The bilinear material model developed exclusively for contact elements (TB,CZM with TBOPT = CBDD or CBDE) is the recommended bilinear model for debonding. The material behavior, defined in terms of contact stresses (normal and tangential) and contact separation distances (normal gap and tangential sliding), is characterized by linear elastic loading followed by linear softening. The slope of the curve depends on contact stiffness and a debonding parameter which is defined in terms of material constants.

Two other cohesive zone material models that were developed for use with the interface elements can also be used to model contact debonding: the bilinear material model (TB,CZM with TBOPT = BILI) and the exponential material model (TB,CZM with TBOPT = EXPO).

Debonding allows three modes of separation:

Debonding is also characterized by convergence difficulties during material softening. Artificial damping is provided to overcome these problems. An option for tangential slip under compressive normal contact stress for mixed-mode debonding is also provided.

After debonding is completed, the surface interaction is governed by standard contact constraints for normal and tangential directions. Frictional contact is used if friction is specified for the contact elements.

The cohesive zone material model with bilinear behavior (TB,CZM with TBOPT = CBDD or CBDE ) is defined as:

where:

To model bilinear material behavior with tractions and separation distances, use TB,CZM with TBOPT = CBDD. You also input the following material constants with the TBDATA command:

Constant	Symbol	Meaning
C1	σmax	Maximum normal contact stress [1]
C2		Contact gap at the completion of debonding
C3	τmax	Maximum equivalent tangential contact stress [1]
C4		Tangential slip at the completion of debonding
C5	η	Artificial damping coefficient
C6	β	Flag for tangential slip under compressive normal contact stress; must be 0 (off)  or 1 (on)

To model bilinear material behavior with tractions and critical fracture energies, use TB,CZM with TBOPT = CBDE. You also input the following material constants with the TBDATA command:

TB,CZM,,,,CBDD      ! bilinear behavior with tractions and separation distances
TBDATA,1,C1,C2,C3,C4,C5,C6

For more information about the cohesive zone material model, see Cohesive Zone Material for Contact Elements in the Material Reference.

This model follows a bilinear law for traction-separation and differs slightly from the bilinear material behavior for contact. See Cohesive Zone Material (CZM) Model in the Mechanical APDL Theory Reference for details on the differences.

To define this material, use the TB,CZM command with TBOPT = BILI. Specify the material constants as data items C1 through C6 on the TBDATA command as described in Material Constants -- Bilinear Law.

This model follows an exponential law for traction separation. To define this material, use the TB,CZM command with TBOPT = EXPO. Specify the material constants as data items C1, C2, and C3 on the TBDATA command as described in Material Constants -- Exponential Law.

For the cohesive zone materials with bilinear material behavior (TBOPT = CBDD, CBDE or BILI on the TB command), you can specify that the cohesive zone interface be “healed” if the surfaces come into contact again after debonding.

To activate this option, use the TBFIELD,CYCLE command to define the CZM material as a function of healing cycle number. For more information, see Post-Debonding Behavior of Cohesive Zone Material in the Contact Technology Guide.