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If FSLIPR is requested, FSLIPR (the magnitude of the slip rate for slave nodes in contact) can be contoured in Abaqus/CAE for each slave surface in a contact pair. In addition, for three-dimensional contact interactions involving an analytical rigid surface and for all two-dimensional contact interactions, components of net slip rate based on local tangent directions (FSLIPR1 and, in three dimensions, FSLIPR2) can also be contoured in Abaqus/CAE for each slave surface in a contact pair if FSLIPR is requested. All of the slip rate variables associated with FSLIPR are zero whenever a slave node is not in contact.
If FSLIP is requested, FSLIPEQ (the length of the overall slip path for a slave node while it is in contact) can be contoured in Abaqus/CAE for each slave surface in a contact pair. In addition, for three-dimensional contact interactions involving an analytical rigid surface and for all two-dimensional contact interactions, components of net slip (FSLIP1 and, in three dimensions, FSLIP2) can also be contoured in Abaqus/CAE for each slave surface in a contact pair if FSLIP is requested. These slip variables are equivalent to the slip rate variables integrated over time: FSLIPEQ, FSLIP1, and FSLIP2 are equivalent to FSLIPR, FSLIPR1, and FSLIPR2 integrated over time, respectively. Therefore, these slip variables account only for relative motions that occur while slave nodes are in contact.
The algorithm used to establish and evolve local tangent directions for contact pairs is described in “Local tangent directions for contact” in “Contact formulations for contact pairs in Abaqus/Explicit,” Section 38.2.2. Unlike general contact, previously accumulated slip components for contact pairs, FSLIP1 and FSLIP2, are not resolved into the new local system before incremental contributions are added to them.
Displacement field output (U) for the entire model is written to the output database automatically when any of the contact field output variables are requested.
History output
Several whole surface contact variables are available as history output. These variables record the contact state of a surface as a set of force (CFN, CFS, and CFT) and moment (CMN, CMS, and CMT) resultants with respect to the origin. Additional variables give the center of pressure (XN, XS, and XT) on the surface (defined as the point closest to the centroid of the surface that lies on the line of action of the resultant force for which the resultant moment is minimal). The last letter of each variable name (except the variable CAREA) denotes which contact force distribution on the surface is used to calculate the resultant: the letter N denotes that the normal contact forces are used to derive the resultant quantity; the letter S denotes that the shear contact forces are used to derive the resultant quantity; and the letter T denotes that the sum of the normal and shear contact forces are used to derive the resultant quantity. These history output variables will be written twice to the output database once for each surface involved in the contact pair.
Each total moment output variable will not necessarily equal the cross product of the respective center of force vector and resultant force vector. Forces acting on two different nodes of a surface may have components acting in opposite directions, such that these nodal force components generate a net moment but not a net force; therefore, the total moment may not arise entirely from the resultant force. The center of force output variables tend to be most meaningful when the surface nodal forces act in approximately the same direction.
The total area in contact at a given time can be requested using output variable CAREA, defined as the sum of all the facets where there is contact force. The contact area reported by CAREA is generally slightly larger than the true contact area for reasonably meshed contact surfaces; therefore, interpretation of CAREA should be done with care. The discrepancy between the CAREA output and the true contact area decreases as the mesh density increases. Using contact inclusions or exclusions to limit CAREA output to smaller contact surfaces may also reduce the discrepancy in some cases. Since the CAREA output is an approximation of the true contact area, deriving force or stress values using this output may not yield accurate values; requesting contact force and stress directly is the most appropriate way to obtain accurate results.
Detailed history output on the status of bonded surfaces is available from an Abaqus/Explicit simulation. Details can be found in “Breakable bonds,” Section 37.1.9.
Obtaining the “maximum torque” that can be transmitted about the z-axis in an axisymmetric analysis
When modeling surface-based contact with axisymmetric (CAX) elements, Abaqus/Explicit can calculate the maximum torque (output variable CTRQ) that can be transmitted about the z-axis. The maximum torque, T, is defined as
T = \iint r ^ {2} p d s d \theta ,
where p is the pressure transmitted across the interface, r is the radius to a point on the interface, and s is the current distance along the interface in the r–z plane. This definition of “torque” effectively assumes a friction coefficient of unity.
36.5.2 ASSIGNING SURFACE PROPERTIES FOR CONTACT PAIRS IN Abaqus/Explicit
Products: Abaqus/Explicit Abaqus/CAE
References
• “Defining contact pairs in Abaqus/Explicit,” Section 36.5.1
• *CONTACT PAIR
• *SURFACE
“Specifying geometric properties for mechanical contact property options” in “Defining a contact interaction property,” Section 15.14.1 of the Abaqus/CAE User’s Guide, in the HTML version of this guide
Overview
This section describes how to modify the surface properties for contact interactions in Abaqus/Explicit defined with the contact pair algorithm, including the surface thickness and offset.
