Products: Abaqus/Standard Abaqus/CAE
With the finite element method, curved geometric surfaces are naturally approximated as a faceted group of connected element faces. The use of a faceted surface geometry rather than the true surface geometry can significantly contribute to contact stress inaccuracy in contact interactions, especially when the magnitude of the differences between the faceted and true surface is not small with respect to the deformation of the components in contact. Contact stress output is of primary importance in many Abaqus/Standard applications; for example, the distribution of contact pressures can be used to identify wear patterns and peak pressure values to determine relative lives of machine parts. Furthermore, discontinuities in the surface normal direction at surface facet boundaries can contribute to convergence difficulties.
Abaqus/Standard offers techniques for overcoming the accuracy and convergence difficulties associated with faceted surfaces in contact interactions. These techniques allow a discretized surface with discontinuous surface normals to more closely approximate the behavior of a smooth surface with continuous normals during an analysis. The smoothing technique used in node-to-surface contact is different from the smoothing technique used in surface-to-surface and general contact:
Node-to-surface contact smoothing is applied by default and affects the entire master surface.
Surface-to-surface contact smoothing is not applied by default, but it can be applied to any surface regions whose geometry is roughly axisymmetric.
Surface smoothing in node-to-surface contact pairs improves numerical stability and sometimes improves solution accuracy. Slave nodes traveling along a master surface tend to “snag” on sharp corners, resulting in convergence difficulties. Because of this behavior, Abaqus/Standard automatically smooths the master surface in node-to-surface contact pairs. This smoothing technique recalculates the master surface normals along facet edges and, depending on the type of surface, may affect the surface geometry. The details of smoothing for node-to-surface contact formulations are discussed in “Smoothing master surfaces for the finite-sliding, node-to-surface formulation” in “Contact formulations in Abaqus/Standard,” Section 36.1.1, and “Using the small-sliding tracking approach” in “Contact formulations in Abaqus/Standard,” Section 36.1.1.
Smooth surfaces are not usually necessary in surface-to-surface contact to ensure analysis convergence; therefore, no smoothing is applied to these surfaces by default. However, an optional smoothing technique is available for improving the contact stress and pressure accuracy for axisymmetric (or nearly axisymmetric) surfaces in surface-to-surface contact interactions.
Surface-to-surface contact smoothing can be applied to specific surface regions. These regions must be roughly axisymmetric (all points on the surface are nearly equidistant from a single axis) or roughly spherical (all points on the surface are nearly equidistant from a single point). The pin insertion model in Figure 36.1.3–1 could benefit from surface-to-surface contact smoothing: the body of the pin and the hole are axisymmetric surfaces, and the head of the pin is a spherical surface. Surface-to-surface contact smoothing would also be effective if the surfaces were not perfectly axisymmetric or spherical; for example, if the pin body were slightly elliptical.
Surface-to-surface contact smoothing for contact pairs is enabled by creating a surface smoothing definition. A contact pair definition references this smoothing definition to apply geometric corrections in the contact formulation (the physical geometry of the model is not altered).
The surface smoothing definition lists all of the faceted regions in the contact pair surfaces that must be smoothed, as well as the geometry correction method that should be applied to each region. Two geometry correction methods can be employed:
A circumferential smoothing method is applicable to surfaces approximating a portion of a circle in two dimensions or a portion of a surface of revolution in three dimensions.
A spherical smoothing method is applicable to surfaces approximating a portion of a sphere in three dimensions.
Each surface-to-surface contact pair refers to a single smoothing definition; therefore, a smoothing definition must list all of the smoothed regions and applicable geometry correction methods for the contact pair. Geometry corrections can be applied to master surfaces and to slave surfaces; you can also apply corrections to selected regions of each surface. A surface smoothing definition can include multiple regions and different geometric correction methods for each region. For each region, you must specify the appropriate geometry correction method and either the approximate axis of revolution (for circumferential smoothing) or the approximate spherical center (for spherical smoothing).
| Input File Usage: | Use both of the following options to apply surface-to-surface contact smoothing: |
*CONTACT PAIR, GEOMETRIC CORRECTION=smoothing_name *SURFACE SMOOTHING, NAME=smoothing_name data lines to define smoothing regions (see below) Use the following data line to apply circumferential smoothing to surface regions with an axis of symmetry passing through points (Xa, Ya, Za) and (Xb, Yb, Zb): slave_region, master_region, CIRCUMFERENTIAL, Xa, Ya, Za, Xb, Yb, Zb Use the following data line to apply spherical smoothing to surface regions with a spherical center at point (Xa, Ya, Za): slave_region, master_region, SPHERICAL, Xa, Ya, Za Repeat the data lines as many times as necessary to define the appropriate geometry corrections for all surfaces in the contact pair. |
| Abaqus/CAE Usage: | Abaqus/CAE can automatically identify any surfaces in a contact interaction that will benefit from contact smoothing and apply the necessary geometry correction methods. |
Interaction module: contact interaction editor: Surface Smoothing: Automatically smooth geometry surfaces Surface-to-surface contact smoothing cannot be applied to surfaces on orphan mesh models in Abaqus/CAE. |
To improve contact pressure accuracy for the model in Figure 36.1.3–1, contact smoothing can be applied to both the master and slave surfaces. Two different geometric correction methods are required for the pin (the slave surface), so additional surfaces are defined corresponding to regions of the slave surface. Spherical smoothing is defined for the tip of the pin. Since the body of the pin and the hole share an axis of revolution, a single circumferential smoothing technique is applied to both of these surfaces. This surface smoothing definition applies even if the cross-sectional shapes of the pin and hole deviate from perfect circles.
