My LS-DYNA Drop and Impact Journey — Course Notes and Reflections
Over the last few weeks, I completed the “Getting Started with LS-DYNA: Drop and Impact Analysis” course — an incredibly hands-on experience that took me from basic setup to interpreting high-energy impact results. Here are my consolidated notes and reflections that double as a quick-reference guide for anyone beginning with LS-DYNA.
Understanding LS-DYNA for Drop and Impact
LS-DYNA is a powerful explicit finite element solver. In drop and impact simulations, time steps are tiny, deformations are large, and contacts are frequent — making this solver ideal. The course focused on how to model, simulate, and interpret such fast events efficiently.
Key Topics I Learned
Non-Linear Material Behavior
Real-world materials don’t always behave linearly under impact. LS-DYNA captures plasticity, strain-rate effects, and failure. For example, simulating an aluminum casing hitting the ground requires defining plastic properties so that it deforms realistically instead of behaving like a perfectly elastic spring.
Rigid Body Constraints
Some parts, such as floors or supports, do not deform significantly during impact. By treating them as rigid, computational time is saved. For example, a rigid floor can act as an immovable object when a component is dropped, allowing focus on the deforming body.
Contact and Contact Force
Contacts are the heart of impact analysis. LS-DYNA automatically detects when two bodies touch or separate and applies realistic forces to prevent penetration. During a phone-drop simulation, for instance, the contact definition helps determine how much force the screen experiences and how it bounces back.
Hourglass Energy
Hourglass modes are non-physical deformations that occur in under-integrated elements. Keeping hourglass energy below 10% of the internal energy ensures simulation stability. It was interesting to observe that when hourglass control was not activated, elements tended to distort unrealistically during impact.
Mass Scaling and CFL Time Step
The CFL condition (Courant–Friedrichs–Lewy) defines how small the time step must be for numerical stability. In explicit analysis, smaller elements lead to smaller time steps. To speed up computation, LS-DYNA allows minor mass scaling, which artificially increases mass slightly so that the simulation runs faster. The key is to ensure that the added mass remains below 5% so that results stay accurate.
Energy Balance
During an impact, kinetic energy is converted into internal energy, contact energy, and sometimes small amounts of hourglass energy. Monitoring this energy balance ensures that no non-physical losses or gains occur. A stable simulation shows total energy remaining nearly constant after the impact settles.
Normal Termination and Tracking
A “normal termination” message at the end of a run indicates a successful and numerically stable analysis. The tracker and global statistics files help monitor progress and energy trends during simulation, confirming that the setup is correct.
Post-Processing Insights
Using LS-PrePost, I explored the deformation shape, contact stresses, and rebound behavior after the impact. Viewing energy graphs and contact forces revealed how material stiffness and damping affected the overall impact response.
Mini Example: Drop of a Steel Ball on a Rigid Plate
In the example problem, a steel ball was dropped from a height onto a rigid plate. The results showed clear conversion of kinetic energy to internal energy upon impact, with rebound behavior dependent on the material’s stiffness. The contact force plot displayed a sharp peak at the moment of impact, gradually reducing as the ball settled.
Key Takeaways
- LS-DYNA is not just about running simulations; it’s about interpreting energy, forces, and stability.
- Keep added mass below 5% and hourglass energy below 10%.
- Always validate with physical intuition — compare rebound height, contact duration, and deformation with expectations.
Final Reflection
This course made me appreciate how explicit dynamics blends mathematics, physics, and engineering into one framework. The exercises gave me real intuition for time stepping, stiffness, and contact mechanics — concepts that extend far beyond LS-DYNA.
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