Hello! Welcome back to the course.
In our last lesson, you successfully set up your first structural analysis in Onshape, defining the necessary constraints, materials, and loads. You ended the lesson by running the simulation and seeing a colorful plot appear on your model.
Today, we'll focus on the most critical part of any analysis: making sense of the output. The learning outcome for this lesson is to interpret FEA results to evaluate stress, displacement, and factor of safety in a component. Your engineering background gives you a head start on the theory; our goal today is to translate that theory into a practical, decision-making workflow within the Onshape environment. By the end of this lesson, you'll be able to look at a simulation result and answer the fundamental engineering question: "Is this design strong enough?"
1. The Sanity Check: Interpreting Displacement
Before you even look at stress values, the first step in interpreting any FEA result is to check the displacement plot. This is your "sanity check" to ensure the simulation behaves as you'd expect.
- Does the part deform in the direction you applied the load?
- Are the constrained areas remaining fixed?
- Does the overall deformed shape make physical sense?
Answering these questions first can save you from drawing incorrect conclusions based on a faulty setup. Onshape makes this easy to visualize.
Onshape Simulation Overview and Demo
Let's watch a short clip from the "Onshape Simulation Overview and Demo" that shows how to visualize and animate displacement.
Watch from 16:50 to 17:25. Notice how the presenter first checks the animated displacement to confirm the load is acting in the intended direction. The ability to animate and exaggerate the deformation (using the deformation scale) is a powerful tool for understanding the model's behavior.
Now, let's look at the official definition of the displacement result in the Onshape documentation.
The Onshape Help page for "Simulation" provides precise definitions for each result type. We'll start with Displacement.
From the resource, navigate to the section titled "Result options." Find the bullet point for Displacement and read its description. Also, review the section just below, "Additional color scale options," to understand how to use the Show deformation checkbox and the Deformation scale slider.
The key takeaway is that the displacement plot tells you how much and in what way your part moves. You can hover your cursor over the model to see specific displacement values at any point. This is often checked against a design requirement—for example, a shaft's deflection might need to be less than 0.1 mm to ensure proper gear mesh.
2. Evaluating Strength: von Mises Stress
Once you've confirmed the model is behaving correctly, it's time to evaluate its strength. For ductile materials like steel and aluminum, the primary metric we use is von Mises stress.
As you'll recall from your engineering studies, von Mises stress is a scalar value that combines the complex, multi-directional stress state at a point into a single number. This number can be directly compared to the material's tensile yield strength to predict whether the material will start to permanently deform (yield).
Let's get Onshape's definition of von Mises stress and see how the results are presented.
Returning to the Onshape "Simulation" help page, let's read the definition for von Mises stress.
In the "Result options" section, read the description for von Mises stress. Pay close attention to how Onshape sets the default maximum value on the color bar. It uses a statistical approach to ignore the absolute highest value, which is often near a computational singularity.
This statistical approach is an important practical detail. Onshape tries to help you focus on meaningful stress values rather than misleading artifacts of the model. However, you still need to be able to identify these artifacts yourself.
3. The Big "Gotcha": Stress Singularities vs. Concentrations
A common pitfall in FEA is misinterpreting extremely high-stress values that appear at sharp inside corners, point loads, or fixed constraints. These are often stress singularities—mathematical artifacts of the model, not real stresses—where the stress value will continue to increase infinitely as you refine the mesh.
A stress concentration, on the other hand, is a real physical phenomenon where stress increases around a geometric feature, like a hole or a fillet. These stress values will converge to a finite number with mesh refinement.
Distinguishing between the two is a critical skill for an analysis engineer.
This excellent video, "Stress Singularity in FEA!" from the channel Enterfea, explains the difference between these two concepts and why you shouldn't always trust the highest stress value in your plot.
Please watch the following segments: 00:00 - 02:20: Introduction to the problem and how modeling simplifications (like sharp corners) can lead to singularities. 11:47 - 13:22: A clear definition of the difference between a stress singularity (a modeling issue) and a stress concentration (a real phenomenon). 15:16 - 18:20: Explanation of why real-world parts don't always fail despite high-stress concentrations (the concept of yielding and stress redistribution). Focus on understanding that singularities are artifacts you might need to ignore, while concentrations are real effects you must design for.
