C-DEMPack2 manual

(Difference between revisions)

Model Section

General Application Data

Domain Options

This section allows the user to choose between a 2D or 3D simulation.

DEM

The DEM section is very similar to that of the non-cohesive DEMPack. Please, refer to G-DEMPack2 manual for a full description of the menus in this section. In this manual only the particular characteristics of this problem type will be explained. For a more practical review, the user is encouraged to check the C-DEMPack Tutorials included in DEMPack Tutorials page.

Cohesive Groups

This subsection allows the user to integrate one or more GiD groups into a single cohesive group with a particular collective behavior. Any GiD group with no cohesive group assigned will be treated as a discontinuum DEM mass.

• Tangency tolerance: Choose if you want to set manually the maximum gap between particles at the beginning of the computation (Absolute) or if you prefer an automatic gap that tries to match a certain average Coordination Number.
• Tangency tolerance value: Choose the maximum admissible gap between two particles to consider them cohesive at the beginning of the computation.
• Accepted gap between neighbours: If the gap between two spheres is bigger than this value, they are no longer neighbours and the bond between them breaks.
• Maximum allowed amplification ratio of the search radius: The search radius must be amplified after the first search because particles separate, but a bond still joins them. In order to keep finding all neighbours, search is amplified according the the mechanical resistance, but up to this limit.
• Virtual mass coefficient: For no mass reduction, set this value to 1.
• Calculate rotations: If activated, torques and rotations will be calculated on the Discrete Elements. Will increase computation time slightly.
• Rolling friction: If activated, rolling friction between particles will be considered.
• Compute stress tensor: If activated, the Stress Tensor will be computed. It will increase computation time and memory consumption.
• Add Poisson Effect: If active, the Poisson effect will be taken into account.
• Add the effect of shear strains parallel to bonds: When active, the shear strains parallel to bonds will be included in the computations.
• Don't search until failure: If activated, search for new particle neighbours will be disabled until the simulation detects the first contact failure.
• Material Analysis: Activate this option to show the material test (UCS, BTS) additional options.
• Thermal Analysis: Changes the default DEM element to Thermal-type elements and shows additional options.

Material Test

When setting the Material Analysis option to Yes, this menu will show up. It allows the user the access to a Material Test Virtual Lab where several kinds of tests are possible, they are: UCS, Triaxial, Oedometric, Hydrostatic and BTS.

• Test type: The user must select here any of the five different test previouly listed.
• Load velocity: The velocity that will be imposed to the presses that will compress the tested specimen.
• Confinement pressure: This field will show up only when selecting the Triaxial or Hydrostatic tests.
• Mesh type: The user can choose to use the current mesh in the problem or a predefined mesh that will be loaded from a file.
• Mesh path: Path to the file when using a Predefined Mesh.
• Specimen length: Length of the specimen given in meters.
• Specimen diameter: Diameter of the specimen in meters.
• Top measuring group: The top press group must be assigned here. The user has also the possibility of printing the forces and/or torques acting in this group.
• Bottom measuring group: The bottom press group must be assigned here. As before, forces and/or torques can be printed for the group.

Thermal results

When setting the Thermal Analysis option to Yes, this menu will show up. It allows the user to set values of the thermal parameters. This section is currently under development.

Results

This problem type has some additional results available for printing, they are:

• Skin Spheres: the contour spheres will be mark with a different color with respect to the rest.
• Poisson Ratio: the local Poisson Ratio will be printed in a sphere-to-sphere-bond fashion.
• Print Virtual Sea Surface: This option can be used to print a virtual sea surface as a mesh in PostProcess. When activated, four pairs of coordinates must be entered which will define the quadrilateral surface representing the virtual sea surface. A z=0 sea level is assumed. This option is related to the simulation of ice in the problem type and is still under development.

When activating the Thermal Analysis, additional thermal post-processing information will be available. Nevertheless, this section is still under development.

• Show bond elements: When activating this option, the bonds connecting the center of every pair of spheres will be shown in the postprocess. The user has then the option of printing the following variables:
• Contact sigma: Normal stress between every pair of spheres.
• Contact tau: Tangential stress between spheres.
• Local contact force: Local force in the contact point of any two spheres.
• Failure criterion state: Shows the current failure state or how close is the considered contact to reach complete failure.
• Failure Id: Labels every contact with its corresponding Failure Type.
• Mean contact area: Displays the calculation area in every contact.

Materials Section

This section is very similar to that of the discontinuous DEM, but with additional fields accounting for several constitutive laws available trying to simulate the behaviour of different cohesive materials, as well as the thermal parameters. Two main classes of constitutive laws exist: the DEMPack and the KDEM. The former is addressed to study materials following a non-linear elastoplastic behavior including damage, while the latter is specially designed to simulate elastobrittle materials. The KDEM constitutive law has several derived laws, which are KDEM_Rankine, KDEM_Mohr_Coulomb and KDEM_Fabric. The first one, the base KDEM, is a standard linear-brittle behaviour constitutive law, the second and third simulate the same behavior of the first but by following the particularities of Rankine and Mohr-Coulomb criteria respectivelly, while the last is addressed to study specific materials like membranes or clothes, as well as cables. Both KDEM and DEMPack laws have the following fields in common:

• Rolling Friction: value for the rolling friction. It may increase computation time slightly.
• Tangential Strength: Maximum shear strength for the material.
• Normal Tensile Strength: Maximum tensile strength of the material.
• Internal Friction Angle Coefficient: value of the static friction coefficient.

When activating the KDEM Mohr-Coulomb law, two specific fields show up that the user must fill: the associated M-C Cohesion and Friction angle. On the other hand, when choosing the KDEM Fabric law, a Fabric Coefficient must be given. The value of this parameter goes from 0 to 1. It refers to the moment resistance, where 0 means no resistance at all and 1 a maximum resistance, which would be equivalent to the base KDEM.

Finally, when activating the DEMPack constitutive law, a list of several parameters show up, which are related to the plastic and damage branches of the stress-strain graph. They are the following:

• LCS1: First stiffness reduction point.
• LCS2: Second stiffness reduction point.
• LCS3: Third stiffness reduction point.
• YRC1: First reduction coefficient.
• YRC2: Second reduction coefficient.
• YRC3: Third reduction coefficient.
• Plastic Young Modulus: Material stiffness in the plastic zone.
• Plastic Yield Stress: Plastic yield point.
• Damage Deformation Factor: Energy stored after bond fracture.
• Shear Energy Coefficient: Energy stored due shear effects.

Details on the previous parameters are shown in the picture that follows:

This section is completed with two standard thermal parameters for the material: the Thermal Conductivity and the Specific Heat.