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

More documents

Recommendations

Info

Oasys LS-DYNA Environment: User Guide (Version 8.1) There are many ways to handle thickness in contact. The old algorithms (i.e, those using node based contact searching) exclude thickness but are not recommended. The old types 3, 5 & 10 can be made to include thickness for the blank only or for both blank and tools; and the new automatic routines (using segment searching) will always include thickness. However, the thickness used in either slave or master side can be scaled or re-defined on the *CONTACT_ card as required and is also affected by flags on the *CONTROL_CONTACT card (which can also be over ruled by Optional Card B). Which option used depends on how the upper and lower tools are defined (whether they are offset from each other, which surface of the tool they represent, etc.). This issue must be handled carefully as an error can cause unrealistic splits due to trapped material. The default algorithm generally recommended for forming simulation is *CONTACT- FORMING_ONE-WAY_SURFACE_TO_SURFACE. This is a fast algorithm that can cope with discontinuities in the tool mesh. Blank thickness should be included but the tool thickness is generally ignored (by setting shlthk = 1 on *CONTROL_CONTACT). However, the tools can be offset by specifying a negative value of thickness in the *CONTACT card. This is very useful if the punch and die are not offset in the CAD data. Alternatively, many pre-processors (e.g. eta/DYNAFORM) can offset a mesh by a specified normal direction to achieve the same result. Friction between tool and blank is specified by setting the Coulomb coefficient on the *CONTACT card. Typical metal to metal contact would be represented by a value of fs ³ 0.15 (higher for aluminium blanks). A lower value would be used if lubrication was being employed. More complex relationships between the friction coefficient and relative velocity and/or pressure can be implemented, if suitable data is available by setting fs = 2 and using *DEFINE_TABLE. User defined subroutines can also be set up. A useful option to consider with penalty based contact methods is contact damping; this can help reduce the oscillations when tool and blank contact at speed and gives improved prediction of punch force. The value to be entered is a percentage; 20 is a typical value. The *CONTACT_ENTITY card must be used if the IGES or VDA options are used. Constraint types of contact can be used in sheet metal forming, but can only be used on one side of the blank. TRIMMING LS-DYNA includes an algorithm to trim material from the blank (and re-mesh to a specified trim line). This has been implemented chiefly for a more realistic springback calculation (as the trimming material can relieve stresses, significantly affecting the results) but can also be used as part of a multistage forming operation. Note that this is not a simulation of the physical trimming process but simply a procedure to delete elements and nodes in the region to be trimmed. Where the trim line passes through an element, that element is adapted, according to a tolerance set by the user. A tighter tolerance will more accurately follow the trim curve but will produce smaller elements. Page 17.6
Oasys LS-DYNA Environment: User Guide (Version 8.1) The trimming option is set up using the *DEFINE_CURVE_TRIM option. The trim data can be either x,y data points (projected into the correct plane using a *DEFINE_VECTOR card as necessary) or IGES or VDA data contained in a file. An arbitrary number of trim curves may be specified as long as curves do not intersect and each element is trimmed by only one curve. Curves may extend outside the part to be trimmed but LS-DYNA always treats a curve as a closed loop. Material can be removed either inside or outside the specified line. Note that pretrimming is also possible using this option by specifying a *ELEMENT_TRIM card; if this card is not in the deck then trimming occurs at the end of the run. A special procedure is required to trim an adaptive mesh - see the *INTERFACE_SPRINGBACK card for details. A users guide to adaptive mesh trimming is available; please contact Oasys for more details. Alternatively, dedicated metal forming pre-processors, such as eta/DYNAFORM, are often capable of trimming a model. CONTROL CARDS Certain control card options are suggested for sheet metal forming. The *CONTROL_SHELL card includes a flag called istupd; this should be set to 1 so that membrane strains in shells cause thickness changes which is important for sheet metal forming simulation. The penalty stiffness value and the thickness flags (including shlthk and ssthk) on the *CONTROL_CONTACT card interact with the contact definitions and should be checked carefully. If a non-continuous mesh is used for the tools it may be necessary to increase the bucket search frequency. Scaling the tool velocities has been mentioned. It is also possible to speed up the analysis using the mass scaling options, with the same caveats about dynamic effects. At the simplest level, mass scaling can be implemented by simply increasing the density of the blank material. However, the *CONTROL_TIMESTEP card allows a minimum time step to be maintained by setting dt2ms = negative value of desired timestep (be aware of the mass increase termination option set on the *CONTROL_TERMINATION card). Mass scaling is easier to implement than velocity scaling as only one value has to be changed. In practice, “a bit of both” is used, with modest levels of velocity scaling combined with mass scaling adjusted to check for dynamic effects. Adaptive remeshing has already been mentioned and is defined using the *CONTROL_ADAPTIVE card (and do not forget to set the appropriate flag on the *PART card for the blank). A tolerance of 5 degrees and three levels of adaptivity are good starting options. Previously, the more expensive, option to repeat the previous cycle after adaption (adpass=0) was advisable otherwise new nodes created by adaptivity could be on the “wrong” side of the contact surface. The latest version of LS-DYNA allows adaptivity to take place before contact between blank and tool takes place. This is achieved by setting adpene to a positive value. The Page 17.7
Page 1:
Oasys LS-DYNA Environment 8.1 VOLUM
Page 4 and 5:
Oasys LS-DYNA Environment: User Gui
Page 6 and 7:
Page 8 and 9:
Page 10 and 11:
Page 12 and 13:
Page 14 and 15:
Page 16 and 17:
Page 18 and 19:
Page 20 and 21:
Page 22 and 23:
Page 24 and 25:
Page 26 and 27:
Page 28 and 29:
Page 30 and 31:
Page 32 and 33:
Page 34 and 35:
Page 36 and 37:
Page 38 and 39:
Page 40 and 41:
Page 42 and 43: Oasys LS-DYNA Environment: User Gui
Page 142 and 143:
Page 144 and 145:
Page 146 and 147:
Page 148 and 149:
Page 150 and 151:
Page 152 and 153:
Page 154 and 155:
Page 156 and 157:
Page 158 and 159:
Page 160 and 161:
Page 162 and 163:
Page 164 and 165:
Page 166 and 167:
Page 168 and 169:
Page 170 and 171:
Page 172 and 173:
Page 174 and 175:
Page 176 and 177:
Page 178 and 179:
Page 180 and 181:
Page 182 and 183:
Page 184 and 185:
Page 186 and 187:
Page 188 and 189:
Page 190 and 191:
Page 192 and 193:
Page 194 and 195:
Page 196 and 197:
Page 198 and 199:
Page 200 and 201:
Page 202 and 203:
Page 204 and 205:
Page 206 and 207:
Page 208 and 209:
Page 210 and 211:
Page 212 and 213:
Page 214 and 215:
Page 216 and 217:
Page 218 and 219:
Page 220 and 221:
Page 222 and 223:
Page 224 and 225:
Page 226 and 227:
Page 228 and 229:
Page 230 and 231:
Page 232 and 233:
Page 234 and 235:
show all

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

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?