Friction

Both explicit cracks and multimaterial mode MPM can model frictional contact between the surfaces.

Introduction

Contact mechanics between surface can be model as frictionless sliding, Coulomb friction with a surface coefficient of friction, or stick contact (no sliding). In frictionless sliding, the tangential surface force is zero while the normal forces is determined by the amount of contact. In frictional sliding, the magnitude of the tangential force is related to the magnitude of the normal force by:

      [math]\displaystyle{ f_t = \mu f_n }[/math]

where μ is the coefficient of friction. But, if the induced tangential force from surface motion is less than this value (e.g., because f_n is low) the the contact is modeled as stick contact instead. In stick contact, the two surface move in the center-of-mass velocity field as if there were one material, but only as long as they remain in contact.

Setting Frictional Contact Properties

The commands below set default frictional properties for all cracks and/or default frictional properties for all material-material contact in multimaterial MPM. If needed, you can always customize each crack or each material-material pair to have their own frictional properties.

Friction on Explicit Cracks

Explicit cracks in NairnMPM can model frictional contact. To choose the default frictional contact options for cracks in scripted files, use the command

Friction (frict)

In XML files, the default frictional contact properties for cracks are define with a <Friction> command within the <Cracks> element in the <MPMHeader>:

<Friction>(frict)</Friction>

The meaning of (frict) is described below.

The above commands set the default frictional contact properties for crack surfaces. When a simulation only has only one crack, this setting is enough. For simulations with more than one crack, you can set different frictional properties for each crack, or convert some cracks to imperfect interfaces, by using settings available when defining the cracks.

Friction in Multimaterial MPM

Contact in multimaterial mode MPM can model frictional contact. To choose the default frictional contact options for material-material contact in scripted files, use the command

FrictionMM (frict)

In XML files, the default frictional contact properties for material contact are defines with a <Friction> command within the <MultimaterialMode> element in the <MPMHeader>:

<Friction>(frict)</Friction>

where (frict) defines the type of friction:

• (frict)=(zero or positive number) or (frict)=none - crack contact is by friction with the supplied coefficient of friction. Enter zero (or "none") for frictionless sliding.
• (frict)=stick (or a number between -1 and -9) - crack contact by stick conditions. They stick when contact is detected, but move freely apart when separated.
• (frict)=ignore or (frict)=single (or a number less than -10) - ignore crack contact when modeling friction on cracks or revert to single velocity field when modeling material contact in multimaterial MPM. This setting will give poor results for cracks that are in contact. For multimaterial mode simulations, this mode reverts to a single velocity field. It is useful in simulations with three or more materials where you want some materials perfectly bonded (set their custom friction to single) while other material contact is by friction or stick conditions.

Note the XML files must always use the numeric option instead of of the text settings.

The above commands set the default frictional properties for contact between any two materials. When a simulation only has two materials contacting by friction, this setting is enough. For simulations with more than two materials, you can set different frictional properties for each material pair, or even combine material pairs interacting by friction with those connected by an imperfect interface, by using a Friction or Interface material property for the secondly-defined material or each pair needing different contact mechanics.