*CONTACT PAIR, TYPE=SURFACE TO SURFACE, INTERACTION=FRICTION1, GEOMETRIC CORRECTION=SMOOTH1 PIN, HOLE *SURFACE INTERACTION, NAME=FRICTION1 *SURFACE SMOOTHING, NAME=SMOOTH1 PIN_TIP, , SPHERICAL, Xb, Yb, Zb PIN_BODY, HOLE, CIRCUMFERENTIAL, Xa, Ya, Za, Xb, Yb, Zb
Contact smoothing can be specified for surfaces in a general contact domain using a surface property assignment. A single surface property assignment specifies all of the surfaces to be smoothed, as well as the appropriate geometry correction method for each surface. General contact uses the same geometry correction methods as contact pairs:
A circumferential smoothing method is applicable to surfaces approximating a portion of a circle in two dimensions or a portion of a surface of revolution in three dimensions.
A spherical smoothing method is applicable to surfaces approximating a portion of a sphere in three dimensions.
| Input File Usage: | *SURFACE PROPERTY ASSIGNMENT, PROPERTY=GEOMETRIC CORRECTION data lines to define smoothing regions (see below) Use the following data line to apply circumferential smoothing to a surface with an axis of symmetry passing through points (Xa, Ya, Za) and (Xb, Yb, Zb): surface, CIRCUMFERENTIAL, Xa, Ya, Za, Xb, Yb, Zb Use the following data line to apply spherical smoothing to a surface with a spherical center at point (Xa, Ya, Za): surface, SPHERICAL, Xa, Ya, Za Repeat the data lines as many times as necessary to define the appropriate geometry corrections for all surfaces in the contact domain. |
| Abaqus/CAE Usage: | Contact surface smoothing can be applied only to native geometry models in Abaqus/CAE. By default, Abaqus/CAE automatically detects all surfaces in the general contact domain that can be smoothed and applies the appropriate smoothing. |
Use the following option to prevent automatic surface smoothing of a model: Interaction module: Create Interaction: General contact (Standard): Surface Properties: Surface smoothing assignments: Edit: toggle off Automatically assign smoothing for geometric faces Use the following option to manually apply smoothing to a surface: Interaction module: Create Interaction: General contact (Standard): Surface Properties: Surface smoothing assignments: Edit: Select surface, click the arrows to transfer surface to list of smoothing assignments. In the Smoothing Option column, select REVOLUTION to apply circumferential smoothing, select SPHERICAL to apply spherical smoothing, or select NONE to prevent smoothing of the surface. |
The surface-to-surface contact smoothing technique assumes that the initial locations of surface nodes lie on the true initial surface geometry, with the exception of midside nodes of higher-order elements. This smoothing technique remains effective even if the midside nodes of higher-order elements do not lie on the true initial geometry (models meshed using Abaqus/CAE always have midside nodes placed on the true initial geometry, but this may not be the case with other meshing preprocessors).
The effects of surface-to-surface contact smoothing tend to be most significant for analyses involving small deformation and coarse mesh discretization with first-order elements in the contact region; however, significant improvements to contact stress solutions are common even when the mesh is quite refined or higher-order elements are used. For analyses with large deformation this smoothing technique typically has an insignificant effect on solutions. However, in some cases the smoothing can degrade the solution accuracy after large deformation; therefore, it is not recommended to use surface-to-surface contact smoothing for large-deformation analyses. The effectiveness of surface-to-surface contact smoothing does not degrade upon relative motion between contact surfaces; for example, the smoothing technique works well for cases involving large sliding but small deformation.
The impact of contact surface smoothing can be demonstrated by a simple model of an interference fit between concentric cylinders modeled with first-order elements of different sizes, as shown in Figure 36.1.3–2.
Discrepancies between the true surface geometry and the faceted surface geometry result in noise in the contact pressure solution. If the interference distance and resulting deformation distance is small with respect to the geometry discrepancy, this noise can have a significant effect on the accuracy of the solution. Although surface-to-surface contact typically handles these discrepancies better than node-to-surface contact, it is not unusual for the maximum deviation from the analytical pressure solution to be upward of 100%. The effects of the noise become less apparent for larger deformations, but they are never completely eliminated.Modeling the interference fit with a surface-to-surface contact pair and using circumferential contact smoothing consistently yields low-noise pressure results that are within 3% of the analytical solution, regardless of the size of the interference distance. The effect is drastically noticeable for small-deformation analyses, but improvements can be observed even for larger deformations.
For a node-to-surface contact pair, increasing the smoothing fraction to the maximum value of 0.5 marginally reduces the noise in the pressure solution in a two-dimensional model. Increasing the smoothing factor in a three-dimensional model has little effect on accuracy, since physical surfaces are not smoothed for three-dimensional node-to-surface smoothing; see “Smoothing master surfaces for the finite-sliding, node-to-surface formulation” in “Contact formulations in Abaqus/Standard,” Section 36.1.1, for more information.
