Making Quasi-Static Simulations Simple with Ansys LS-Dyna
Quasi-static simulation sounds technical, but the objective is simple: simulate slow, controlled loading conditions where dynamic effects like inertia and high-speed impacts aren’t the main concern. Think about things like roof crush tests, bolt pre-tensioning, or structural deflection under gradually applied loads.
This kind of analysis is crucial for evaluating the real-world behavior of mechanical systems under long-duration loading. The good news? You can run these simulations efficiently using LS-Dyna’s explicit solver, if you know the right tricks!
Here, we explore how to run quasi-static simulations using LS-Dyna explicitly, and why that approach makes a lot of sense in complex engineering workflows.
Understanding LS-Dyna’s Power
LS-Dyna is a full multiphysics simulation solution. Initially developed for short-duration dynamics using explicit time integration, it has grown to include implicit solvers, CFD, thermal, electromagnetic, NVH (noise, vibration and harshness), and more. Everything is wrapped into one cohesive solver environment, so coupling physics doesn’t require separate platforms or multiple inputs.
For quasi-static analysis, LS-Dyna’s explicit solver is surprisingly well-suited. That might sound counter-intuitive, since explicit solvers are often associated with crash or drop tests. But with a few smart adjustments, such as controlling kinetic energy and choosing suitable time steps, you can use LS-Dyna’s speed and stability to model slow deformation very effectively.
Time Integration: Implicit vs. Explicit
To simulate motion or deformation over time, engineers need to solve equations of motion. Implicit and explicit time integration schemes offer different approaches:
- Implicit methods solve future states (n+1) using iterative techniques. They’re unconditionally stable and support long time steps but can be computationally expensive.
- Explicit methods solve the current state (n) directly and don’t require iterations. They’re faster per step but limited by stability constraints, which means small time steps.
Explicit time integration becomes viable for quasi-static problems when the loads are applied very slowly. This minimizes inertia and kinetic energy so even though it’s a dynamic solver, it behaves like a static analysis.
How to Ensure Quasi-Static Conditions
Running a truly quasi-static simulation with LS-Dyna’s explicit solver requires careful setup:
- Slow load application: Apply displacements or velocities gradually. Avoid applying forces directly. Prescribed motion gives better control and minimizes noise.
- Minimize kinetic energy: The aim is for inertial effects to be negligible. You want displacement and stiffness terms to dominate over velocity and acceleration.
- Use load ramping: Smooth load curves help reduce transients and achieve a more stable solution.
A common method is to start with a fast simulation to debug the model and understand time scale. Then, gradually increase the analysis end time and observe if the results stabilize. If results no longer change with longer runtimes, the solution is likely in the quasi-static regime.
Comparing applied loads with reaction forces also helps confirm quasi-static behavior. If these values closely match, the simulation is not dominated by dynamic effects.
Using Mass Scaling Smartly
Explicit solvers are constrained by the smallest element in the mesh, therefore the smaller the elements, the shorter the time step. Mass scaling increases element mass artificially, reducing the speed of sound and allowing longer time steps.
Done carefully, mass scaling improves simulation performance without introducing significant error. As long as kinetic energy stays low and inertial effects remain negligible, mass scaling can be a powerful enabler for quasi-static analysis.
Preloading Structures: Why and How
Many real-world structures are preloaded (think bolts under tension or press-fitted joints). Simulating these accurately is important for realistic results.
There are two main methods to introduce preload:
- Direct loading in the main analysis: This adds preload as the first load step. It’s simple but introduces transients.
- Dynamic relaxation: This runs a short pre-analysis where loads are applied and the system is allowed to settle. Once stable, the main analysis begins with those preloads in place.
[NB Dynamic relaxation can be run with either explicit or implicit solvers and gives more controlled, cleaner initial conditions]
Sequential Loading with Restarts
Many engineering scenarios involve stepwise loading: preload, side load, unload, top load, etc. Rather than cram all these into one simulation, LS-Dyna allows restart analysis, where each load stage builds on the results of the previous.
Three restart types offer flexibility:
- Simple restart: Extend the time without model changes.
- Small restart: Modify existing boundary conditions or delete parts.
- Full restart: Full model modification; add parts, contacts, materials.
This modular approach is efficient and lets engineers iterate quickly, especially when load assumptions change.
Efficient Setup in Ansys Workbench
Quasi-static workflows are supported in Ansys Workbench via LS-Dyna integration. Here’s what makes setup smooth:
- Material definitions: Use engineering data or insert LS-Dyna keyword snippets for advanced materials.
- Contact setup: Automatic or manual contact detection, with support for soft constraints and segment-based contacts.
- Mesh control: Prime meshing with mesh connect ensures continuity across unshared surfaces.
- Element selection: Shells and solids can be scoped to use the most accurate element types, for example, fully integrated shells or nodal pressure tets for rubber.
- Bolt pretension: Add bolt loads directly to dynamic relaxation setup using coordinate systems.
Using mechanical’s Workbench interface, engineers can manage model definitions and run sequential restarts, all within a traceable and repeatable environment.
Evaluating Results: Quasi-Static or Not?
After running the simulation, you’ll want to check energy plots. If kinetic energy remains small compared to internal energy, your analysis is quasi-static.
Plotting reaction forces gives insight into noise and transient effects. A smooth force response and stable displacements over time further indicate success.
You can also experiment with end times. For example, comparing simulations with 0.1s, 0.5s, and 1s load durations will help identify when results stabilize which is an indicator of sufficient time for static behavior.
Why It Matters
Simulating slow-load scenarios with explicit solvers brings multiple benefits:
- Avoids convergence issues common in implicit methods
- Handles highly nonlinear materials and contacts better
- Offers flexibility in model setup and stepwise simulation
- Enables efficient debugging and iteration through restart workflows
This makes explicit quasi-static analysis especially valuable in scenarios like crash safety, structural collapse, bolt modeling, and rubber deformation.
With smart control of loading rates, mass scaling, and sequential restarts, LS-Dyna’s explicit solver becomes a robust tool for simulating quasi-static problems. You get the best of both worlds: stability, speed, and control, all within a cohesive, multiphysics-ready environment.
And with intuitive setup in Ansys Workbench, even complex simulations like bolt preload and ROPS (rollover protection system) testing become manageable.
Need help getting started or want to explore how this could work in your engineering process?
