SolidWorks Simulation: What is von Mises Stress? - Part 1 of 2


Many companies are already reaping the benefits of working with the extensive simulation packages available with SolidWorks. Others are just now beginning to realize the use of these tools. A proper finite element analysis of a part can drastically cut down on prototyping time, and provide design validation or justification for changes. This is because simulation within SolidWorks allows us to actually add the forces that your components would be subject to, and see how it will respond. We can run this in parallel with our design process to create better, safer parts. The body and title are included in case you want to use this page as a landing page.

Today I want to talk about results of a SolidWorks Simulation study, and one of the criteria we often use to verify part design- stress. If we want to make sure a part doesn't break under a given loading, we'd want to make sure that the maximum stress in the part is below the yield strength of the material. But what does that even mean? How does SolidWorks measure the stress in the part, and how can we say by a simple comparison of stress values that the part isn't failing? That is exactly what I'm going to attempt to explain in fairly simple terms.

With a B.S. in mechanical engineering from Lehigh University, I have a fairly strong background in strength of materials. It's something that really interests me. I've taken classes in college covering material including concentration factors, transformation of stress tensors, and strain energy deflection calculations. That being said, if you have no interest in that material there's absolutely no reason to learn it to that level. Nor is that knowledge necessary to understand and properly use the tools available in SolidWorks Simulation. However, a basic level of understanding of what your stress plots are showing will go a long way in interpreting your results. That's my goal here today - to explain something called von Mises stress to you in a top-level way, avoiding as much of the dense mathematics that I can. This will allow you to really dig into your simulation results, and give you confidence that they do in fact validate your design.

What is Stress?

First, let's start with what will likely be review. Stress is simply a measurement of the internal forces in a body, as a result of the externally applied loads. The units of stress are the same as those of pressure; force per unit area (psi, since you're likely working in English units). As a result, we can very roughly calculate a stress value just that way; divide the total force load by the cross sectional area of the part at a location to get the stress spot.

Let's make things a little more complicated...

Force is a vector - it has both magnitude and direction. As a result, stress actually has direction as well. So what happens when you have complex (multi-directional) loading? Your stress inside your part will have components in each of these different directions. Let me give you an example. Assume a part centered at the origin. If you pull on it only in the direction of the X-axis, it will only have stress values normal to that direction. However, if you also pull on it in the Y-direction, it has stress normal to X from the original force in the X-direction, but also stress normal to Y from the Y-direction force. In addition, an angled force load with respect to geometry will causes the material try to slide past, and shear the part. This contributes to what are called shear stresses.

As a result of this, looking at a 3-D part we can actually have up to 3 different stress normal directions, as well as up to 3 different shear directions (X on Y, Y on Z, and X on Z). This means we can in theory have 6 different stress values. That's a lot to try to calculate or interpret!

Royblog 1.2.13

The picture above shows this scenario. There are normal or "principal" stresses (denoted by sigma, σ) with respect to each of the 3 axes, as well as three shear stresses (denoted by tau, τ), which are located on the different faces. Please note opposite shears are the same, for example X on Y, and Y on X. This is how we arrive at only three, rather than six shear stresses.

Each of these stresses can be solved for based on the loading conditions, by plugging into strength of materials equations. This is where the heavy math comes in, so especially for the purpose of this article let's leave that to SolidWorks. It's truly incredible how SolidWorks runs through thousands of iterations of those calculations, and all in just a few minutes or even seconds. It certainly is a powerful too.

But let's get back to the situation at hand. I've run the calculations and I've got my six numbers. So now what? Is my design validated, or is the part breaking?

We know the strength of a given material in terms of stress. In many cases, it's listed as a single value: yield strength. For example, our material properties list the yield strength of AISI 1020 steel as 51,000 psi. This means that at that stress value, our material will begin to yield, which is the first step in part failure. This is clearly the number we want to compare to. However, we cannot simply compare each normal direction or each shear to the yield value and know for certain that the part won't fail. This is where our problem arises: how do we compare stress values in scenarios of complex loading? We'll discuss the solution to that, and what exactly a resolved stress value like von Mises actually is.

If you have any questions or need anything clarified please leave a comment.

* To read part 2 of this series please click here.

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3 Comments:

Thank you for such a great and well redacted post/article. It is simple, clear and straight to the point.
March 21, 2013 09:03
Steven said...
Hello there, can you suggest to me where to start when dealing with the Finite Element Analysis. My Final Project is fully mechanical design which fully dealing with the Mechanic of material. I have done with the geometry and some of manual calculation. And after that I have to run the finite element analysis on my design which I just do the finite element analysis without understanding what I am doing and even when people asking me what is von mises stress I have no idea to answer. I hope you can give me any suggestion where to start i mean maybe from the tutorial or the book or any website that i can use for the finite element analysis. Thank YOu
October 30, 2013 00:10
Roy M. said...
Hi Steven, There are a few resources to help you get started with SolidWorks Simulation. First off, there's a nice student guide that SolidWorks has published, which walks you through some examples and discusses briefly what each step is doing. This is certainly a good starting point, and available online. Please see the link below: http://www.solidworks.com/sw/docs/simulation_student_wb_2011_eng.pdf In addition, there are indeed tutorials for SolidWorks simulation as well. These range from your basic linear-static analysis through examples of fatigue, and non-linear studies. They can be found from the Help menu, SolidWorks tutorials. I hope this helps!
October 30, 2013 08:10
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