What happens when a structure is hit by a sudden shock? This is not just a theoretical question. It’s a real challenge faced in industries ranging from defense to aerospace to heavy machinery. To explore this, we created a simple but representative test case in Simcenter 3D (Siemens): a cantilever beam with a heavy mass attached to its free end, fixed at the other end, and exposed to a shock at its base.

The purpose of the study was to understand how the duration of a shock pulse affects the way the structure responds. While the peak acceleration of the shock was kept constant at ten times gravity (10 G), the time span of that pulse was varied. This allowed us to see clearly how different “styles” of impact — fast, sharp hits versus slower, broader pushes — lead to very different demands on the structure.

Building the Model

The model itself could hardly be simpler. A fixed beam, free on one side, with a 90-kilogram mass at the tip. This setup is a stand-in for countless real applications: a piece of equipment mounted on a support, a component subjected to transportation shocks, or even a structure reacting to a sudden blast.

Figure 1: Tested geometry, a beam and a concentrated mass at the free end.

Before applying any shocks, we began with a modal analysis. This is the natural first step, because it tells us the frequencies at which the beam “likes” to vibrate. Just as a guitar string has its preferred notes, a structure has its preferred frequencies. For our beam, the first bending mode was found at 17.7 Hz, which corresponds to a vibration period of roughly 56 ms. This number already hinted at where we might see the most dramatic responses when we applied shock loads.

Figure 2: First bending mode around z-axis at 17.73Hz.

With that in hand, we moved on to transient response simulations. In this stage, we applied half-sine shaped acceleration pulses at the base of the beam. Each pulse had the same peak acceleration of 10 G, but we changed the duration: first just a single millisecond (1 ms), then 5, 10, 20, 40, 50, 70, 90, and finally 120 ms. For each case we tracked the reaction moment at the fixed support — in other words, the internal response the structure must resist.

What the Analysis Revealed

The results painted a very clear picture. For the shortest pulses, only one or a few milliseconds long, the structure barely had time to react. The acceleration may have looked impressive on paper, but the pulse ended before the beam could bend significantly. The reaction moments stayed relatively low.

Figure 4: Response of the bearing reaction force at the fixed end of the beam around z-axis.

As we increased the duration to around 20 ms, the beam began to “feel” the shock more strongly. Now there was enough time for the pulse to interact with the beam’s natural flexibility. The reaction moments climbed higher.

The most interesting behavior appeared around forty to 50 ms. Here the pulse length was similar to the natural vibration period of the beam, and the structure responded vigorously. This is where the reaction moment reached its maximum plateau. The pulse was effectively tuned to the beam’s natural rhythm, and as a result the structure absorbed more of the shock.

Beyond that point, something perhaps counter-intuitive happened. As the pulse lengthened further — to 70, 90, or even 120 ms — the reaction moments actually began to drop. Instead of amplifying the load, the longer pulses acted more like a slow push. The frequency content of the load shifted downward, away from the structure’s resonance, and the response became more quasi-static. The structure simply followed the imposed motion without being shaken to its core.

Why These Results Make Sense

Although at first it may seem odd that longer shocks lead to smaller peak reactions, the explanation is straightforward when you look at it from a structural dynamics perspective. Structures respond most strongly when they are excited close to their natural frequencies. Very fast shocks contain mostly high-frequency content, which the beam cannot follow. Very slow shocks contain mostly low-frequency content, which the beam can follow quasi-statically. Only when the shock’s timescale aligns with the structure’s own vibration period does the maximum amplification occur.

This is why performing a modal analysis before running transient simulations is so important. Knowing the natural frequencies of your system tells you immediately which kinds of shock loads are likely to be the most damaging.

Lessons Learned

Although at first it may seem odd that longer shocks lead to smaller peak reactions, the explanation is straightforward when you look at it from a structural dynamics perspective. Structures respond most strongly when they are excited close to their natural frequencies. Very fast shocks contain mostly high-frequency content, which the beam cannot follow. Very slow shocks contain mostly low-frequency content, which the beam can follow quasi-statically. Only when the shock’s timescale aligns with the structure’s own vibration period does the maximum amplification occur.

This is why performing a modal analysis before running transient simulations is so important. Knowing the natural frequencies of your system tells you immediately which kinds of shock loads are likely to be the most damaging.

Final Thoughts

By combining modal analysis with transient dynamic simulations, we created a workflow that allows engineers to see not just how a structure reacts to shocks, but why it reacts that way. The insights are directly useful: they help in designing more robust supports, in planning qualification tests, and in ensuring that equipment can survive real-world impacts.

When it comes to shock loads, it is not enough to know how hard the hit is. You also need to know how long it lasts, and whether it strikes the structure in its most vulnerable rhythm.

We at FEAC hope that this blog post has been interesting and that you will be able to create a nice FE Analysis after reading this. If you have any questions, you are always welcome to contact us at support@feacomp.com.

Ioannis Georgiadis – CAE Engineer | FEA Specialist | Mechanical & Aeronautics Engineer M.Sc e-mail: g.georgiadis@feacomp.com LinkedIn Profile: Ioannis Georgiadis