Deformable particles

By default a Rockable particle is rigid: it has six degrees of freedom, three of translation and three of rotation. The soft particles extension adds a homogeneous deformation to each body, so that a particle can change shape under load without being discretised into a mesh.

The intent, stated in the design of the feature, is not to resolve a full kinematic field with many degrees of freedom, but to make a few of them possible: enough to model soft grains, polymers or biological tissues, while keeping the cost of a discrete element method.

Important

This feature requires

cmake .. -DROCKABLE_ENABLE_SOFT_PARTICLES=ON

Without it, the keyword is accepted but only prints SOFT_PARTICLES_NOT_ENABLED when Rockable was compiled.

Activating the feature

• useSoftParticles (double) Young (double) Poisson

  Activates the homogeneous straining of the particles and builds the compliance from the Young modulus \(E\) and the Poisson ratio \(\nu\) of the material the bodies are made of.

input.txt

useSoftParticles 1e6 0.0

Both parameters are global: all the bodies share the same compliance, whatever their group. A Poisson ratio of zero, as above, means the body shortens along the loading direction without bulging sideways, which is the simplest case to interpret.

Note

The compliance relates the average stress carried by a body to its uniform strain. The stiffness of the contacts is a different matter, set by knContact and ktContact as usual: a soft particle in a stiff contact and a stiff particle in a soft contact are two different models, and both are available.

Current status

Warning

The extension is under active development, in the postdoctoral work of Mukesh Singh Bisht, and it is not yet complete. In particular, the uniform transformation of the particles is not written to the conf-files: the code that saved it is commented out, and the corresponding field is not read back either.

The practical consequence is that a computation using soft particles cannot be restarted exactly from a dump. The positions, velocities and orientations are restored, but the accumulated deformation of each body is lost, and the bodies restart undeformed. Run such a simulation in one go, or check the state of the feature in the source before relying on a restart.

Tip

rockable -b prints ROCKABLE_ENABLE_SOFT_PARTICLES among the compilation options, which is the quickest way to tell whether the binary at hand supports the feature.