Cars, even when driven by careful drivers, will almost definitely experience misuse, peak load or strength events, such as driving over a curb or skidding against a curb a few times during their lifetimes. These durability load cases play can significantly influence the design for several components. At Volvo, the “driving over a curb” and “skid against a curb” strength events are classified into two categories, Level 1 and 2. Level 1 represents extreme customer usage and requires that all functions remain intact with no visible or noticeable deformation of any component of the vehicle. Level 2 involves user misuse, and in this case a certain amount of damage is accepted with a safe failure mode – structural deformations are acceptable but there should be no separation or breakage. Here, a chain of failure is established so that in most conceivable scenarios, a predetermined inexpensively replaceable component deforms and protects neighboring components.
The ability to perform peak load simulation with a high level of confidence is very importance to setting the design loads for components and studying vehicle behavior in these events. Volvo uses Adams multibody dynamics software to simulate Level 1 load cases for driving over and skidding against a curb. The components of interest are modeled as linear flexible bodies in Adams. This allows for linear material response for flexible bodies so this method is only valid up to small plastic strains which is a good fit for Level 1 load cases. On the other hand, Level 2 load cases involve plasticity and buckling of flexible bodies for which there has not been a way to simulate with sufficient accuracy till the release of the Adams-Marc co-simulation.
MSC’s Adams-Marc co-simulation capability makes it possible to include geometrically and materially nonlinear structural behavior in multibody dynamics simulation. Any Adams model and any Marc model can be used in co-simulation with this tool. Post processing is done separately, Adams results in Adams and Marc results in the Marc postprocessor, or using Computational Engineering International’s (CEI Inc.) EnSight post-processor which can import both Adams and Marc results. When setting up the co-simulation model for the skid against curb load case, the Marc model contains the lower control arm and bushings connecting the LCA to the subframe whereas the rest of the half vehicle model are included in the Adams/Car model. Because of the extreme nature of a peak load event, component modeling is absolutely critical to simulation accuracy. All components have to be described within their full range of excitation.
Adams runs a dynamics analysis while Marc runs a quasi-static analysis which means that mass and inertia of the component is not accounted for. It would also be possible to run a transient analysis in Marc that would take mass effects into account. Adams leads the co-simulation and then feeds its results to Marc. Marc interpolates the Adams results to catch up and passes the results to Adams which extrapolates them in taking the next step. The simulated event has a duration of 0.7 seconds in clock time. The communication interval is 5e-4 seconds in clock time. The total simulation time was a very reasonable 40 minutes on a Dell laptop with 16 Gigabytes of RAM and a 2.7 GHz CPU.
The Adams – Marc co-simulation of the Volvo S80 front suspension accurately predicted the behavior of a Level 2 skid against a curb load case. The low velocity impact (Level 1) and high velocity impact (Level 2) cases showed the same behavior as the physical tests.
The ability to accurately simulate Level 2 load cases makes possible to substantially improve the product development process. Co-simulation allows Volvo to quickly and easily look at alternatives to develop a lighter suspension and improve the fuel economy of the vehicle while reducing the cost and time involved in suspension development.