Contact Diagnostics in an Abaqus/Explicit Analysis

Reviewing the Adjustments of Initially Overclosed Surfaces

Contacting surfaces that are overclosed in the initial configuration of the model are adjusted automatically by Abaqus/Explicit to remove the overclosures (see Contact Initialization for General Contact in Abaqus/Explicit and Contact Initialization for Contact Pairs in Abaqus/Explicit). There are three sources of information on the adjustments of overclosed surfaces: the status (.sta) file, the message (.msg) file, and the output database (.odb) file.

Obtaining the Adjustments of Overclosed Surfaces in the Status and Message Files

By default, Abaqus/Explicit writes all nodal adjustments and—for general contact surfaces—contact offsets to the message (.msg) file along with a summary listing of the maximum initial overclosure and the maximum nodal adjustment to the status (.sta) file for the contact pairs defined in the first step of a simulation. You can choose to suppress the information written to the message file and write only the summary information to the status file. The information written to the message and status files is also written to the output database (.odb).

Visualizing the Precise Initial Clearances for Small-Sliding Contact Pairs

Abaqus/Explicit does not adjust the coordinates of the secondary surface when precise initial clearances are specified for small-sliding contact pairs (see Contact Initialization for Contact Pairs in Abaqus/Explicit). Therefore, the specified clearances cannot be seen in a postprocessor. Thus, depending on the initial geometry of the surfaces and the magnitude of the clearances or overclosures, the surfaces may appear open or closed in the postprocessor when they are actually just in contact in the simulation.

Detecting Crossed Surfaces in a General Contact Domain

If a secondary surface initially penetrates a double-sided main surface by a distance greater than the main surface's thickness, the severely overclosed secondary nodes will see the back side of the main surface as the appropriate contact force direction. These secondary nodes in these crossed surfaces effectively become trapped behind the main surface. This issue is discussed in more detail in Contact Initialization for General Contact in Abaqus/Explicit and Contact Initialization for Contact Pairs in Abaqus/Explicit.

For general contact definitions, diagnostic testing that identifies regions in which surfaces are crossed in the initial configuration is activated by default. When the diagnostic tests are activated, a warning message is issued to the message (.msg) file if two adjacent secondary nodes (connected by a facet edge) are detected on opposite sides of a main surface. No such warning is issued for node-based surface nodes on opposite sides of a main surface, because adjacency cannot be determined among the node-based surface nodes. In some cases involving corners of main surfaces this warning message may be issued even though adjacent secondary nodes are really on the same side of a main surface. The CPU cost of performing diagnostic testing on large models is potentially significant. You can choose to deactivate the diagnostic testing and avoid the extra CPU cost in such cases.

Excessive Penetrations between General Contact Surfaces

As described in Contact Constraint Enforcement Methods in Abaqus/Explicit, the penalty constraint enforcement method used by the general contact algorithm in Abaqus/Explicit allows slight penetrations of one surface into another surface. A “spring” stiffness is applied automatically to the surfaces to resist these penetrations. If the nodes involved in general contact do not have adequate mass, the default “spring” stiffness chosen automatically by Abaqus/Explicit may not be sufficient to prevent large penetrations. Such a situation can arise, for example, when a cloud of massless nodes, fully constrained by a kinematic coupling definition, contacts a fully constrained rigid face with no mass.

By default, if during node-to-face contact, the penetration of a node into its tracked face exceeds 50% of the typical face dimension in the general contact domain, the penetration is regarded as excessive and Abaqus/Explicit issues a diagnostic message to the status (.sta) file. A node set containing deeply penetrated nodes is also written to the output database (.odb) file. You can control the fraction of the typical face dimension used to trigger the diagnostic message.

Warning Messages for Highly Warped Surfaces

Calculating the correct contact conditions along a surface that is highly warped is very difficult, and Abaqus/Explicit employs a specialized algorithm to enforce contact between warped surfaces; this specialized algorithm is more expensive than the default contact algorithm (see Contact Controls for Contact Pairs in Abaqus/Explicit). By default, Abaqus/Explicit checks for highly warped surfaces every 20 increments.

Abaqus/Explicit writes a warning message in the status (.sta) file the first time that it detects that a surface is highly warped. The message is brief; it states only which surface has a highly warped facet. If additional facets on this surface become highly warped later in the analysis, no additional warning messages are issued.

You can request more detailed diagnostic warning messages, if desired. In this case the message file will contain a warning every time a warped facet is found on a particular surface. The warnings will give the parent element associated with the warped facet (the parent element is the element whose face forms the facet) and the warping angle of the facet.

The computation time and the size of the message file can increase significantly if detailed warnings are requested. You can switch back to the summary warnings in subsequent steps or suppress the warped surface warnings entirely.

If the analysis terminates with a fatal error, the preselected output variables will be added automatically to the output database as field data for the last increment.