Shell, membrane, or rigid element thickness and shell or rigid element offset
To define surfaces on shell, membrane, or rigid elements such that they are in contact at the start of the analysis, the element thicknesses must be considered when defining the nodal coordinates; otherwise, the surfaces in the contact pair will be overclosed. Surface thickness and surface offset are properties that are inherited from underlying shell and membrane elements by default. For a surface based on rigid elements, the default surface thickness and offset correspond to the thickness and offset defined for the rigid body to which the elements belong (see “Rigid elements,” Section 30.3.1). The surface thickness and offset are zero for surfaces based on solid elements.
By default, the nodal thickness for surfaces based on shell, membrane, or rigid elements equals the minimum thickness of the surrounding elements (see Figure 36.5.2–1 and Table 36.5.2–1). The surface thickness within a facet is interpolated from the nodal values; the interpolated surface thickness never extends past the specified element or nodal thickness, which may be significant with respect to initial overclosures.
If a spatially varying nodal thickness is defined for the underlying elements (see “Nodal thicknesses,” Section 2.1.3), the nodal surface thickness may not correspond exactly to the specified nodal thickness (see node 4 in Figure 36.5.2–2 and Table 36.5.2–2). The nodal surface thickness distribution will tend to be more diffuse than the specified nodal thickness distribution (because the specified nodal thicknesses are averaged to compute the element thicknesses, and the minimum of the surrounding element thicknesses is the nodal surface thickness).
Effects of surface thickness and offsets, as well as methods for modifying the surface thickness and for avoiding surface offsets, are discussed below.
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specified element thickness (constant over element) nodal surface thickness interpolated surface thickness 1 a 2 b 3 c 4 d 5
Figure 36.5.2–1 Continuous variation of surface thickness across facet boundaries.
Table 36.5.2–1 Thicknesses corresponding to Figure 36.5.2–1.
| node | element | specified element thickness | nodal surface thickness (minimum of adjacent element thicknesses) |
| 1 | 0.5 | ||
| a | 0.5 | ||
| 2 | 0.5 | ||
| b | 0.5 | ||
| 3 | 0.5 | ||
| c | 0.9 | ||
| 4 | 0.9 | ||
| d | 0.9 | ||
| 5 | 0.9 |
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element thickness (constant over element) nodal surface thickness specified nodal thickness interpolated surface thickness 1 a 2 b 3 c 4 d 5 e 6
Figure 36.5.2–2 Small discrepancy between the nodal surface thickness and the specified nodal thickness.
Table 36.5.2–2 Thicknesses corresponding to Figure 36.5.2–2.
| node | element | specified nodal thickness | element thickness (average of specified nodal thickness) | nodal surface thickness (minimum of adjacent element thicknesses) |
| 1 | 0.5 | 0.5 | ||
| a | 0.5 | |||
| 2 | 0.5 | 0.5 | ||
| b | 0.5 | |||
| 3 | 0.5 | 0.5 | ||
| c | 0.7 | |||
| 4 | 0.9 | 0.7 | ||
| d | 0.9 | |||
| 5 | 0.9 | 0.9 | ||
| e | 0.9 | |||
| 6 | 0.9 | 0.9 |
Effects of surface thickness and offsets
Accounting for thickness in the contact pair algorithm will cause the surface to extend past the parent element boundary in the plane of the element by an amount equal to one-half its thickness. For example, this surface extension, which is semi-circular in shape, will cause contact to be established between the edge of a shell and an opposing surface before the node on the shell boundary reaches the opposing surface. The extension is present for both single-sided and double-sided surfaces. Figure 36.5.2–3 demonstrates this concept. Such “bull-nose” extensions are avoided when the general contact algorithm (“Defining general contact interactions in Abaqus/Explicit,” Section 36.4.1) is used. The effect of a shell or rigid offset on a surface is shown in Figure 36.5.2–4. Poorly defined surfaces can result near corners if large offsets are present, as shown in Figure 36.5.2–5. You should consider this when defining a model. A warning message will be issued if the offset magnitude is greater than one-half of any of the parent shell element edge lengths. However, at acute corners it is possible for an offset less than one-half of the parent element size to result in a poorly defined contact surface (and in this case no warning will be given).
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contacting surface surface extension t shell nodes shell reference surface contact established
Figure 36.5.2–3 Extension of contact surface for edge contact without zero surface thickness.
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midsurface t/2 t/2 offset contact surface, same as shell outer surface except at edges reference surface
Figure 36.5.2–4 Extension of contact surface if a shell offset is present.
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nodal offset adjusted nodal position shell midsurface reference surface
Figure 36.5.2–5 Example of a poorly defined surface near a corner when a large shell offset is present.
Controlling the effects of surface thickness and offset in contact calculations
You can control the thickness and offset used in the contact calculations only; they do not affect surfacebased constraints. These settings are intended primarily for self-contact surfaces since you cannot force zero thickness for these surfaces, as described below.
Self-contact surfaces should not contain facets that are thicker than their edge or diagonal lengths. Extremely large thicknesses will cause nodes to appear to be penetrating nearby facets in even a flat self-contact surface due to the algorithmic use of a semi-circular tube with a radius of half the contact thickness around the edge of each facet (see Figure 36.5.2–6).
