*SURFACE BEHAVIOR

Keyword type: model definition, surface interaction

With this option the surface behavior of a surface interaction can be defined. The surface behavior is required for a contact analysis. There is one required parameter PRESSURE-OVERCLOSURE. It can take the value EXPONENTIAL, LINEAR, TABULAR, TIED or HARD.

The exponential pressure-overclosure behavior takes the form in Figure 131. The parameters $c_0$ and $p_0$ define the kind of contact. $p_0$ is the contact pressure at zero distance, $c_0$ is the distance from the master surface at which the pressure is decreased to 1 % of $p_0$. The behavior in between is exponential. A large value of $c_0$ leads to soft contact, a small value to hard contact.

The linear pressure-overclosure behavior (Figure 132) simulates a linear relationship between the pressure and the overclosure. At zero overclosure the pressure is zero as well. For node-to-face penalty contact the user should specify the slope of the pressure-overclosure curve (usually 5 to 50 times the typical Young's modulus of the adjacent materials; the default is the first elastic constant of the first encountered material in the input deck multiplied by 50) and the tension value for large clearances $\sigma_{\infty}$ (should be small, typically 0.25 % of the maximum stress expected; the default is the first elastic constant of the first encountered material in the input deck divided by 70,000). The value of $c_0$, from which the maximum clearance is calculated for which a spring contact element is generated (by multiplying with the square root of the spring area, cf. Section 6.7.5) can be specified too (default value $10^{-3}$). For face-to-face contact only the slope of the pressure-overclosure relationship is needed.

The tabular pressure-overclosure relationship is a piecewise linear curve. The user enters (pressure,overclosure) pairs. Outside the interval specified by the user the pressure stays constant. The value of $c_0$, from which the maximum clearance is calculated for which a spring contact element is generated (by multiplying with the square root of the spring area, cf. Section 6.7.5) takes the value $10^{-3}$ and cannot be changed by the user. Due to programming restraints the use of a tabular pressure-overclosure relationship in a thermomechanical calculation implies the use of a *GAP CONDUCTANCE card defining the thermal conductance across the contact elements.

The tied pressure-overclosure behavior simulates a truly linear relationship between the pressure and the overclosure for positive and negative pressures. At zero overclosure the pressure is zero. It can only be used for face-to-face contact and similates tied contact between the slave and master face. Notice that all slave faces will be tied to opposite master faces, if any, irrespective whether there is a gap between them or not. The only parameter is the slope of the pressure-overclosure relationship. However, tied contact requires the specification of the stick slope on a *FRICTION card.

Hard pressure-overclosure behavior is internally reduced to linear pressure-overclosure behavior with the default constants.

First line:

• *SURFACE BEHAVIOR
• Enter the parameter PRESSURE-OVERCLOSURE and its value.

Following line if PRESSURE-OVERCLOSURE=EXPONENTIAL:

• $c_0$.
• $p_0$.

Following line if PRESSURE-OVERCLOSURE=LINEAR:

• slope $K$ of the pressure-overclosure curve ($>0$).
• $\sigma_{\infty}$ ($>0$, irrelevant vor face-to-face contact).
• $c_0$ ($>0$, irrelevant for face-to-face contact, optional for node-to-face contact)

Following line if PRESSURE-OVERCLOSURE=TABULAR:

• pressure.
• overclosure.

Repeat this line as often as needed.

Following line if PRESSURE-OVERCLOSURE=TIED:

• slope $K$ of the pressure-overclosure curve ($>0$).

Example:

*SURFACE BEHAVIOR,PRESSURE-OVERCLOSURE=EXPONENTIAL
1.e-4,.1

defines a distance of $10^{-4}$ length units at which the contact pressure is .001 pressure units, whereas the contact pressure at loose contact is 0.1 pressure units.

Example files: contact1, contact2.