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Academic year: 2022

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PCONT – Contact Property Description

Defines properties of CONTACT interface.


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PCONT 34 0.3 0.25

Enforced stick condition:


Field Contents

PID Property Identification number.

No default (Integer > 0)

GPAD “Padding” of the interface to account for additional layers, such as shell thickness. This value is subtracted from the contact gap opening as calculated from the location of nodes. See comment 1.

Default = THICK (Real, NONE or THICK)

STIFF Relative stiffness of contact interface. See comment 2.

Default: AUTO (AUTO, SOFT, HARD or Real > 0.0)

MU1 Coefficient of static friction (µs). See comments 3 and 4.

Default = 0.0 (Real > 0.0 or STICK or FREEZE)

MU2 Coefficient of kinetic friction (µk). (Ignored in linear analysis).


Default = MU1 (0.0 < Real < MU1)

CLEARANCE Prescribed initial gap opening between master and slave, irrespective of the actual distance between the nodes. See comment 11.

Default = not prescribed. (Real or blank).

FRICESL Frictional elastic slip – distance of sliding up to which the frictional transverse force increases linearly with slip distance. Specified in physical distance units (similar to U0 and GPAD). See Comment 7.

• Non-zero value or blank activates respective friction model based on Elastic Slip Distance.

• Zero value activates friction model based on fixed transverse stiffness KT.

Default = AUTO (Real > 0.0 or AUTO) Comments

1. The initial contact gap opening is calculated automatically based on the relative location of slave and master nodes (in the original, undeformed mesh). To account for additional material layers covering master and slave objects, the GPAD entry can be used. GPAD option THICK automatically accounts for shell thickness on both sides of the contact interface (this also includes the effects of shell element offset ZOFFS or composite offset Z0).

2. Option STIFF=AUTO determines the value of normal stiffness for each contact element using the stiffness of surrounding elements. Additional options SOFT and HARD create respectively softer or harder penalties. SOFT can be used in cases of convergence difficulties and HARD can be used if undesirable penetration is detected in the solution.

3. Prescribing MU1=STICK is interpreted in OptiStruct as an enforced stick condition – such contact interfaces will not enter the sliding phase. Of course, the enforced stick only applies to contacts that are closed. Note that, in order to effectively enforce the stick condition, frictional offset may need to be turned off (See comment 8).

4. Prescribing MU1=FREEZE enforces zero relative motion on the contact surface – the contact gap opening remains fixed at the original value and the sliding distance is zero.

Also, rotations at the slave node are matched to the rotations of the master patch. The FREEZE condition applies to all respective contact elements, no matter whether open or closed (hence, GPAD is of no relevance in this case). Also, this condition is effective irrespective of the frictional offset setting.

5. The CONTACT element force-displacement behavior is different in linear and nonlinear analysis (see Nonlinear Gap and CONTACT Analysisfor more information on nonlinear solutions). In linear analysis, the contact stiffness is constant and depends on the initial contact gap opening U0 as calculated from the positions of Slave and Master (and

considering padding GPAD). Note that for open contact elements, a very small stiffness value of KB=10-14*STIFF is used to avoid numerical singularities.


CONTACT element force deflection curve for linear analysis

The CONTACT force displacement behavior in nonlinear analysis is illustrated in the figure below. While the contact is open, its normal stiffness is essentially zero (a small value of KB=10-14*STIFF is used to avoid singularities). When the contact element closes, the stiffness becomes STIFF.

CONTACT element force deflection curve for nonlinear analysis

6. When contact is open, there is no transverse stiffness. When the contact is closed, friction is activated and the contact has stiffness KT=mu1*STIFF in the transverse direction (KT=0.1*STIFF in case of STICK). This acts as a linear spring in linear solution sequences. For nonlinear solution sequences, frictional force increases with sliding distance in proportion to KT until it reaches static friction force MU1 * Fx, Fx being the normal force in the contact element. With further transverse deformation, friction becomes kinetic and the friction force is MU2 * Fx. See the figure below for a one- dimensional illustration.


CONTACT element frictional behavior in nonlinear analysis

Note that the nonlinear contact element's force-displacement behavior may produce negative contributions to the compliance of the structure. For example, when slave and master bodies have initial overlap and the contact releases elastic energy during the solution.

7. Effective in Release 12.0, two models of friction are available in nonlinear analysis:

(a) Model based on fixed slope KT (previously existing),

(b) Model based on Elastic Slip Distance FRICESL (introduced in v12.0 and current default).