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outer boundary of node penetration outer boundary of overall surface outer boundary of facet reference surface
Figure 36.5.2–6 Undesired penetration resulting from a large thickness in a self-contact surface.
You can scale the effective thickness used for all of the facets on a surface by a single factor, f. Alternatively, you can adjust only the thicknesses for surface facets in which the thickness to minimum edge or diagonal length ratio exceeds a specified value, r ; the amount by which a facet thickness is
adjusted may vary during an analysis because of changes in the facet size. If the thickness to element size ratio exceeds 1.0 in the initial configuration for a self-contact surface, an error message recommending that you adjust the thickness will be issued.
You should not specify extremely small values for f or r for double-sided surfaces or surfaces that will be involved in self-contact since these surfaces must have a contact thickness that is significant compared to the facet size. For surfaces involved only in two-surface contact it is acceptable to set { f } { = } 0 . 0 ; however, it is computationally more efficient to use the method described below to force a zero surface thickness. It is also possible to enforce the offset but not the thickness in the surface model by setting the scale factor, f, equal to zero.
Input File Usage: Use the following option to scale the surface thickness by a single factor:
* \text { SURFACE, NAME } = \text { name, SCALE THICK } = f
Use the following option to adjust the thickness to element size ratios:
* \text { SURFACE }, \text { NAME } = \text { name }, \text { MAX RATIO } = r
Abaqus/CAE Usage: You cannot scale the thickness of a contact surface in Abaqus/CAE.
Forcing zero surface thickness and offset
You can force the surface thickness and offset to be zero, rather than inherit the thickness and offset of underlying shell, membrane, or rigid elements. In this case the contact surface is taken as the reference surface (see Figure 36.5.2–7).
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midsurface t/2 t/2 shell surfaces reference surface and contact surface
Figure 36.5.2–7 Contact surface with zero thickness and offset.
You cannot ignore the thickness for a surface that is used as a contact surface for single-surface (self) contact. If one of the surfaces in a contact pair is a double-sided surface, zero thickness can be forced on only one of the two surfaces: at least one surface in a contact pair involving double-sided surfaces must have a nonzero thickness. The ability to force zero surface thickness is useful for performing parameter studies on the thickness or offset of a model since you can change the thickness and offset without also having to move the mesh to control the initial separation between the surfaces.
Input File Usage: *SURFACE, NAME=name, NO THICK
Abaqus/CAE Usage: You cannot force a surface thickness to be zero in Abaqus/CAE.
Example
Contact calculations are generally most accurate with the default treatment of thickness and offset. However, when a shell offset of half the original shell thickness has been specified, forcing zero surface thickness will give an accurate representation of one side of the surface. This approach can be more accurate near a corner (especially on the exterior side of a corner) than if the offset and thickness are enforced for the surface, as shown in Figure 36.5.2–8.
flowchart
graph TD
A["reference surface"] -->|default surface| B["adjusted nodal position"]
A -->|desired midsurface| C["midsurface"]
A -->|contact surfaces| D["surface if zero thickness is forced"]
B --> E["Shell model with offset equal to half the thickness"]
C --> E
D --> E
Figure 36.5.2–8 Forcing zero surface thickness when the shell offset is half the original shell thickness.
Forcing zero surface offset
For situations in which it is desirable to ignore the effect of the offset but when it is still necessary to model the thickness in the contact calculations, you can force only the surface offset to be zero without affecting the surface thickness. In this case the contact surface is the outside surface of an imaginary shell, membrane, or rigid element whose midsurface is at the reference surface (see Figure 36.5.2–9). This method could be used for a self-contact surface that would be poorly defined if the offset were enforced (thickness must be enforced for self-contact surfaces).
Input File Usage: *SURFACE, NAME=name, NO OFFSET
Abaqus/CAE Usage: You cannot force a surface offset to be zero in Abaqus/CAE.
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midsurface t/2 t/2 shell surfaces contact surface reference surface
Figure 36.5.2–9 Contact surface with zero offset.
Defining additional contact thicknesses for a contact pair interaction
You can specify a contact offset for a contact pair interaction in addition to any element thicknesses or midsurface offsets already defined for the elements underlying the contact pair surfaces. For small sliding this includes contact offsets defined by initial clearances (see “Specifying initial clearance values precisely” in “Adjusting initial surface positions and specifying initial clearances for contact pairs in Abaqus/Explicit,” Section 36.5.4). The specified offset value will be applied as an additional thickness of a layer separating the two surfaces, not as an additional thickness for each surface in the contact pair. This value can be positive or negative. This technique is often used in conjunction with softened behavior (see “Contact pressure-overclosure relationships,” Section 37.1.2) to model the thickness of a thin layer between two contacting surfaces.
Input File Usage: *SURFACE INTERACTION, PAD THICKNESS=value
Abaqus/CAE Usage: Interaction module: contact property editor: Mechanical→Geometric Properties: toggle on Thickness of interfacial layer (Explicit): value