Introduction to Stress Analysis Simulation
Stress analysis is an essential tool used by mechanical engineers during the design process. It allows them to predict how a mechanical part will behave under different loading conditions and identify potential failure points before the part ever reaches manufacturing. By simulating the real-world stresses a part will endure, engineers can evaluate its structural integrity and make any necessary modifications early on to ensure adequate strength and durability.
Traditionally, stress analysis testing required building physical prototypes and placing them under loads in a controlled lab environment. However, modern CAD (computer-aided design) software provides a cost-effective alternative: the ability to simulate stress on 3D CAD models. With a powerful finite element analysis (FEA) engine, CAD allows users to see how stresses distribute across the part under static, dynamic, or thermal loads. The visual representation of stress enables a comprehensive evaluation of design weaknesses.
This guide will provide an overview of the end-to-end workflow for simulating stress analysis on mechanical parts using CAD software. We will cover the required steps from importing a model into the CAD tool, applying appropriate loads and boundary conditions, meshing, running the simulation, understanding the results, and using those insights to optimize the design. Whether you’re new to FEA simulation or looking to deepen your skills, this guide aims to help you get started with stress analysis on mechanical parts in your CAD tool of choice. The ability to simulate real-world stress can take your engineering designs to new heights!
Understanding Stress and Failure in Mechanical Parts
All mechanical parts experience different types of stresses that can lead to failure if not properly designed. The three primary types of stresses are:
Normal Stress - Stress perpendicular to the surface of the part. This is usually caused by tensile or compressive forces.
Shear Stress - Stress parallel to the surface of the part. This is usually caused by shear forces or torsional loads.
Bearing Stress - Localized stress where two parts contact each other, like bolted joints or gear teeth. The compressive stress can deform and damage surface areas.
Several common failure modes can occur when stresses exceed the strength of the material:
Yielding - When normal or shear stresses exceed the yield strength of the material, it undergoes permanent plastic deformation. This can cause work-hardening and eventual fracture.
Fracture - Cracks form and rapidly propagate through the part, causing complete separation. This is often preceded by yielding.
Fatigue - Failure occurs gradually over time as cyclic stresses induce crack growth during repeated loading and unloading. High cycle fatigue can happen even below yield strength.
Buckling - Slender parts like columns and beams can buckle and deform suddenly under high compressive loads. The buckling mode depends on the loading conditions and geometry.
Creep - Plastic deformation occurs slowly over time when parts are subjected to stresses at high temperatures. This can lead to necking and rupture.
Proper design is key to minimizing risk of failure. Using the right material, including generous fillets and radii, and avoiding stress concentrations can improve the strength and fatigue life of mechanical parts. Performing stress analysis during design can predict areas of high stress and allow the engineer to optimize the design.
Modeling the Part Geometry
CAD (Computer Aided Design) software allows engineers to create detailed 3D models of mechanical parts and assemblies. The modeling process requires understanding the geometry, dimensions, and features of the real-world component so that the CAD model accurately represents the physical part.
There are a few key steps involved in modeling geometry for stress analysis:
Review CAD Drawings/Models: If available, use existing 2D CAD drawings or 3D models of the part. Review the dimensions, features, and any specifications that impact the analysis.
Model the Geometry: Use CAD software like SolidWorks, NX, Creo, Inventor, CATIA or others to model the 3D geometry. Make sure to include all the fine details - holes, fillets, threads etc.
Check Accuracy: Compare the CAD model to the drawings and real part to ensure it is dimensionally accurate. The model must match the real geometry for stress analysis results to be valid.
Simplify When Possible: Look for areas where small features like fillets or threads can be removed to simplify the meshing process later on. But don't remove anything that impacts the structural integrity.
Save/Export Formats: Save the CAD model in IGES, STEP or Parasolid formats which can be imported into finite element analysis (FEA) software. The model may need cleaning up or idealizing further in the FEA pre-processor.
Idealize Geometry: Some idealization of the geometry may be required to simplify the mesh. For example, small holes might be combined into one big hole or very small fillets removed. But only idealize features that will not affect the stress distribution.
Proper modeling of the part geometry in CAD is an important first step before simulating stress analysis. Having an accurate 3D model ensures the FEA results will match the real-world behavior. Simplifications can be made, but without compromising the fidelity of the analysis.
Meshing and Discretization
The next step in the simulation process is meshing and discretization. Meshing converts the 3D CAD geometry into a finite element model made up of discrete elements or "mesh."
There are a few key steps in the meshing process:
Select element types - The type of element depends on the analysis and geometry. Common types are solid elements for 3D parts, shell elements for thin structures, and beam elements for slender parts like rods. Solid tetrahedral elements are commonly used
Apply mesh controls - Mesh controls specify the overall element size and any local sizing needed. You want a fine mesh in areas of interest and a coarser mesh elsewhere. Refining the mesh increases accuracy but also increases solve time.
