Contact parameters
The default settings for these parameters should be used as a starting point, but often non-default values are appropriate depending on the behavior of the contact. The following sections describe the most common contact parameters and make general recommendations regarding their use. Contact parameters may be set using the commands *CONTROL_CONTACT, *CONTACT, and *PART_CONTACT. Certain parameters may be set using more than one command and so a command hierarchy must exist. Parameters set with *CONTROL_CONTACT redefine default settings for all contacts in the model. Contact parameters set in *CONTACT_ will override default settings for individual contacts. Contact parameters set in *PART_CONTACT supercede settings in *CONTACT for contact involving a specific part.
Thickness offsets, SLTHK (card 1, *CONTROL_CONTACT and Optional Card A in *CONTACT_option) AUTOMATIC (*CONTACT_option)
In crashworthiness analysis, sheet metal components are represented using shell elements with the nodal points at the mid-plane surface. Each shell has a thickness, ts, that by default is equal to the thickness of the sheet metal. When these components are included in the contact treatment, shell thickness offsets are used to project the mid-surface of the shell to create the surface for contact. The choice of the contact type determines whether shell thickness offsets are considered.
In LS-DYNA the non-automatic contact types:
*CONTACT_SURFACE_TO_SURFACE*CONTACT_NODES_TO_SURFACE*CONTACT_ONE_WAY_SURFACE_TO_SURFACE
use two different treatments depending on the parameter SHLTHK. This parameter can be specified globally on the *CONTROL_CONTACT card and locally for a given contact definition on optional card B of the *CONTACT input. If SHLTHK=0, an incremental search technique is used to determine the closest master segment and shell thickness offsets are not included. If SHLTHK=1, LS-DYNA considers the shell thickness offsets for deformable nodes but ignores the offsets for the nodes of rigid bodies. If SHLTHK=2, then LS-DYNA considers the thickness for both deformable and rigid nodes. For SHLTHK set to 1 or 2 a global bucket search is used to identify contact pairs. After contact is established, incremental searching is used to track the position of the slave nodes on the master surface. An advantage of global bucket searching is that the master and slave surfaces can be disjoint. This is impossible if incremental searching is used since incremental searching assumes that the contact surfaces are fully connected. In these contact types, it is important to orient the contact segment normals, based on the right-hand-rule, towards the contacting surface before the calculation begins. This is called oriented contact. An optional automatic orientation feature may be invoked using the parameter ORIEN on the *CONTROL_CONTACT card; however, for this option to work a gap must exist between opposing shell mid-plane surfaces. AUTOMATIC and single surface contact types always consider shell thickness offsets as shown in Figure 6.1. These contact types use both global bucket sorting and local incremental searching in determining the contact pairs. AUTOMATIC contacts are generally more robust than their nonautomatic counterparts since this contact type has no orientation requirement, i.e., contiguous segments do not obey the right-hand-rule. This is important in crash analysis since metal part can fold over and change the orientation. The contact search algorithm checks for penetration from either side of the shell mid-plane.
Shell Thickness Offset Recommendations
The AUTOMATIC contact types, which consider shell thickness offsets, are recommended for impact and crash analysis. If it is desired that shell thickness offsets of rigid components be disregarded, a non-automatic contact type may be used with the parameter SHLTHK set to 1 in either *CONTROL_CONTACT or on Optional Card B of *CONTACT. Additionally, it is important to ensure that the finite element mesh is constructed so that the shell mid-plane surfaces of the opposing parts are set apart by at least (ts+tm)/2 with meshes of similar density around sharp changes in curvature. If this condition is not satisfied, LS-DYNA will issue warning messages to indicate that penetrations were detected and that the penetrating nodes were moved to eliminate the penetrations. Sometimes the modification of the geometry can change the results. In version 960 of LS-DYNA, an option exists whereby penetrating nodes are not moved but rather the initial penetrations become the baseline from which additional penetration is measured. This option of tracking initial penetrations is invoked by setting the parameter IGNORE equal to 1 on Card 4 of *CONTROL_CONTACT or on optional card C of *CONTACT. We recommend that this option be used in most calculations. See Sections 6.4 and 6.5 for more on shell thickness offsets. In those sections, the term “contact thickness” refers to the magnitude of the shell thickness offsets.
Contact Sliding Friction, FS and FD (card 2, *CONTACT_option)
Contact sliding friction in LSDYNA is based on a Coulomb formulation and uses the equivalent of an elastic-plastic spring. Friction is invoked by giving non-zero values for the static and dynamic friction coefficients, FS and FD, respectively, in the *CONTACT or *PART_CONTACT input. For a detailed description of the frictional contact algorithm, please refer to Section 23.8.6 in the LS-DYNA Theory Manual.
