Implicit and Explicit Finite Element Methods for Crash Safety Analysis

Explicit method is commonly used in crashworthiness analysis due to its capability to solve highly non-linear problems without numerous iterations and convergence problems. However, the time step for explicit methods is limited by the time that the physical wave crosses the element. Therefore, to avoid large amount of CPU time, the explicit method is usually used for non-linear dynamic problems with a short period of simulation duration. For problems under quasi-static loading conditions at pre-crash and post-crash, implicit method could be more efficient than explicit methods because the required computation time is much shorter. Due to the recent advance of crash codes, which allows both implicit and explicit computations to be performed in the same code, crash engineers are able to use explicit computation for crash simulation as well as implicit computation for some of the pre-crash quasi-static loading or post-crash spring back simulations. Hence, the advantage of implicit method for saving computation time and cost and the advantage of explicit method for its stability in solving highly non-linear crash problems can be combined. In this paper, pre-crash occupant setting and vehicle gravity setting are simulated utilizing the implicit method. Incessantly, the rear impact and frontal impact simulations are conducted using the explicit method. Therefore, the elastic deformations of seat and vehicle structure due to gravity obtained from implicit simulations are carried over as the initial conditions for explicit crash simulations. The results of pre-crash simulations from implicit method are discussed and compared with those from explicit method.