When running problems using Implicit solution sheme in LS-DYNA, the default iterative non-linear solver used is the BFGS method that employs a ‘Quasi-Newton’ method in which the global stiffness matrix is reformed only every ILIMIT steps and in between these a relatively inexpensive update to the stiffness matrix is performed. This default stiffness matrix update…
When solving static or quasi-static type problems, the default Implicit Static solver (IMAS=0 in CONTROL_IMPLICIT_DYNAMICS) requires a well-conditioned model, with no rigidbody modes, to get good convergence behavior. It is often difficult to prevent rigidbody modes especially when its dependent on contact-impact conditions. In such cases, use of Implicit Dynamics solver (IMAS=1) can help us…
For non or poorly converging implicit solutions, the parameter D3ITCTL parameter in *CONTROL_IMPLICIT_SOLUTION may come handy to isolate regions of interest. When D3ITCTL is non-zero, LS-DYNA outputs the model information at each Iterative step into a binary file named D3ITER which is in the same format as D3PLOT. The number of steps for which the…
The known advantage with implicit simulations is that the solution is unconditionally stable allowing larger values of timestep. In implicit static simulations (IMASS=0 in *CONTROL_IMPLICIT_DYNAMICS), the simulation time has no real significance but is rather an indication of the applied load magnitude. For example if we have a simulation where a load is applied linearly…
When migrating input files or using internal switching from Explicit to Implicit solution type, the influence of strain-rate effects, if defined for the materials used, should always be considered. If the problem is intended to be static or quasi-static during the implicit solution, it is recommended that strain-rate effects be ignored. LS-DYNA now offers a…
Under-integrated elements are widely popular in the explicit field for their robustness and computational efficiency. However, their use in Implicit without appropriate stabilization methods can cause singularities and may result in poor convergence behavior. LS-DYNA offers parameters such as ISHELL, ISOLID, and IBEAM in the keyword *CONTROL_IMLPICIT_EIGENVALUE which can be used to switch to an…
There are two classical solution methods available in LS-DYNA to solve a given problem. The widely popular “Explicitâ€? solution scheme, using the central-difference method, is applied to short-duration transient dynamic problems while the “Implicitâ€? solution scheme is used for static problems. Both of these methods have their advantages and disadvantages which depend entirely on the…
