When engineers perform structural finite element analysis (FEA), there are three types of nonlinearities they need to consider in their simulation. Material nonlinearities are needed to predict plastic strains in metal parts or extreme deformation of plastic or rubber materials. Contact nonlinearities are required to predict changes in status or sliding friction between assembly parts. The third type, geometric nonlinearity, is not always easy to identify, but it involves changes in stiffness due to changes in the shape of the parts.
For ANSYS Mechanical simulation users, it’s important to note that if you’re not sure whether you need large deflection or not, you should turn it on. Without performing a comparison study with and without it, there’s really no way to know for sure if it’s needed.
So, what are large deflection effects? In simple terms, the inclusion of large deflection means that ANSYS accounts for changes in stiffness due to changes in the shape of the parts you’re simulating. Let’s take the example of a fishing rod to better understand this concept.
When a fishing rod is not loaded, it’s very flexible at the tip. But with a heavy fish on the end of the line, the rod will deflect downward, and the stiffness of the rod will increase. This means that a small amount of force will cause a certain amount of downward deflection at the top when the rod is lightly loaded. However, when the rod is heavily loaded, a much larger amount of force will be needed to cause the same amount of deflection.
This change in the force amount required to achieve the same change in displacement shows that there is no linear relationship between force and displacement.
Let’s talk about Hooke’s law, also known as the spring equation. It’s a fancy way of saying that if you apply a force (let’s call it F) to something like a spring or a fishing rod, it will bend or deform (let’s call that x), and the amount it bends depends on how stiff the thing is (let’s call that K).
But here’s the thing: in a simple system, like a spring, if you double the force, you double the deformation. Easy peasy. But with something like a fishing rod, it’s not that simple. You might have to triple the force to get the same amount of deformation, depending on how the rod is built and how it’s loaded. And to double the deformation again, you might have to use four times the force.
So, when we’re dealing with a fishing rod, we can’t just use Hooke’s law in its simple form. We need to take into account the fact that the stiffness of the rod changes as it bends. This means we have to recalculate the stiffness as the rod bends, which we can do using a computer program called ANSYS.
To make ANSYS recalculate the stiffness as the rod bends, we have to turn on something called “large deflection effects.” Without this turned on, ANSYS will just use the original stiffness no matter how much the rod bends, which won’t give us an accurate result.
But why not just have “large deflection effects” turned on all the time? Well, it turns out that this adds extra work for the computer program, so it’s turned off by default. It’s like how your phone might have a bunch of fancy features that are turned off by default to save battery life. When we turn on “large deflection effects,” ANSYS has to solve the problem in a different way, which takes more time and computer power.
Let’s talk about fishing rods. Imagine a fishing rod that has been simplified to make it easier to understand. We can see two images: one of the rod before any force is applied, and one of the rod after a downward force is applied to the right end. The second image shows that the rod has bent, with the tip of the rod now 34 inches from its original position.
Now, let’s compare that to a situation where we turn off the effects of large deflection. When we apply the same force, the tip of the rod moves 40 inches, which is 15% more than when large deflection is turned on. This happens because the rod’s stiffness changes as it bends, and accounting for this change in stiffness gives us a more accurate result.
We can visualize this by looking at a graph that shows the relationship between the force applied to the rod and the deflection of the rod. This graph is not a straight line, which means that the stiffness of the rod changes as it bends. The more force we apply, the less the rod moves for each additional unit of force. This matches what we observe in real life when we use fishing rods.
So, the question is, do we always need to account for large deflection effects? The answer is no. Sometimes it’s not necessary, but when in doubt, it’s best to run simulations with and without large deflection effects to compare the results.
For example, when we look at an idealized compressor vane, we see that the deflections and stresses with and without large deflection effects are nearly the same.
Large Deflection On:
Small Deflection:
It’s important to note that turning on large deflection effects in ANSYS actually activates four different behaviors: large rotation, large strain, stress stiffening, and spin softening. All of these behaviors involve changes in stiffness due to deformation. So, remember to try it out, with and without large deflection effects, to get the most accurate results. In ANSYS Mechanical, you can turn large deflection effects on or off in the Analysis Settings branch.
Good Luck!
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