements.
FEA Guideline
ASME Section VIII Div. 2 provides comprehensive guidelines for conducting FEA. Key components of these guidelines include:
(1) Stress Categorization
ASME VIII-2 Figure 5.1 outlines the maximum allowable stresses for various locations within a component. This chart, in conjunction with the output from the stress classification tool, facilitates pass/fail assessments of the component being analyzed.
a) General Primary Membrane Equivalent Stress (Pm)
b) Local Primary Membrane Equivalent Stress (PL)
c) Bending Stress (Pb)
d) Primary Membrane (General or Local) Plus Primary Bending Equivalent Stress(PL+Pb)
These categories help ensure that the component adheres to safety and performance standards, providing a structured approach to evaluating stress within mechanical systems
Stress Categories and Limits of Equivalent Stresses
(Sourced From Figure 5.1 in ASME VIII Div. 2, 2013)
(2) Stress linearization
In the finite element method, the total stress distribution is obtained. To extract membrane and bending stresses, this total stress distribution shall be linearized on a stress component basis to calculate the equivalent stresses.
a) Selection of stress classification lines (SLC): SCLs are typically positioned at gross structural discontinuities. These lines are straight and extend from the inside to the outside of a vessel, perpendicular to both the inner and outer surfaces, as illustrated in Figure 5-A.5 from ASME VIII Div. 2 (2013).
b) Stress results obtained from a finite element analysis
c) Calculation of the membrane stress tensor
d) Calculation of the bending stress tensor
e) Calculation of the peak stress tensor
f) Calculation of the three principal stresses at the ends of the SCL, derived from the components of membrane and membrane plus bending stresses
g) Calculation of the equivalent stresses at the ends of the SCL, again based on components of membrane and membrane plus bending stresses.
This systematic approach to stress linearization is crucial for accurately assessing the structural integrity of components under various loading conditions.
Finite Element Model Stress Classification Line for the Structural Stress Method
(Sourced From Figure 5 - A.5 in ASME VIII Div. 2, 2013)
Analysis Software
Typical FEA software includes Ansys, Abaqus, and SAP2000. SolidWorks Simulation is also used for simple FEA analysis. At CCPGE, we utilize Ansys, SAP2000, and SolidWorks Simulation, depending on the complexity of the problem being investigated.
- Ansys is a comprehensive and widely-used FEA software that offers a broad range of simulation capabilities, including structural, thermal, and fluid dynamics analyses. It is known for its powerful solvers and user-friendly interface, making it suitable for various engineering applications.
- Abaqus is highly regarded for its advanced capabilities in nonlinear analysis, contact mechanics, pipe-to-soil interaction, and complex material modeling. It is commonly used in industries where precise simulations are essential.
- SAP2000 is a structural analysis and design software widely used in structural engineering for analyzing and designing buildings, bridges, dams, towers, and other structures. Its intuitive interface and robust analysis features make it a favorite among structural engineers.
- SOLIDWORKS Simulation is an integrated FEA tool within the SolidWorks CAD environment. It is user-friendly and ideal for designers and engineers who need to perform structural and thermal analyses as part of their design workflow, allowing for seamless integration of simulation into the design process.
