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Preload general

A "dynain" file contains a set of keyword data that can be inserted into an input deck in order to initialize deformation, shell thickness, element history variables (stress, plastic strain, material-model-dependent extra history variables), and tensorial strains. LS-DYNA will write a dynain file at the end of an analysis if the keyword command *INTERFACE_SPRINGBACK_LSDYNA is included in the input deck (see Users Manual for command syntax). Alternately, LS-Prepost can write a dynain file corresponding to any output state stored in the D3PLOT database by selecting Output > dynain ASCII.

If the preload induces contact forces, dynain is not an ideal approach for initializing a subsequent analysis since dynain does not initialize the contact forces. This omission will disrupt the state of equilibrium attained in the preload analysis if contact forces are produced by the preload. Dynamic oscillation of response is expected in the 2nd analysis until contact forces are reestablished. Furthermore, the new state of equilibrium will be slightly different than the original state of equilibrium.

A hypothetical, unproven approach to account for contact forces in the dynain approach would be to set the contact birthtime (BT) on card 2 of *CONTACT to a small value, e.g., 1e-10, and also set the shooting node logic flag SNLOG on Optional Card B of *CONTACT to 1. Furthermore, IGNORE should be 0 and SOFT should be 0 or 1. The idea here is that the initial penetrations would not be removed during initialization and then, after one time step, the contact is birthed and contact forces corresponding to the penetration are created.   In this approach, equilibrium is absent in the first time step as contact forces are zero but then equilibrium is approximately restored in the second time step.

This issue of uninitialized contact forces is circumvented in alternate approaches such as a dynamic relaxation approach, all-in-one (preload followed by other loadings in a single transient analysis) approach, or a restart.

A better way to approach the problem may be to use dynamic relaxation (DR) to preload the system. The dynamic relaxation phase can be explicit transient, implicit transient, or implicit static.

 

To run implicit Dynamic Relaxation:

  1. Set IDRFLG=5 and set the dynamic relaxation termination time DRTERM in *CONTROL_DYNAMIC_RELAXATION
  2. Add *CONTROL_IMPLICIT commands as necessary to govern implicit analysis in DR phase. As a minimum, you'll need *CONTROL_IMPLICIT_GENERAL with DT0 set to a positive value.

  3. Ramp preload linearly using *DEFINE_CURVE command(s) with SIDR=1. The ramp time for preload is typically equal to DRTERM (see step 1).

  4. Include *DATABASE_BINARY_D3DRIF to write output from the implicit dynamic relaxation phase (recommended).  The output interval is given in units of cycles, not time.

  5. *CONTROL_IMPLICIT_DYNAMICS governs whether implicit analyses are static or transient.

  6. To initialize velocity as the conclusion of the dynamic relaxation phase, set iphase=1 on *INITIAL_VELOCITY_GENERATION

 

As an alternative to implicit DR is switching from implicit to explicit via a curve (1st parameter in *CONTROL_IMPLICIT_GENERAL is negative curve ID). This does NOT allow for initialization of velocities at the beginning of the explicit phase.

Thermal loads (*LOAD_THERMAL) are sometimes used to thermally shrink bolts in order to induce preload due to bolt torque.