# Built-in Contact Models

The contact models provided with PFC are listed in the table below, together with their typical usage. The linear, rolling resistance linear, linear contact bond, and linear parallel bond contact models share many characteristics, and thus, are referred to as linear-based models. The linear contact bond, linear parallel bond, smooth joint, and flat joint contact models utilize the bonding concept wherein shear and/or tensile forces may develop as a consequence of relative motion. These models may be used to model Bonded-Particle Materials (BPMs).

Contact ModEL nAME kEYWORD bEHAVIOR SUMMARY
Built-in Contact Models
Null null No mechanical interaction.
Linear linear Linear elastic law with viscous dashpots.
Linear Contact Bond linearcbond Linear model with contact bonding for BPM.
Linear Parallel Bond linearpbond Linear model with parallel bonding for BPM.
Hertz hertz Non-linear elastic law with viscous dashpots for impact problems.
Hysteretic hysteretic Non-linear elastic law with viscous dashpots for impact problems—directly specify the normal restitution coefficient.
Smooth Joint smoothjoint Frictional/bonded interface for BPM.
Flat Joint flatjoint Frictional/bonded interface for BPM.
Rolling Resistance Linear rrlinear Linear elastic law with viscous dashpots and rolling resistance mechanism for granular applications.

Generic Adhesive Contact Model

arrlinear

Linear model, rolling-resistance mechanism and adhesion to give a cohesive granular material.

Burger's burger Creep mechanisms using a Kelvin model and a Maxwell model connected in series in both normal and shear directions.
Soft Bond sofbond Linear softening bond model for BPM or granular applications.

## Linear-Based Models

The linear, rolling resistance linear, linear contact bond, and linear parallel bond contact models share many characteristics, and thus, are called linear-based models. The linear-based models were also available in PFC 4.0. The distinct-element modeling framework within which they are embedded has been generalized and expanded in PFC 5.0 and PFC 6.0 so that their implementation differs from that in PFC 4.0; however, the contact mechanics embodied in these models remains the same so that their behavior in PFC 4.0 can be reproduced in PFC 5.0 and PFC 6.0.

The linear-based models provide two standard bonding behaviors embodied in the contact bonds and parallel bonds. These bonds can be installed at both ball-ball and ball-facet contacts. Both bonds can be envisioned as a kind of glue joining the contacting pieces. The contact-bond glue is of a vanishingly small size that acts only at the contact point, while the parallel-bond glue is of a finite size that acts over a {rectangular in 2D; circular in 3D} cross-section lying between the contacting pieces. The contact bond can transmit only a force, while the parallel bond can transmit both a force and a moment. By default, pieces are not bonded. Bonds are created by invoking the bond method. Bonds are removed when their strength is exceeded or by invoking the unbond method.

## Bonded-Particle Materials (BPM) and Interfaces

The bonded-particle modeling methodology defines materials and interfaces based on the contact models that are employed. The following materials and interfaces are defined. A contact-bonded material is a granular assembly with all contacts using the linear contact bond model. A parallel-bonded material is a granular assembly with all contacts using the linear parallel bond model. A flat-jointed material is a granular assembly with all contacts using the flat-joint model. A smooth-jointed interface can be inserted into the contact-bonded, parallel-bonded and flat-jointed materials by identifying the contacts near the interface and replacing their contact models with the smooth-joint model.

A BPM material is created by bonding selected contacts of a packed particle assembly. Contacts are bonded by invoking the bond method of the contact model. One can ensure the existence of contacts between all pieces with a contact gap less than a specified bonding gap (gb) by specifying gb as the proximity in the Contact Model Assignment Table (CMAT).