Generate mesh - The software generates a mesh based on the controls. It breaks down the geometry into a finite number of elements with nodes connecting them. This process is called discretization.
Check mesh quality - Review the mesh to ensure there are no issues like distorted elements or poor aspect ratios. The mesh directly impacts result accuracy.
Run mesh convergence study - Mesh density affects results, so run studies with progressively finer meshes. When results stop changing significantly, an appropriate mesh is reached.
Proper meshing and discretization is crucial for accurate stress analysis results. It converts the CAD geometry into a finite element model that can be mathematically solved to simulate real-world stresses and deformations. Taking the time to generate a quality mesh will pay off in the accuracy of your simulations.
Defining Material Properties
Most CAD tools have material libraries that contain the properties for common engineering materials like metals, plastics, and composites. When setting up your simulation, you will need to define the material that matches your mechanical part.
The three key properties needed for stress analysis are:
Young's Modulus - Defines how much a material will deform under load. A high modulus means the material is very rigid.
Poisson's Ratio - Defines how much a material will compress laterally when under tension. Most materials have a value between 0.2 and 0.5.
Yield Strength - The maximum stress a material can withstand before permanent deformation. The higher the yield strength, the stronger the material.
You also need to specify whether the material is isotropic or anisotropic. Isotropic means the material properties are the same in all directions. Anisotropic means the properties change based on direction - common in composites.
For more advanced analysis like thermal stress or vibration, you may need temperature-dependent properties. Metals like steel and aluminum change modulus and yield strength at high/low temperatures. Your CAD software should allow defining these temperature-based changes.
Select a material in your CAD library that closely matches the actual material used for your mechanical part. Make sure the orientation is correct for anisotropic materials. Accurate material properties are crucial for realistic stress analysis results.
Applying Loads and Boundary Conditions
In the real-world, mechanical parts and structures are subjected to different types of loading that induce internal stresses. These need to be simulated in your CAD stress analysis model to get accurate results. The main types of loads are:
Forces - Loads applied at a particular point on the part. Common examples are tensile, compressive, bending, torsional, and shear loads.
Pressures - Uniform loads applied over a surface area. For example, internal pressure in a cylinder or external atmospheric pressure.
Thermal Loads - Non-uniform temperature distributions that induce thermal stresses. Can simulate steady-state or transient heat transfer.
Inertial Loads - Dynamic loads induced by acceleration, such as centripetal forces. Important for analysis of rotating parts.
Moment Loads - Rotational forces applied perpendicular to the plane of rotation. Cause torsional stress.
Nonlinear Loads - Loads that vary in magnitude and direction during the simulation, like pressure from impact or blast.
It is critical to accurately determine the types, locations, directions and magnitudes of loads acting on your part. Refer to the engineering specifications or conduct experiments to quantify the operational loading.
In addition to loads, boundary conditions are needed to constrain the motion of the part. Common constraints include:
Fixed Constraints - Zero displacements and rotations at a point. Simulates an anchored/welded joint.
Sliding Constraints - Restricts motion along a plane or axis. Used for idealized bearings or guides.
Contact Constraints - Restricts penetration of joined surfaces. Needed for interfaces between parts.
Symmetry Constraints - Used to reduce the modeled domain by exploiting symmetry.
Properly simulating the operating constraints ensures the model behaves realistically. Misapplication of loads and constraints is a common source of error in stress analysis.
Running the Simulation
Once the model setup is complete, the next step is to run the simulation solver to calculate the stress and deformation results. There are a few key aspects to consider when running the solver:
Solver Settings and Controls
The solver settings allow you to control the type of analysis and desired accuracy of the results. For a linear static stress analysis, you will define parameters like:
Analysis type (static, modal, etc)
Iterative solver controls
Convergence criteria
Load stepping options
Set these parameters carefully based on your model and accuracy needs. Using tighter convergence tolerances and smaller load steps will improve accuracy but increase solve time.
Estimating Solve Time
The solve time is highly dependent on the model size and complexity. Models with finer mesh discretization, non-linear materials, and complex contact conditions will take longer to converge on a solution. Monitor the estimated solve time displayed by the solver and set your convergence criteria appropriately - tighter tolerances may significantly increase solve time.
For large models, you can utilize parallel processing capabilities in most solvers to speed up the solution by splitting the analysis across multiple CPU cores. Cloud-based solvers also provide access to increased compute power.
Monitoring Convergence
During the solution phase, monitor the convergence metrics to ensure the solver is steadily progresses towards the specified tolerance. If convergence is stalling, you may need to relax the tolerances or review the model setup. Plotting the stress and displacement results as the solution progresses is also useful to identify any issues early.
