Simulating Drum Collapse with Explicit Dynamics in Autodesk Inventor Nastran
Engineering is not always about serious stuff.
Recently, my client Vacvator, was having fun imploding oil drums with their powerful vacuum excavator. So, I decided to have some fun too by simulating it in the Autodesk Inventor Nastran.
I have had a strong interest in the simulation of the rapid deformation processes and structural instability since university. So, when Matthew Cudaj, in our conversation about buckling, mentioned that they have a good example, I could not resist the temptation.
Background
At the beginning of my career, 20 years ago, making simulations like this was a very serious challenge because Explicit Dynamics requires enormous computing power and FEA products with such capabilities were very expensive.
Today it has become available to everyone. First, Autodesk Inventor Nastran with Explicit Dynamics capabilities comes as a part of the Product Design Collection. Second, a modern workstation can crack this analysis with ease. For instance, it took less than 2 days to solve it on my laptop with a 24-core i9-13950HX.
Step 1: Linear Buckling – The Starting Point
When it comes to the loss of stability, there are two major methods to find out what is going to happen. Linear Buckling and Explicit Dynamics.
Linear Buckling helps to predict the critical load and shape of deformation. This method works on the assumption that the geometry of the model remains unchanged. In this example, it cannot consider the second-order effect related to top/bottom sag under pressure. Like all linear analysis types, this method is lightning fast!
So, it quickly helped me to understand what to expect from this model.
To my own surprise, I had to discard all analysis results because it found lots of modes under positive pressure around the lids, irrelevant to the goal of my analysis.
For the next run, I tweaked the analysis settings to give me only positive load multipliers.
From the 2nd run results, I had to brush off the first two modes as irrelevant and peek 3rd as something that I expected.
It gave me a prediction for critical pressure=-8.2217932 * 0.01=82.217932 kPa (-0.8113 atm).
This result clearly shows three waves forcing the drum to collapse like a triangle.
Step 2: Why Not Nonlinear Buckling?
For a structure like this, the linear buckling is a good starting point. However, it cannot tell us what is going to happen next and is a linear model good enough to predict buckling?
Perhaps, there are second-order effects that can influence the stability of the drum walls. In this case, the classical answer would be – Non-Linear Buckling. In this analysis, we can calculate the prestressed model using more accurate Static Non-Linear analysis and then make a prediction for the buckling load.
Speaking honestly, I did not even want to try. These days, static non-linear solver works faster on new processors, but do not utilise all available power, because they perform many single-threaded operations. But the most significant turn-off for me – it often turns into a convergence nightmare!
Step 3: Running the Explicit Dynamics Simulation
Therefore, my choice immediately turns to Explicit Dynamics. Yes, this analysis type is a computational nightmare – a good way to overload any CPU. But, like I said earlier, the models became solvable on 16 or more core processors.
My usual strategy to solve non-linear analysis starts with a draft model. Something light-weight, but close to what I need. Its purpose is to test the ground without spending much time on the analysis.
So here are my tips:
- 1
Keep the mesh node count as low as possible
- 2
If you are expecting dramatic changes in the model, like in this example, use multi-stage loading. For example, I bring the model quickly (20% time) up to 80% of the critical load (calculated by Linear Buckling analysis) and then slowly increase the load to 125% of the critical load.
- 3
Monitor results from the beginning of the analysis. In Autodesk Inventor Nastran, you can check intermediate results on the fly by right-clicking on the results node in the model browser and choosing “Load”.
- 4
Run the Explicit solver as an independent process via Command Line. Here is my example: “C:\Program Files\Autodesk\Inventor Nastran 2026\Explicit\aexp_app.exe” -i “” -f 1 -license_type 2 -timeLimit 691200 -outfile
- 5
Set the Explicit Solver process priority to Below Normal or Idle. This will allow you to minimise the effects of the processor overload, so you can use your PC as usual. It’s easy to do through the task manager. My personal choice is Process Lasso Pro, which sets it automatically on the process startup.
The first run revealed that the solution diverged due to highly distorted elements. Therefore, I set Element Deletion Criteria = NEGVOLUME
In the end, it required 2 more iterations to get perfect results.
Step 4: Comparing Simulation vs Reality
At the final stage my model became more deformed than the real drum on the video, but it could be explained by modelling inaccuracies:
- we did not use precise measurements to make an accurate replica of the drum (the top-side connection ring should be harder in reality);
- constraints applied to the drum mouth – different to the wide suction hose of the Vacuum Excavator.
Anyway, let’s come to a conclusion.
The first chart, Deformation vs Load, makes it clear that we are observing collapse. Sharp increase in deformation, compared to a very small increase in pressure.
The most interesting fact here is that the Lenear Buckling prediction was quite accurate.
Linear Buckling Critical load prediction was 82.217932 kPa.
Data behind this plot demonstrates the beginning of collapse at 83.09 kPa, which gives 1.01% difference.
However, it still takes some time due to the inertial forces. As you can see from the deformation vs time plot, collapse occurs in less than 15 milliseconds. The video of the real drum collapse only confirms that it happens in less than 33 milliseconds between the video frames.
Step 5: Key Takeaways
- Linear Buckling gives a fast and accurate estimate of critical load.
- Nonlinear or Explicit Dynamics analysis reveals what happens next by capturing large deformations, plasticity, and element failure.
- With modern hardware, Explicit Dynamics is no longer out of reach for everyday engineers.
Conclusion
Modern engineering technologies like Autodesk Inventor demonstrate the power of modern FEA tools. What was once a high-end research problem is now something that any engineer can do from their desk. And that’s exactly what makes engineering so exciting today.
