Contact Interaction of Cracked Element Surfaces

When an element is cut by a crack, the compressive behavior of the crack surfaces has to be considered. The formulation that governs the compressive behavior is very similar to that used for surface-based small-sliding penalty contact (About Mechanical Contact Properties). A more general small-sliding surface interaction model between the cracked element surfaces can be defined in user subroutine UXFEMCRACK.

For an element intersected by a stationary crack or a moving crack with the linear elastic fracture mechanics approach, it is assumed that the elastic cohesive strength of the cracked element is zero. Therefore, compressive behavior of the crack surfaces is fully defined with the above options when the crack surfaces come into contact. For a moving crack with the cohesive segments method, the situation is more complex; traction-separation cohesive behavior as well as compressive behavior of the crack surfaces are involved in a cracked element. In the contact normal direction, the pressure-overclosure relationship governing the compressive behavior between the surfaces does not interact with the cohesive behavior, since they each describe the interaction between the surfaces in a different contact regime. The pressure-overclosure relationship governs the behavior only when the crack is “closed”; the cohesive behavior contributes to the contact normal stress only when the crack is “open” (that is, not in contact).

If the elastic cohesive stiffness of an element is undamaged in the shear direction, it is assumed that the cohesive behavior is active. Any tangential slip is assumed to be purely elastic in nature and is resisted by the elastic cohesive strength of the element, resulting in shear forces. If damage has been defined, the cohesive contribution to the shear stresses starts degrading with damage evolution. Once maximum degradation has been reached, the cohesive contribution to the shear stresses is zero. The friction model activates and begins contributing to the shear stresses.

ENRICHMENT, INTERACTION=interaction_property_name
SURFACE INTERACTION, NAME=interaction_property_name
SURFACE BEHAVIOR

ENRICHMENT, INTERACTION=interaction_property_name
SURFACE INTERACTION, NAME=interaction_property_name, UXFEMCRACK

Interaction module: crack editor: toggle on Specify contact property

A general-purpose finite-sliding contact capability is available for XFEM-based crack surfaces within the general contact framework in Abaqus/Standard (About General Contact in Abaqus/Standard). Contact pairs do not support XFEM-based crack surfaces. Crack surfaces can be included in the general contact domain to participate in contact within the compressive regime. Contact between crack surfaces and contact between crack surfaces and other types of surfaces in the model can be considered. The general contact algorithm includes crack surfaces when the default all-inclusive exterior surface defines the contact domain. Alternatively, you can specify pairwise inclusions and exclusions to control more precisely the regions of the model that can potentially come into contact. In both cases the enrichment regions where cracks occur and participate in general contact have to be associated with at least one XFEM-based crack surface. In the former case the association of an enrichment region with an XFEM-based crack surface triggers general contact on cracks in that enrichment region. In the latter case the named XFEM-based crack surface can be explicitly used with contact inclusions and exclusions akin to other types of surfaces to precisely define the general contact domain (Defining the General Contact Domain).

General contact always takes precedence over the small-sliding contact formulation when both are included to model contact between opposing surfaces of a crack within the same analysis. The small-sliding formulation cannot be used to model contact between the crack surfaces and other surfaces; in that case, only general contact supports contact interactions. General contact is applicable only for a crack propagation analysis.

ENRICHMENT, NAME=region-1, TYPE=PROPAGATION CRACK, ELSET=set-1
ENRICHMENT, NAME=region-2, TYPE=PROPAGATION CRACK, ELSET=set-2
SURFACE, TYPE=XFEM, NAME=crack_surf
region-1
region-2

CONTACT
CONTACT INCLUSIONS, ALL EXTERIOR
SURFACE INTERACTION, NAME=interaction_property_name

CONTACT
CONTACT INCLUSIONS
crack_surf,
SURFACE INTERACTION, NAME=interaction_property_name

Interaction module: Create Interaction: General contact (Standard): Included surface pairs: All* with self

Optionally, you can specify thermal contact interaction properties (gap conductance, gap radiation, and gap heat generation) in a contact property definition for cracked element surfaces using the small-sliding formulation or using the general contact finite-sliding formulation. You can include all three types of thermal properties in the same contact property definition. General contact always takes precedence over the small-sliding contact formulation when both are included to model thermal contact between opposing surfaces of a crack within the same analysis.

SURFACE INTERACTION, NAME=interaction_property_name
GAP CONDUCTANCE
GAP RADIATION
GAP HEAT GENERATION

Interaction module: contact property editor: ThermalThermal Conductance, Heat Generation, and/or Radiation