*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 . 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. For mortar face-to-face contact the exponential pressure vs. penetration curve is move parallel to the pressure axis such that zero penetration corresponds to zero pressure. Furthermore, negative pressure values are set to zero.

The linear pressure-overclosure behavior (Figure ) 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 ) 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 ) 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 penalty 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. In case of mortar contact true hard behavior can be simulated by omiting the *SURFACE BEHAVIOR card within the *SURFACE INTERACTION card.

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.