# Defining the constitutive response of cohesive elements using a traction-separation description

 The features described in this section are primarily intended for bonded interfaces where the interface thickness is negligibly small. In such cases it may be straightforward to define the constitutive response of the cohesive layer directly in terms of traction versus separation. If the interface adhesive layer has a finite thickness and macroscopic properties (such as stiffness and strength) of the adhesive material are available, it may be more appropriate to model the response using conventional material models. The former approach is discussed in this section, while the latter approach is discussed in Defining the constitutive response of cohesive elements using a continuum approach. Cohesive behavior defined directly in terms of a traction-separation law: can be used to model the delamination at interfaces in composites directly in terms of traction versus separation; allows specification of material data such as the fracture energy as a function of the ratio of normal to shear deformation (mode mix) at the interface; assumes a linear elastic traction-separation law prior to damage; can be used in combination with linear viscoelasticity in Abaqus/Explicit (Defining viscoelastic behavior for traction-separation elasticity in Abaqus/Explicit) to describe rate-dependent delamination behavior; assumes that failure of the elements is characterized by progressive degradation of the material stiffness, which is driven by a damage process; allows multiple damage mechanisms; and can be used with user subroutine UMAT in Abaqus/Standard or VUMAT in Abaqus/Explicit to specify user-defined traction-separation laws. The following topics are discussed:
 Related Topics About cohesive elements Defining the constitutive response of cohesive elements using a continuum approach In Other Guides *COHESIVE SECTION *DAMAGE EVOLUTION *DAMAGE INITIATION Defining damage Adhesive joints and bonded interfaces

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