Simulating Transient Dynamic Events
By Bill Kelly
In order to accurately and efficiently simulate the engineering mechanics of actual events, a structural analyst must use the proper tools for the job. The time scale of an event is a key factor in the selection of the computational algorithms that should be used. In the context of finite element analysis of transient events, there are two primary solvers to choose from: implicit or explicit. The most commonly used method is implicit, and engineers will often default to using that technique. Perhaps their software does not provide an explicit solver, or they are unfamiliar with the advantages of an explicit solver. Explicit solvers are far better than implicit for handling high speed events like collisions and ballistics. Also, highly nonlinear events with significant body interactions, or which involve highly nonlinear material properties or behavior also benefit from an explicit solver.
Transient events require a time-stepping finite difference solution to the equations of motion. Without getting into too much detail, the explicit solver handles the time integration such that the solution at a specific time point depends only on the solution of the previous time points. In doing so, it requires a very small time step for convergence, but once that is established, the mathematics required is straightforward and quick. An implicit scheme allows for larger time steps, but it requires an inversion of the stiffness matrix at each step. So for any particular time step solution, the explicit solver is tremendously faster than the implicit. The issue is that the time step itself must be tremendously short. In fact, the time step needs to be less than the travel time of an acoustic wave across the smallest element. So to reap the benefits of an explicit solver, in general, the event to be simulated must be short duration. Analysts therefore use it for simulating things like ballistic impacts, explosions, and car crashes. There are some advanced techniques for broadening the applicability of the explicit solver, and improving the solution efficiency of longer term highly nonlinear events too.
As the animations depict, at MSI we’ve used the LS-DYNA explicit solver (recently acquired by ANSYS) for a variety of simulations including nonlethal bullet clay penetration, explosion effects on a tethered manhole, long distance sliding frictional contact, anti-reverse ratchet pin impact, and biological soft tissue deformations. It would have been impractical and inefficient to simulate these events with an implicit solver. The explicit solver greatly reduced computational time for these problems, and also circumvented common issues that implicit solvers have with nonlinear convergence.
How a ratchet really works
Manhole cover explosion tether modeling
Non-lethal projectile penetration modeling