In a linear static analysis (which is what Onshape runs), if the calculated von Mises stress in a region exceeds the material's yield strength, it simply means the linear model is no longer valid in that area. In reality, the material would yield, deform, and redistribute the load to adjacent areas. A small, localized area of yielding is often acceptable in static applications with ductile materials. A large zone of yielding, however, could lead to failure.
4. The Decision-Making Tool: Factor of Safety
While stress plots are informative, the Factor of Safety (FoS) plot is often the most direct tool for making a design decision. It's calculated as:
An FoS greater than 1 means the stress is below the yield strength, and the part is not expected to permanently deform. An FoS less than 1 indicates that yielding is predicted to occur.
Your target FoS is a critical design requirement. For static industrial machinery, an FoS of 2 to 4 might be typical to account for uncertainties in loads, material properties, and manufacturing. For a weight-critical aerospace part, the target might be as low as 1.25.
Let's read how Onshape defines and presents the Factor of Safety plot.
In the "Result options" section of the help page, read the description for Safety factor. Note that the color scale is inverted: high safety factors (good) are on the left, and low values (bad) are on the right, with a standard target range highlighted.
The FoS plot instantly shows you which parts of your design are over-stressed (typically shown in red), under-stressed (deep blue), or within the target range.
Test your understanding!
You run an analysis on a steel bracket (Yield Strength = 250 MPa). In a small area around a sharp, un-filleted inside corner, the von Mises stress reads 800 MPa, giving a Factor of Safety of 0.31. What is the most appropriate next step?
- Immediately conclude the design is a failure and needs to be completely redesigned.
- Change the material to a much stronger steel alloy.
- Recognize this is likely a stress singularity. Add a realistic fillet to the corner and re-run the analysis to get a more accurate stress value.
Show answer
The most appropriate next step is 3. The combination of a sharp inside corner and an extremely high stress value that is multiples of the yield strength is a classic sign of a stress singularity. Before making any major design changes, you should first refine the model to be more realistic (by adding a fillet) to determine the true stress concentration.
5. A Systematic Interpretation Workflow
To bring this all together, here is a systematic workflow for interpreting your FEA results:
- Check Displacement: Animate the deformation. Does it move as expected? Is the magnitude of movement acceptable for your design requirements? This is your sanity check.
- Examine von Mises Stress: Look at the stress plot. Where are the "hot spots" (high-stress areas)? Are these located where you would expect (e.g., at bends, holes, or areas with smaller cross-sections)?
- Qualify High Stress: Critically evaluate the hot spots. Is the peak stress located at a sharp corner or point constraint? If so, it might be a singularity. Consider if the result is trustworthy or if the model needs refinement (like adding a fillet).
- Evaluate Factor of Safety: Switch to the FoS plot. Does the entire part meet your minimum required FoS? The regions with the lowest FoS are your primary candidates for design improvement.
This structured approach ensures you don't just look at the prettiest picture, but perform a proper engineering evaluation, which is exactly what's needed for your portfolio.
Conclusion
Congratulations! You are now equipped to move beyond just setting up an FEA to actually interpreting the results like a design engineer. You can now use simulation not just to get a number, but to gain insight into how your design is performing.
Key Takeaways:
- Displacement is the first result to check for a "sanity check" of your model's behavior.
- von Mises Stress is the primary metric for predicting yielding in ductile materials and should be compared against the material's yield strength.
- Be critical of results! Distinguish between real stress concentrations (which need to be addressed) and artificial stress singularities (which are modeling artifacts).
- Factor of Safety is your ultimate decision-making tool, providing a direct comparison of your part's performance against its design requirements.
Preview of the Next Lesson:
Identifying a problem is only the first step. In our next lesson, "Refine the Project 1 design based on structural analysis results to meet performance criteria," we will act on the insights you've gained today. You'll learn how to modify your design to strengthen weak areas and improve its performance, completing the iterative loop of the engineering design process.