A cohesive zone material model is needed to model debonding in a contact analysis. This material is defined using the data table method (TB and TBDATA commands). Temperature dependent data is also allowed (TBTEMP command).
The following cohesive zone materials are available:
These models are also described in Cohesive Zone Material (CZM) Model in the Theory Reference.
This model follows a bilinear law for traction-separation and allows for two different ways of defining the input material data. This is the recommended bilinear material model for use with contact elements.
This is a linear elastic material behavior with linear
softening characterized by maximum traction and maximum separation. To define
this material, use the TB,CZM command with
TBOPT = CBDD. Specify the material constants as
data items C1 through C7 on 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) |
| C7 | α | Power law exponent for mixed-mode debonding (defaults to 2) |
Sample command input for this material is shown below.
TB,CZM,1,2,,CBDD TBDATA,1,σmax,,τmax,
,η,β TBDATA,7,α
This is a linear elastic material behavior with linear
softening characterized by maximum traction and critical energy release rate. To
define this material, use the TB,CZM command with
TBOPT = CBDE. Specify the material constants as
data items C1, through C7 on the TBDATA command:
| Constant | Symbol | Meaning |
|---|---|---|
| C1 | σmax | maximum normal contact stress [1] |
| C2 | Gcn | critical fracture energy density (energy/area) for normal separation [2] |
| C3 | τmax | maximum equivalent tangential contact stress [1] |
| C4 | Gct | critical fracture energy density (energy/area) for tangential slip [2] |
| C5 | η | artificial damping coefficient |
| C6 | β | flag for tangential slip under compressive normal contact stress; must be 0 (off) or 1 (on) |
| C7 | α | Power law exponent for mixed-mode debonding (defaults to 2) |
For contact elements using the force-based model (see the description of KEYOPT(3) for CONTA175 and CONTA177), input a contact force value for this quantity.
For contact elements using the force-based model (see the description of KEYOPT(3) for CONTA175 and CONTA177), this quantity is critical fracture energy.
Sample command input for this material is shown below.
TB,CZM,1,2,,CBDE TBDATA,1,σmax,Gcn,τmax,Gct,η,β TBDATA,7,α
Debonding involves separation of surfaces forming an interface. The direction of separation determines the debonding mode. The program detects the debonding mode based on material data that you input for normal and tangential directions:
Mode I debonding involves separation normal to the interface. It is activated by inputting data items C1, C2, and C5 on the TBDATA command.
Mode II debonding involves slip tangent to the interface. It is activated by inputting data items C3, C4, and C5 on the TBDATA command.
Mixed-mode debonding involves both normal separation and tangential slip. It is activated by inputting data items C1, C2, C3, C4, C5, C6, and C7 on the TBDATA command.
Normal Contact Stiffness
Normal contact stiffness dictates the relationship between contact gap/penetration and contact pressure. For debonding, the default program behavior is to use the same normal contact stiffness for both opening contact (gap) and closing contact (penetration). The normal contact stiffness value is based on real constant FKN of the contact element.
In order to accurately reflect the material adhesion strength at the interface, you may want to use a large normal contact stiffness for closing contact to prevent any non-physical penetration but use a much smaller normal contact stiffness for opening contact to relate gap and contact pressure during debonding. To achieve this, specify real constant FKOP to dictate the gap vs. pressure relationship and specify real constant FKN to dictate the penetration vs. pressure relationship. Note that if you do not specify FKOP, the normal contact stiffness determined by FKN will be used for both opening and closing contact at the debonding interface.
Artificial Damping
Debonding is generally accompanied by convergence difficulties in the Newton-Raphson solution. Artificial damping can be used to stabilize the numerical solution. It is activated by specifying the damping coefficient η (input on TBDATA command as C5). The damping coefficient has units of time and should be smaller than the minimum time step size so that the maximum traction and maximum separation (or critical fracture energy) values are not exceeded in debonding calculations.
Tangential Slip under Normal Compression
An option is available to control tangential slip under compressive normal contact stress for mixed-mode debonding. By default, no tangential slip is allowed for this case, but it can be activated by setting the flag β to 1 (input on TBDATA command as C6).