Be patient with larger models - depending on model complexity, solutions may take minutes to hours to fully converge. Allow the solver to run until the specified tolerances are met to ensure accurate results.
Interpreting the Results
Once the simulation has completed, it is time to interpret the results to gain insights into your design. The CAD software will provide detailed stress, strain, and deformation results that require careful analysis.
The first thing to look at is the stress results. Stress indicates the internal forces experienced by different regions of the part under load. Excessive stress can lead to permanent deformation or catastrophic failure. The stress will be displayed using color maps, with red indicating dangerously high stress. Review areas of high stress and determine if the part can withstand them without failure.
Strain results show the amount of deformation experienced by the material. Like stress, high strain in certain areas may indicate potential failure points. The strain is represented by color gradients, with higher strain in red.
The CAD software will also show overall deformation of the part under the applied loads. Excessive deformation can impact the function and fit of an assembly. Analyze the deformation shape and magnitude to assess if it is acceptable.
Use the CAD software's visualization tools like animated displacements and cut section views to thoroughly examine the stress distribution and deformation. Rotate and slice through the part model for deeper insights.
Once the stress, strain and deformation metrics are understood, focus your attention on areas of concern. Look for:
Highly localized stress concentrations
Areas where yield strength is exceeded
Deformation that impacts functionality
Intersections of load paths
Geometric discontinuities
These high-risk areas are prime candidates for design optimizations and improvements to ensure the part will perform safely in the real world. The stress simulation highlights weaknesses early so they can be addressed before manufacturing.
Validating the Model
Once you have completed your simulation and reviewed the results, it is important to validate that your model produces accurate, reliable results that can be trusted for making design decisions. There are a few techniques for validating a simulation model:
Mesh Sensitivity Studies
A mesh sensitivity study checks how much the simulation results change based on the density of the mesh applied to the part geometry. A properly converged mesh will produce consistent results even as the mesh is refined. To perform a mesh sensitivity study:
Run the initial simulation with a standard mesh density
Refine the mesh globally by 50% and re-run the simulation
Compare the results between the two mesh densities - they should be reasonably close
Continue refining and re-running until the results stabilize within an acceptable tolerance
If the results vary widely based on mesh density, the model may not be properly converged and could contain errors.
Correlation to Hand Calculations or Test Data
Another validation approach is to compare your simulation results to simplified hand calculations for the same loading scenario. While hand calcs make simplifying assumptions, the results should reasonably match.
Alternatively, comparing to any physical test data for the part under similar loads is even better for benchmarking the accuracy of the simulation. There will always be some deviation between simulation and real-world tests, but major discrepancies could indicate issues with the simulation assumptions.
### Sources of Errors
During validation, identify any sources of error that could lead to inaccuracies in the simulation:
Idealized geometry, lacks small features
Approximate material model doesn't match supplier data
Coarse mesh density insufficient to resolve stress gradients
Boundary conditions do not precisely match real load application
Contacts, connectors, joints overly simplified
By scrutinizing these potential error sources, you can determine if any model improvements are required to produce more reliable results. If discrepancies are minor, the model may be useful for design optimization despite small inaccuracies.
Optimizing the Design
Once you have the results from your initial stress analysis simulation, the next step is to optimize your design to improve its performance under load. There are several ways you can modify your CAD model to reduce stress concentrations and avoid potential failure points:
Modifying Geometry
One of the most direct ways to optimize your design is by changing its geometry. You can modify feature sizes, add or remove material, and alter the overall shape of the part. Common geometry improvements include:
Increasing fillet radii to minimize stress concentrations
Adding ribs or gussets to strengthen highly stressed regions
Removing any unnecessary mass to reduce inertial loads
Smoothing sharp corners and edges prone to cracking
Optimizing wall thicknesses and part dimensions
Running additional simulations after geometry changes will show you their impact on stress distribution. Iterate on the modifications until the geometry optimization reduces stresses sufficiently.
Choosing Materials Strategically
The material choice for your part can also help minimize stresses. Consider stronger and more ductile metals, composite materials, or even anisotropic materials with directionally tailored properties. Stress analysis results can pinpoint where such optimized materials will help the most.
Switching to a higher strength alloy or composite layup in the most affected regions will lower stresses. You may also consider graded materials with varied properties through the part thickness. Analyze how material changes propagate through the overall stress state.
Iterating the Design
It often requires multiple design iterations to arrive at an optimal solution. After modifying geometry and materials, rerun the simulation to evaluate the changes. The new results will guide what further refinements are necessary.
With each design iteration, look to reduce the peak stress values and create a more uniform stress distribution. It's an iterative process guided by the physics of the simulation results. Often 4-5 design iterations are needed to minimize stresses for a robust final design.