Contact Sliding Friction Recommendations
Figure 6.1 Automatic Contact Segment Based Projection
When setting the frictional coefficients, physical values taken from a handbook such as Marks, provide a starting point. Note that to differentiate static and dynamic friction, FD should be less than FS and the decay coefficient DC must be nonzero. For numerically noisy problems such as crash, the static and dynamic coefficients are frequently set equal to avoid the creation of additional noise. The decay coefficient determines the manner in which the instantaneous net friction coefficient is ransitioned from FS to FD. The parameter, VC, provides a means to limit the frictional contact stress based on the strength of the material. The suggested value for VC is SIGY/sqrt(3) where SIGY is the minimum yield stress of the materials in contact. In LS-DYNA, version 960, the optional parameter FRCENG on card 4 of *CONTROL_CONTACT may be set to write the frictional contact energy to the binary interface database (*DATABASE_BINARY_INTFOR). Routinely, one automatic, single-surface contact with numerous dissimilar materials, are used in full vehicle simulations. In these cases, using a uniform value for FS and FD may be inappropriate. In such instances, it is recommended that the frictional parameters be specified part by part using the contact option in the part definition, *PART_CONTACT. It is helpful in understanding the sensitivity contact friction in a calculation by making two runs utilizing lower-bound and upper-bound friction coefficients.
6.3 Penalty Scale Factors, SFS and SFM (card 3, *CONTACT_option)
So-called penalty scale factors provide a means of increasing or decreasing the contact stiffness. SLSFAC in *CONTROL_CONTACT scales the stiffness of all penalty-based contacts, which have the parameter SOFT set equal to 0 or 2. SLSFAC is applied cumulatively with SFS, i.e., the actual scale factor is the product of SFS and SLSFAC, the slave penalty scale factor, or SFM, the master penalty scale factor, defined on card 3 of the *CONTACT input. SSF, when defined in *PART_CONTACT, is cumulative with the aforementioned penalty scale factors. For contacts with SOFT=1, the aforementioned penalty scale factors have no affect; rather SOFSCL on optional card A is used to scale the contact stiffness when SOFT=1. (SOFT is the first parameter specified on optional card A of *CONTACT.)
Penalty Scale Factors Recommendations
The default values (SFS=SFM=1.0; SLSFAC=0.1) generally work well for contact between similarly refined meshes of comparably stiff materials. For contacts involving dissimilar mesh sizes and dissimilar material constants, non-default values penalty scale factors may be necessary to avoid the breakdown of contact if SOFT=0. Generally, a better alternative than setting scale factors is to set SOFT=1 and leave all penalty scale factors at their default values.
Contact Thickness SST and MST (card 3, *CONTACT_option)
SST and MST on card 3 of *CONTACT allow users to directly specify the desired contact thickness. When the default value of SST=MST=0, is used, the contact thickness is equal to the element thickness specified in the *SECTION_SHELL card.
Contact Thickness Recommendations
Nonzero values of SST and MST are sometimes used to decrease the contact thickness and thus eliminate initial penetrations. This is a poor substitute for accurate mesh generation. When using nonzero values of SST and MST, it is highly recommended to use reasonable values. Specifying a very small thickness value, such as 0.1 mm, will result in contact breakdown owing to the fact that contact thickness goes into determining the maximum penetration allowed before the contact releases a penetrating node. Often, by increasing the contact thickness, breakdown of contact involving very thin materials can be averted. Based on experience, SST and MST should not be less than 0.6-0.7 millimeters. Since nonzero values of SST and MST are applied to all the parts defined in the contact, it may be more prudent to use the OPTT or SFT parameter in *PART_CONTACT to control the contact thickness for individual parts in cases where many parts of widely ranging thickness are included in a single contact.
Contact Thickness Scaling , SFST and SFMT, (card 3, *CONTACT_option)
As an alternative to directly specifying the contact thickness as described above, SFST and/or SFMT may be defined to serve as contact thickness scale factors. These factors are applied to the shell thickness specified in *SECTION_SHELL in order to obtain a contact thickness. The default values of SFST and SFMT are 1.0.
Contact Thickness Scaling Recommendations
The same concepts discussed in Contact Thickness Recommendations apply here. Care must be taken though not to assign contact thickness scale factors so small as to result in a contact thickness that is less than 0.6-0.7 mm.
Viscous Damping, VDC (Card 2, *CONTACT_option)
The viscous contact damping parameter, VDC, on card 2 of *CONTACT is zero by default. Originally, contact damping was implemented to damp out the oscillations that existed normal to the contact surfaces in sheet metal forming simulations. It has been found that contact damping is often beneficial in reducing high-frequency oscillation of contact forces in crash or impact simulations.
Viscous Damping Recommendations
In contacts involving soft materials such as foams and honeycombs, frequent instabilities exist due to contact oscillations. Using a value of VDC between 40-60 (corresponding to 40 to 60% of critical damping), it is found that the model stability improves; however, it may be necessary to reduce the scale factor for the time step size. Generally, a smaller value of 20 is recommended when metals, which have similar material constants, interact.