Pinball Radius and Mesh Density
When using a fine mesh for underlying elements of bonded surfaces, you may need to increase the pinball radius (PINB) for contact elements so that it is greater than the maximum separation value in the normal direction (contact gap when normal contact stress goes to zero). The default value for PINB is based on the depth of the underlying element. If PINB is smaller than the maximum separation value, debonding calculations will be bypassed when the contact gap exceeds PINB.
Debonding under the Presence of Friction
When friction is defined between contact surfaces undergoing debonding, tangential stress is calculated as the maximum between the tangential stress as governed by the debonding model and the tangential stress as governed by the friction law.
All applicable output quantities for contact elements are also available for debonding: normal contact stress (PRES), tangential contact stress (TAUR, TAUS, SFRIC), contact gap (GAP), tangential slip (TASR, TASS, SLIDE), etc. In addition, the following debonding specific output quantities are available as NMISC data: debonding time history (DTSTART), debonding parameter (DPARAM), and critical fracture energy (DENERI, DENERII).
For more information on how to review results in a contact analysis, see Reviewing the Results in Surface-to-Surface Contact (Pair-Based).
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 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.
| Constant | Meaning | Property |
|---|---|---|
| C1 | σmax | Maximum normal traction [2] |
| C2 |
| Normal displacement jump at the completion of debonding |
| C3 | τmax | Maximum tangential traction |
| C4 |
| Tangential displacement jump at the completion of debonding |
| C5 |
| Ratio of  to  , or ratio of  to 
|
| C6 [1] | β | Non-dimensional weighting parameter |
The program detects the debonding mode based on material data that you input for normal and tangential directions:
Mode I debonding involves separation normal to the interface. It is activated by inputting data items C1, C2, C3, C4, and C5 on the TBDATA command, where C3 = -τmax.
Mode II debonding involves slip tangent to the interface. It is activated by inputting data items C1 C2, C3, C4, and C5 on the TBDATA command, where C1 = -σmax.
Mixed-mode debonding involves both normal separation and tangential slip. It is activated by inputting data items C1, C2, C3, C4, C5, and C6 on the TBDATA command, where C1 = σmax and C3 = τmax.
Artificial Damping
Artificial damping can be used to stabilize the numerical solution when convergence difficulties are encountered. Use the viscous regularization model (TB,CZM,,,VREG) to specify the viscous regularization. For more information, see Viscous Regularization of Cohesive Zone Material for Interface Elements and Contact Elements in the Material Reference.
Pinball Radius and Mesh Density
The same considerations apply as described for the bilinear material behavior for contact.
All applicable output quantities for contact elements are also available for debonding: normal contact stress (PRES), tangential contact stress (TAUR, TAUS, SFRIC), contact gap (GAP), tangential slip (TASR, TASS, SLIDE), etc. In addition, the following debonding-specific output quantities are available as NMISC data: debonding time history (DTSTART), damage parameter (DPARAM), total debonding energy (DENER).
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, through C3 on the TBDATA command.
| Constant | Meaning | Property |
|---|---|---|
| C1 | σmax | Maximum normal traction at the interface [1] |
| C2 | δn | Normal separation across the interface where the maximum normal traction is attained |
| C3 | δt | Shear separation where the maximum shear traction is attained |
Artificial Damping
Artificial damping can be used to stabilize the numerical solution when convergence difficulties are encountered. Use the viscous regularization model (TB,CZM,,,VREG) to specify the viscous regularization. For more information, see Viscous Regularization of Cohesive Zone Material for Interface Elements and Contact Elements in the Material Reference.
Pinball Radius and Mesh Density
The same considerations apply as described for the bilinear material behavior for contact.
All applicable output quantities for contact elements are also available for debonding: normal contact stress (PRES), tangential contact stress (TAUR, TAUS, SFRIC), contact gap (GAP), tangential slip (TASR, TASS, SLIDE), etc. In addition, the following debonding-specific output quantity is available as NMISC data: total debonding energy (DENER).