Oasys LS-DYNA Environment 8.1 VOLUME 3 ... - Oasys Software

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Oasys LS-DYNA Environment: User Guide (Version 8.1) INTERIOR CONTACT Interior contact is used with meshes of very soft solid elements (eg foams) which are prone to invert under severe loading conditions. It is equivalent to a single surface contact in which contact segments of a finite thickness are attached to every internal and external solid element face. When the elements are severely crushed, these segments contact each other and resist further crushing, thus preventing element inversion (negative volumes). An alternative method is to coat the outer surfaces with null shell elements, and define contact between the null shells. OUTPUT Two options are available for recovering forces on contact surfaces. Time histories of total force against time for surface-to-surface and nodes-to-surface contact types can be obtained from the .XTF file via OASYS T/HIS. This output is always generated automatically. The forces are time-averaged over the interval between outputs. Plots of the contact stress distribution can be obtained by requesting a .CTF file when submitting a job. In this case contact stresses are dumped to the .CTF file at the same time as complete state data is dumped to the .PTF file. Contact stresses (which are instantaneous values, not timeaveraged) can then be plotted by OASYS D3PLOT. The total force on a single surface contact is zero by definition, since the action and reaction of every penetrating node is included. However, the total force on any subset of a single surface contact can be obtained using the *CONTACT_FORCE_TRANSDUCER option. This contact type does not generate any force itself: it outputs the total force due to all the contacts on the slave set to which it refers. CONTACT SEARCH METHODS There are two methods used by LS-DYNA for detecting which segment is being contacted by each node. Further details are given in the theory manual. In summary, the methods are: - Mesh Connectivity Tracking, in which neighbouring segments can be identified because they share nodes. When one segment is no longer contacted by a node, the neighbouring segments are checked. - Bucket sort, in which the 3-D space occupied by the contact surface is divided into cubes ("buckets"). Nodes can contact any segment in the same bucket or a next-door bucket. The Mesh Connectivity method is used by contact types 3, 5, 8, 9 and 10 (unless SHLTHK on *CONTROL_CONTACT is non-zero). The main disadvantage is that the mesh must be continuous for the contact surface to work correctly. These contacts are rarely used. Page 9.6
Oasys LS-DYNA Environment: User Guide (Version 8.1) The Bucket Sort method is applied in all the commonly used contact surfaces. It is much more robust but can be slow if the contact surface contains large numbers of segments (or elements). The bucket sort is used by types a3, a5, a10, 14, 16 and all single surface types; in addition, types 3, 5 and 10 use the bucket sort if SHLTHK on *CONTROL_CONTACT is non-zero. CONTACT ORIENTATION Contact segment orientation defines which side of the surface is "solid" and which is "air". Contacting nodes can move freely on the "air" side but penetrations into the "solid" side are resisted. If the contact surface is on solid elements, LS-DYNA automatically sets the contact orientation correctly whatever the contact type and input method. The rest of this section describes what happens with contacts on shell elements. All single surface contact types, and types a3, a5, a10, 14 and 16, check for contact on both sides of the shell element. This is possible because the contact plane is always offset from the nodal plane and the contact depth does not cross the nodal plane. Therefore there is no requirement to orient the elements or contact segments in any particular way. The orientation of contact types 3, 5 and 10 can be set automatically by LS-DYNA, provided that the slave and master surfaces are initially separated by a gap. To do this, either define the contact surface by a list of parts and a box, or set ORIEN on *CONTROL_CONTACT to 2. Thus, in summary, contact orientation needs to be checked by the user only for contact types 3, 5 and 10 on shell elements. If defining the surfaces by list of parts, ensure initial gaps are present. If defining the surfaces by segments, either create the model with the segments correctly oriented or include initial gaps and set ORIEN to 2. CONTACT DEPTH Contact types 3, 5 and 10 For contact types 3, 5 and 10, the contact depth is 1 x 10 10 by default (in effect, infinite). Therefore any slave node which passes behind a master segment (or vice-versa) will feel a force proportional to the distance from the segment. Sometimes this is due to the geometry of the situation rather than a genuine case of contact - e.g. Figures 9.2, 9.3. Where the distance is large, the force will be large and can cause the model to blow up. To prevent this, it is possible to specify a maximum depth, beyond which contact penetrations are to be considered spurious and ignored by LS-DYNA. There are two ways to do this: either - define the distance as MAXPEN on the *CONTACT cards or - set PENCHK (on *CONTACT) to 1. In this case the maximum depth is XPENE*thk, where XPENE can be set on *CONTROL_CONTACT (or simply left as default = 4.0), and thk is the shell element thickness. For solid elements thk is volume/area. XPENE applies to the whole model and cannot be adjusted Page 9.7
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Oasys LS-DYNA Environment 8.1 VOLUME 3 ... - Oasys Software

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