Contact Segment Extension, MAXPAR (Optional Card A, *CONTACT_option)
MAXPAR on Optional Card A of *CONTACT controls the enlargement of each contact segment that is needed to combat an inherent flaw in segment-based projection. This parameter is no longer used in the AUTOMATIC contact options, except for *AUTOMATIC_GENERAL, starting with version 950d of LS-DYNA. Figure 6.2 shows the contact surface that is projected from the shell mid-plane when using the segment-based projection scheme. It can be seen that at corners of convex surfaces, an open space or gap is present in the contact surface through which a slave node could freely enter without any contact detection. This can result in contact instability, negative contact energy, etc. due to a sudden, large penetration of a node that has entered through a gap. To combat this problem, the contact surface is automatically extended a slight distance parallel to the plane of the contact segment (as well as projected normally from the segment). This slight extention serves to close the gap in the contact surface. In versions starting with 950d, a cylindrical surface is created in the valley which is used as the contact surface with the forces acting normal to the surface.
Segment Extension Recommendations
The default value of MAXPAR (1.025) works well for most analyses, as most sheet metal components are not much greater than 3-4 mm. However, contact instabilities may develop when a part with a very large thickness (> 5-10mm) or having an angular surface is present in the contact definition. Such an instability may be corrected by reducing the contact thickness (discussed in earlier sections) or by increasing the segment enlargement parameter MAXPAR (to as high as, but no greater than, a value of 1.2). Refining the mesh to reduce sharp angles in the contact surface will also help. A certain cost penalty is paid for MAXPAR values greater than default.
Bucket-Sort Frequency, BSORT (Optional Card A, *CONTACT_), NSBCS, (Card 2, *CONTROL_CONTACT)
Bucket sorting refers to a very effective method of contact searching to identify potential master contact segments for any given slave node. This sorting is an expensive part of the contact algorithm so the number of bucket sorts should be kept to a minimum to reduce runtime. If thickness offsets are considered, then all contact types use the bucket sort approach to track the most probable contacting segments. BSORT specifies the number of time steps between bucket sorts. Depending on the contact type, the default bucket sort interval is between 10 and 100 cycles. Except for high speed impact, this interval is almost always adequate. The contact bucket searching frequency should increase, i.e., BSORT should be reduced, if nodes move from one disconnected surface to another in short time intervals or if the surface is folding onto itself. If two relatively smooth simply-connected surfaces are moving across each other without folds, the bucket sorting can be done at larger intervals. Note that if the surfaces are more than several segment widths away from each other, no information is stored related to future contact, and later bucket searching is required to pick up future contacts. Once a slave node is in contact, local searching tracks the motion, and bucket sorting for the nodes, which are in contact, is not necessary.
Bucket-Sort Frequency Recommendations
In certain contact scenarios where contacting parts are moving relative to each other in a rapid fashion, such as airbag deployment, more frequent (than default) bucket sorting intervals may improve the contact behavior. A tell-tale sign inadequate bucket sorting is the appearance of certain penetrating nodes inexplicably being bypassed in the contact treatment. In such cases, using the BSORT parameter
a) MAXPAR = 1.0 b) MAXPAR = 1.2
Figure 6.2 Segment extension using MAXPAR. This option is now obsolete in the AUTOMATIC contact types.
in *CONTACT or NSBCS in *CONTROL_CONTACT, the user can decrease the cycle interval between bucket sorts. Rarely will a value of less than 10 be required.
6.9 Maximum Penetration, PENMAX (Optional card B, *CONTROL_CONTACT), XPENE (Card 2, *CONTROL_CONTACT)
To avoid instability in models, slave nodes that penetrate too far are eliminated from the contact algorithm; however, they remain in other calculations. This is done so that very high forces, which are proportional to large penetration values, are not applied to the penetrating nodes that might lead to instabilities. It’s also necessary for contacts that consider shell thickness offsets to prevent a sudden reversal in direction of contact force as a penetrating node passes through the shell midplane. In non-automatic types and SHLTHK=0, the default maximum penetration is set to 1e+20. In other words, no nodes are released at all. When SHLTHK=1 or 2, the XPENE parameter determines the nodal release criteria and is given as follows:
Max Distance (Solids) = XPENE (default=4.0) * (thickness of the solid element), SHLTHK=1Max Distance (Solids) = 0.05 * (thickness of the solid element), SHLTHK=2Max Distance (Shells) = XPENE (default=4.0) * (thickness of shell element), SHLTHK=1Max Distance (Shells) = 0.05 * (minimum diagonal length), SHLTHK=2
In AUTOMATIC types and single surface, excluding AUTOMATIC_GENERAL, the maximum allowable penetration is a function of PENMAX that is set to a default value of 0.4 (40%). The maximum allowable penetration in these cases are shown below:
Max Distance = PENMAX * (thickness of the solid)Max Distance = PENMAX * (slave thickness + master thickness)
For *AUTOMATIC_GENERAL only, the default value of PENMAX is set to 200 and provides an almost no nodal release criteria.
6.9.1 Maximum Penetration Recommendations
It is generally recommended that parameters affecting maximum penetration not be changed from the default values. If nodes penetrate too far and are released, the preferred solution is to increase the contact stiffness, change the penalty formulation (SOFT), or increase the contact thickness.
sb 2001