Friction

 

Friction defines the friction of Contact. And the user can also define the type as the following table.

 

Friction Type

Entities

Sliding and Stiction Type

Element Assembly

2D Segment Assembly

Sliding Type

Segment Assembly

 

Sliding / Sliding & Stiction

Friction type can be selected. The default value is sliding type. The difference between them is shown in Figure 1, 2 and Table 1

 

 

Figure 1  Sliding Type

 

 

Figure 2  Sliding & Stiction Type

 

     Static Threshold Velocity (): This value is a criterion between static and dynamic friction. If the relative velocity is less than this value, the friction is on static friction. If not, the friction is on dynamic friction. The friction coefficient of static friction region is calculated from following equation for the sliding type. The mathematical definitions of all friction types are defined in Table1.

 

 

     Dynamic Threshold Velocity (): If the relative velocity between a contact pair is greater than this value, the friction coefficient is same as the specified dynamic friction coefficient. If the relative velocity between a contact pair is greater than “Static Threshold Velocity” and less than this value, the friction coefficient is defined as following. The detail mathematical definitions is defined in Table1.

 

     Static Friction Coefficient (): Specifies a static friction coefficient.

     Maximum Friction Force (): If this is checked, the contact friction force is limited to this value. 

Figure 3  Relationship between relative velocity and friction coefficient in the sliding friction type 

 

The friction force of contact entities is determined by the following equations.

 

where, the ,  and  are the contact normal force, the friction coefficient and the maximum friction force, respectively. The friction coefficient of  is determined by a relative and tangential velocity on the contact point.

 

Type

Sliding

Sliding & Stiction

Static friction condition ()

consticslip3

consticslip4

Mathematical definition for Static friction condition

Note1)

In steady state, the slip is constant. Because, the friction force is function of relative and tangential velocity ()

In steady state, the slip is almost zero. Because, creep generate the stiction friction force for a stick effect.

Note2)

Mathematical definition for Dynamic friction condition

Table 1  Comparison in inclined plane model

 

     Maximum Stiction Deformation: This controls the stick-slip effect on the contact. The value must be greater than or equal to zero. If the tangential velocity is smaller than the static threshold velocity, then the stick effect can be occurred. On the other hand, if the tangential velocity is greater than the static threshold velocity, then the friction model is on the slip condition.

 

Note

The friction coefficient of Sliding & Stiction Type is function of relative velocity and creep () while the friction coefficient of Sliding Type is function of relative velocity.