Designing for driver mistakes

Credit: ti-ja

Driver mistakes, such as running over a curb or skidding against a traffic barrier, can test a vehicle’s peak load or strength capacities. These durability load cases play a major role in the product development process since they potentially drive the design for several components.

At Volvo, 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 the requirement is that all functions remain intact with no visible or noticeable deformation of any component of the vehicle. Level 2 covers customer misuse and a certain amount of damage is accepted with a safe failure mode. Structural deformations are acceptable but there should be no separation or breakage. For Level 2 events, designers try to create predetermined, inexpensively replaceable components that deform and protect neighboring components, a design principle known as chain of failure.

Simulating vehicle misuse

To simulate peak load with a high level of confidence is of great importance to setting the design loads for components and studying vehicle behavior in these events. Volvo uses Adams multibody dynamics software from MSC Software Corp. to simulate Level 1 load cases for driving over a curb 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, a good fit for Level 1 load cases.

On the other hand, Level 2 load cases involve plasticity and buckling of flexible bodies that until recently Adams software had not been able to simulate with sufficient levels of accuracy. The skid-against-a-curb load case is verified with physical testing with a known mass hitting the vehicle at a specified velocity and impact angle. These tests require prototype hardware that is expensive to build and only available later in the product development cycle.

“We wanted the capability to simulate Level 2 load cases to be able to evaluate design of suspension components earlier in the development cycle, without having to build hardware for each design alternative,” says Anders Wirje, technical expert CAE Durability at Volvo Car Corp.

Software upgrade

MSC recently introduced the Adams-Marc co-simulation capability that 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. Post processing is done separately – Adams results in Adams, and Marc results in the Marc post-processor. Or using Computational Engineering International’s EnSight post-processor, both Adams and Marc results can be imported.

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 lower control arm (LCA) to the subframe. The rest of the half-vehicle model are included in the Adams/Car model. Due to the extreme nature of a peak load event, component modeling is critical to simulation accuracy. All components have to be described within their full range of excitation.

Key components and behavior to model include:

• Contacts between curb and tire and between curb and rim
• Elastomers (bushings)
• Camber stiffness of the suspension
• Flexibility and plasticity/buckling of structural components

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.

Simulations of skids in Volvo cars showed no structural deformation during extreme maneuvers.

The simulated event has a duration of 0.7 sec. in clock time. The communication interval is 0.0005 sec. in clock time with total simulation time of 40 min. on a Dell laptop with 16GB of RAM and a 2.7GHz CPU.

Simulation results

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 will make it possible to substantially improve the product development process.

“From the early stages of the development process, we will be able to evaluate the performance of alternative designs in terms of their performance under Level 2 loads,” Wirje says. “The ability to quickly and easily look at alternatives at a time when we are not locked into any particular approach should make it possible to meet performance requirements with a lighter suspension that can improve the fuel economy of the vehicle. At the same, we should be able to reduce the cost and time involved in suspension development by performing product development more accurately from the beginning so fewer prototype verification cycles are required. Of course, full physical verification will be performed at the end of the project.”

MSC Software Corp.

www.mscsoftware.com

Volvo Car Group

www.volvo.com