SUMMARY
In this blog post the use of the LS-DYNA capability ‘Adaptive Solid to SPH’ as an object in Ansys Mechanical is presented. The use of SPH formulation combined with classic solid elements is explained and through a simple example, the setup and its main parameters are developed.
What is SPH?
Smoothed Particle Hydrodynamics (SPH) is a computational technique employed in structural analysis to simulate the behavior of complex structures subjected to dynamic loads, such as impacts or explosions. In SPH, the structure is represented by a collection of particles, each possessing attributes like position, velocity, and stress. The method evaluates structural properties at any point by averaging over neighboring particles within a defined smoothing length. This approach allows for the modeling of irregular geometries and dynamic interactions between structural components without the constraints of a fixed grid, making SPH particularly advantageous for simulating scenarios where traditional grid-based methods face challenges, such as large deformations or material failure.
Why connect particles to my solid elements?
Even if you love solid elements and continuum theory, there are some cases where limitations arise. In structural analysis involving large deformations, such as material failure or impact simulations, maintaining mesh quality and avoiding element distortion becomes challenging for FEM. Simulating crack propagation and fracture mechanics are additional challenges.
SPH, being a Lagrangian method, naturally handles large deformations by tracking the motion of individual particles, making it well-suited for simulating material failure and impact scenarios, due to its ability to handle material discontinuities and fragmentation. This makes it valuable for analyzing failure mechanisms in materials and structures, such as brittle fracture or fatigue failure, where crack propagation plays a significant role.
In failure analysis, is important to maintain the highest accuracy possible. When a classic Lagrangian element is too distorted it can be eroded from the analysis to simulate cracking. But this method will reduce the total mass, affecting accuracy. Replacing the element by mass equivalent particles will help maintaining accuracy.
EXAMPLE
A rigid sphere impacts a flexible steel disc fixed by its perimeter. Initial velocity is high enough to traverse the disk causing failure.
Adaptive Solid to SPH:
This object allows the user to create particles to posteriorly replace solid elements. It’s possible to use scope to create particles in intended bodies. The number of particles per element is important to keep similar accuracy respect to solid elements. Using the option ‘Coupled to Solid Element’ will create the transition from eroded solid to particle.
The SPH particles replacing the failed solid Lagrangian elements inherit all the Lagrange nodal quantities (like displacement, velocity and acceleration) and all the Lagrange integration point quantities (like stress and strain) of these failed solid elements. Those properties are assigned to the newly activated SPH particles. The newly created SPH part can have different material properties using the Material Assignment field.
Material setup:
The other half of the adaption from solid to particles is the eroding of the solid elements. It’s necessary to add erosion definition in Engineering data and use some criteria to eliminate the initial elements. In this example effective strain is used, but there are several additional options.
RESULTS
Once the simulation starts, is possible to find a message that indicates the erosion criteria has been reached.
That means the first solid element has been replaced by its equivalent particles. In the next timestep some elements have been eliminated and result values can be seen in the particles.
A displacement plot shows how the particles separated from the main body, their energy is considered in the whole analysis giving more accuracy to the solid model.
FINAL COMMENTS
When Material Assignment is set to Program Controlled, the SPH part will have the same material as the solid part. Is possible to use any material available in Engineering Data.
Shell and beam elements are not allowed for this object.
CONCLUSIONS
The "Adaptive solid to SPH" model in Mechanical LS-DYNA offers several advantages for structural analysis. Firstly, it enhances accuracy by dynamically converting solid elements to Smoothed Particle Hydrodynamics (SPH) elements as needed, ensuring precise simulations. Secondly, its versatility enables seamless simulation of complex structural behavior, making it applicable across a wide range of engineering scenarios. Additionally, it minimizes the need for manual intervention, streamlining the simulation process and saving time. Lastly, it provides a more realistic representation of material response under varying conditions, contributing to more reliable results in structural analysis simulations.