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

More documents

Recommendations

Info

Oasys LS-DYNA Environment: User Guide (Version 8.1) The distortional kinetic energy is defined as: Distorsional k.e Total k.e Total mass (mass averaged velocity) 2 Typical values are 1E-3 (loose) - 1E-6 (tight) convergence. The degree of error associated with a given convergence tolerance depends on the structure and the loading history. If only displacement results are required, a looser tolerance may be used than if stresses are wanted. For preload applications a loose tolerance is generally sufficient. The damping constant is a number which multiplies all nodal velocities at each timestep. Its value must lie between 0 and 1 (the closer to 1, the less the damping). It is important to choose the damping constant carefully; ideally the solution should be critically damped. Over damping will result in the calculation taking too long to converge, sometimes by several orders of magnitude. Under damping will result in overshoot, sometimes resulting in large errors. Overshoot can also cause errors with path-sensitive systems, e.g. models involving friction. To calculate the correct damping constant, it is necessary to estimate the lowest natural frequency of the structure, and the solution time step. The damping constant (k) is given by: k = 1 - 2cω n dt where c = desired fraction of critical damping (normally 1.0) ω n = lowest natural frequency of oscillation (rad/s) dt = timestep LOADING Any of the LS-DYNA loading methods involving load vs time functions can be used (e.g. point or pressure loads, prescribed velocities, etc). Initial velocity conditions cannot be used to load the dynamic relaxation phase; the velocities are initialised to zero, and user-specified initial velocities are stored for use in any transient analysis which follows the dynamic relaxation. Load curves may be set to act during dynamic relaxation only, during normal transient analysis only, or during both phases (SIDR on *DEFINE_CURVE). Thus it is possible to use different methods for each phase. Load curves for dynamic relaxation should contain a constant load to allow a converged solution to be reached. It is good practice to ramp the load initially to avoid sudden loading and over-excitation of higher modes. 2 Page 13.2
Oasys LS-DYNA Environment: User Guide (Version 8.1) Figure 13.1 Typical Load vs Time Curve for Dynamic Relaxation TERMINAL VELOCITY If a constant load is applied to an unrestrained dynamic relaxation then the body will reach a “rigid body” terminal velocity that is given by the following equation. V term F dt k m (1 k) where F = desired fraction of critical damping (normally 1.0) dt = timestep k = damping constant m = mass of body Although this calculation may not appear to be of much use in itself it can be used as a guide to the velocity that will be obtained in the analysis if parts have to move together. OUTPUT By default no output (except convergence data in the .LOG and .OTF files) is produced during dynamic relaxation, and the time is reset to zero when convergence is reached. Complete State and Time History data are always output on convergence. The subsequent transient analysis will then show non-zero displacements and stresses (the static solution) at time zero. The finish time specified on *CONTROL_TERMINATION does not apply to the dynamic relaxation phase. The finish time will determine how long the subsequent transient phase lasts. A finish time of zero will give a static solution only. To obtain plot file information during dynamic relaxation, include a *DATABASE_BINARY_D3DRFL card. A binary file (jobname.rtf) will then be written, containing plot states at each convergence check. This file can be read into OASYS D3PLOT 7.0 for viewing, and in structure is identical to the PTF file. Page 13.3
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: Oasys LS-DYNA Environment: User Gui
Page 118 and 119:
Page 120 and 121:
Page 122 and 123:
Page 124 and 125:
Page 126 and 127:
Page 128 and 129:
Page 130 and 131:
Page 132 and 133:
Page 134 and 135:
Page 136 and 137:
Page 138 and 139:
Page 140 and 141:
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

Create successful ePaper yourself

Delete template?

Save as template?