This latter model typically shows better performance in solution of frictional problems thanks to more stable handling of transitions from stick to slip. Key differences between the two available models are illustrated in the figure below (F1and F2represent two different values of normal force Fx):

Comparison of the two available friction models for contact elements.

• Model (a), based on fixed stiffness KT, is relatively simple, yet has certain drawback in modeling nonlinear friction. Namely, in Coulomb friction the frictional resistance

depends upon normal force. Using fixed KT will predict different range of stick/slip boundary for different normal forces, and thus may qualify the same configuration as stick or slip, depending on normal force.

• Model (b), based on Elastic Slip Distance, provides unique identification of stick or slip and generally performs better in solution of problems with friction. This model does require prescribing elastic slip distance FRICESL – for contact interfaces this value is determined automatically as 0.5% of typical element size on all Master contact surfaces.


The model (b), which is currently the default, is recommended for solution of nonlinear problems with friction. For backwards compatibility, the model based on fixed KT can be activated by prescribing FRICESL=0 on PCONT or CONTPRM card.

8. The model of friction in OptiStruct is relatively simple. The frictional force is always directed back to the point where the slave and master first came into contact (changed status from open to closed). Its location is estimated using proportional interpolation between the current position and the last converged solution before penetration.

OptiStruct CONTACT should not be used for modeling frictional problems with complex deformation paths and changing sliding directions.

9. The presence of friction can introduce moment loadings and counter-intuitive results into the problem by way of frictional offset. The reason for this is that, for contact elements with non-zero length (distance between slave node and master segment), the actual location of the contact interface is presumed to be in the middle of the contact element’s length (see figure below).

CONTACT presumed contact surface

The frictional forces act along this contact surface. Transferring these forces to the slave and master objects requires an offset operation that produces both forces and moments at slave and master. Similarly, the sliding distance at the contact interface is a result of nodal displacements and rotations of the slave node and master segment (see figure below).

CONTACT sliding with friction

Master segments, which consists of several nodes, can effectively resist these offset forces and moments. However, for slave bodies that do not support moments (nodes of solid elements, for example), this offset may render friction ineffective because the free rotations at slave nodes offer no effective resistance to friction. With the stick condition formally satisfied, for example, slave and master can move relative to each other (see figure below).


CONTACT stick (zero sliding distance)

In practice, for contact interfaces that are initially open, AUTOSPC will effectively fix respective unsupported rotations. However, for contacts that are initially closed (for example, pre-penetrating contact with MORIENT=NORMAL) the frictional terms will prevent AUTOSPC from being effective. Hence, respective SPC on rotations need to be applied manually to respective slave nodes.

Effective in the Release 12.0, to avoid such counter-intuitive behavior, the frictional offset operation is by default turned off if the model involves friction or stick and contains at least one nonlinear subcase (of NLSTAT type). (Note that for consistency, this affects both linear and nonlinear contact elements.) This produces more intuitive results with friction.

However, it may violate the rigid body balance of the body.

Note that FREEZE condition is enforced using a special formulation where the above caveat does not apply and the offset operation is always applied.

The above default setting can be changed via the GAPOFFS command on the GAPPRM card.

10. The presence of friction, due to its strongly nonlinear, non-conservative nature, may cause difficulties in nonlinear convergence, especially when sliding is present. If frictional resistance is essential to the solution of the problem and convergence problems are

encountered, enforcing the stick condition (by prescribing KT>0 and MU=0) may be a viable solution that will often result in better convergence than with Coulomb friction.

Please note however, that this only applies to problems in which minimal sliding is expected. In the case of larger sliding motions, the stick condition may lead to divergence through a "tumbling" mode.

11. Prescribing CLEARANCE overrides the default contact behavior of calculating initial gap opening from the actual distance between Slave and Master. CLEARANCE now becomes the distance that Slave and Master have to move towards each other in order to close the contact. Negative value of CLEARANCE means that the bodies have initial pre-penetration.

Warning: When prescribing CLEARANCE, it is important to correctly restrict the contact zones and pick search distance

SRCHDIS so that only desired Slave-Master pairs are involved. With prescribed CLEARANCE, all contact

elements created on a given interface, even those where Slaves are geometrically distant from the respective Master surface, will be considered to be at prescribed initial gap and participate in resolving the contact condition.

Note: CLEARANCE cannot be prescribed together with (non-zero) GPAD. Blank GPAD field in presence of CLEARANCE is interpreted as NONE.

12. This card is represented as a propertyin HyperMesh.


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