Common Usages of Cohesive Contact Behavior
Cohesive contact can be used in a variety of workflows. Cohesive contact behavior often is one of many possible approaches to modeling interface behavior. Common usages of cohesive contact include:
- Modeling a permanently bonded interface.
- Modeling a bonded interface in which the bond might damage and fail.
- Approximating interface behavior in a simplified form while a model is being built (and other aspects of the model are being refined).
These usages are discuss in more detail below.
Modeling a Permanently Bonded Interface
In it simplest form, cohesive contact can be used as an alternative to surface-based tie constraints (which are discussed in Mesh Tie Constraints) or other modeling methods. There is no need to specify stiffness or damage properties of the contact cohesive behavior in this case; you can allow Abaqus to assign default interfacial stiffness components. Bonded regions remain bonded throughout a simulation if cohesive damage characteristics are not specified. Unlike surface-based tie constraints, cohesive contact will not constrain rotational degrees of freedom.
Modeling a permanently bonded interface as a type of contact behavior rather than as a surface-based tie constraint has the following advantages:
- Enables contact output variables to be used to evaluate interface stresses and other quantities.
- Enables numerical softening to be introduced in the constraint enforcement, which avoids the potential for numerical issues associated with overconstraints where different types of strictly enforced "hard" constraints overlap.
- Optionally, allows a specific interface stiffness representative of physical behavior to be specified.
Permanent cohesive bonds with default cohesive stiffness or user-specified cohesive stiffness at least as stiff as the default cohesive stiffness have the following characteristics for general contact in Abaqus/Standard:
- No regular contact constraints act in parallel to cohesive contact constraints: Conditions for regular contact constraints acting in parallel to cohesive contact constraints are discussed in Interaction between Cohesive Properties and Regular Contact Properties. However, those conditions are not relevant to stiff, permanent cohesive constraints, and regular contact constraints are avoided in these cases to improve convergence behavior and performance.
- Details of the cohesive contact formulation are enhanced for stiff, permanent cohesive bonds. Results for stiff cohesive bonds might differ, even prior to cohesive damage, depending on whether or not a cohesive damage evolution model is specified.
Modeling a Bonded Interface That May Fail
Specifying a damage model for the contact cohesive behavior allows for modeling of a bonded interface that might fail as a result of the loading. This modeling approach is an alternative to using cohesive elements or other element types that directly discretize the cohesive material for the simulation. Comparisons of cohesive-contact versus cohesive-element approaches are discussed below in High-Level Comparison of Cohesive-Element and Cohesive-Contact Approaches.
Approximating and Modifying Interface Behavior While a Finite Element Model Is Built
Using different interface modeling strategies across different stages of building and refining a finite element model is sometimes a good strategy for improving your efficiency. For example, during an initial stage of a model build, you might choose to model interfaces as permanently bonded to enable more focus on noninterface modeling details. You can switch to more physically representative interface behavior (such as regular contact or bonded contact with the possibility of damage and failure) in later stages of the model build. The later stages often require more care to avoid unconstrained rigid body modes and other types of static instabilities.
Analysts sometimes use surface-based tie constraints (Mesh Tie Constraints) in early stages of building a model and then switch to contact specifications as the model becomes more mature. An alternative is to specify cohesive contact behavior with a permanently bonded interface and default stiffness in the early stages, and then reassign a more realistic contact behavior as the model becomes more mature. This alternative of reassigning the contact behavior as the model matures, rather than switching from a constraint option to a contact option during the model evolution, might result in greater consistency across different stages of the model build.