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Crash codes pave the way to safer vehicles.

Manufacturers of all types of transportation equipment are increasingly making their suppliers responsible for design and fabrication of entire subassemblies that must meet stringent performance specifications. At the same time, suppliers are beginning to assume responsibility for aspects of subsystem design that they have not handled in the past. Without a doubt, the most important and challenging of these areas is vehicle safety.

As suppliers attempt to come to grips with the challenges of subsystem safety engineering, they face a field that is undergoing tremendous change. The automobile and other transportation industries are focusing more attention than ever before on reducing occupant injuries during crashes. For example, federal regulations that became effective in 1989 require that manufacturers of mass transportation vehicles must submit all designs whose purchase will involve funds from urban mass transit authorities to the Bus Testing Facility in Altoona, Pa., for a series of stringent structural tests. in addition, more stringent Federal Motor Vehicle Safety Standards (FMVSS) are expected soon; these will include side-impact crash standards as well as basic standards for light trucks.

Fortunately, a variety of crash analysis software tools are coming to the aid of the design engineer. Performing crash analysis in software is intended to serve as a low-cost alternative to destructive testing using fully dressed prototype vehicles. Full-sized crash prototypes must typically be built with custom non-production tooling and amply equipped with instrumentation and high-speed photographic gear. Costs can often range from $250,000 to $500,000 for a single vehicle.

Computerized crash simulation technology, in contrast, allows an unlimited number of simulations to be run from a single model, with each simulation taking only a matter of hours. While measurements in physical testing of actual crash models are hampered by the practical limitations of sensors and associated instrumentation, computer simulation enables the engineer to examine velocity, acceleration, and stress data at as many different points in the structure as desired. Computer simulation also allows ready computation of relative velocities between any pair of points.

Nevertheless, computer modeling is not meant to replace physical testing, but to complement it. Often, physical testing leads to the discovery or isolation of specific problems, indicating the need for a redesign. At that point, a computer model can be used to analyze the problem in more detail and try out prospective solutions.

Different Stages

It is important to note that different types of simulation are used at different stages of the product development cycle. In the early concept phase, simplified models can help direct engineers along the right design path. For example, performing a static finite element analysis on an automobile door frame can help engineers understand how the door will perform in a side crash. Simplified system models consisting solely of masses, springs, and dampers used to simulate a frontal crash can also provide valuable guidance at this stage. The data to construct these models are frequently available through physical testing of earlier models that shared similar components.

During the later preprototype and prototype stages, crash simulation can provide even greater benefits. Critical subsystems that perform energy-absorbing functions in a crash can be independently evaluated to determine their effectiveness. Examples include bumpers, doors, the instrument panel and knee bolster assembly, seats, and hoods.

Automotive crash analysis is often handled by special-purpose codes such as PAM-Crash from Engineering Systems International (Rungis-Cedex, France); DYNA3D, developed by Lawrence Livermore National Laboratory in Livermore, Calif. (see January ME, pp. 56-60); Radioss Crash from Mechalog (Paris); and Madymo from TNO Research Institute (Delft, the Netherlands). These programs apply the finite element method to model vehicle structures using a wide range of elements including volumetric, shell, beam, and slide-contact types. Special-purpose crash analysis programs typically use an explicit time integration scheme that makes it possible to solve large dynamic problems.

Bumper Testing

One of the first subsystems for which responsibility for meeting safety standards has been assigned to suppliers is automotive bumpers. Current U.S. government requirements call for bumpers to withstand a 2 1/2-mile-per-hour impact test without damage. But many carmakers are asking their suppliers to meet their own corporate standards, which are more stringent. For example, General Motors Corp. (Detroit) has instituted standards geared to a 5-mph impact. Many insurance companies are also pressing for 5-mph bumpers. At the same time, with the advent of air bag systems and their sensing requirements, automobile companies are increasingly requiring that bumpers contribute to the deceleration of the automobile in high-speed crashes, where in many cases bumpers instantly disintegrate. Even-higher-speed impact tests for bumpers are being considered for future safety regulations.

At the concept level of the product design cycle, a relatively coarse finite element model can be used for dynamic transient analysis and design optimization loops. A systems approach is required that takes into account the responses of fascia, beam, brackets, and shocks. One example of an automated pre- and post-processor routine to format a bumper system for nonlinear static and dynamic transient finite element analysis with a minimum of effort is Easi-Bumper, developed by Engineering Analysis Services Inc. (Auburn Hills, Mich.). Three impact cases contained in FMVSS No. 581 can be considered: a fixed-barrier impact; a pendulum striking the center of the bumper, which simulates a head-on crash with another car; and a pendulum striking the corner of the bumper, which simulates an oblique corner collision. With such software, a materials supplier can use a PC to screen a variety of bumper material candidates for suitability. The best candidate can then be selected for use in the final design and development effort.

During the detailed design stage, full-blown crash analysis is required. A detailed model of the bumper and supporting structure is normally created while the rest of the vehicle is represented with a coarse model or approximated with a lumped mass system. The bumper and the support structure can then be redesigned, so that the bumper makes the necessary contribution to the overall deceleration pulse. Here. simulation of quick early deceleration is essential to ensure timely air bag sensing and deployment.

The instrument panel and knee bolster is another vehicle subsystem for which suppliers are increasingly being asked to take responsibility for safety engineering. Present regulations require that the secondary impact at 15 mph of the occupant with the knee bolster during a frontal collision cannot exceed 2250 pounds of force. The force requirement arises from the fact that the human femur might be fractured if a load of over 2250 pounds is applied. An excessively stiff structure will result in high knee loads, and a very soft structure will result in high knee displacements and "submarining" of the occupant. The optimal stiffness can be arrived at by exploring alternate materials and geometry configurations using a crash model.

During a recent study conducted by Engineering Analysis Services to develop a knee bolster, a variety of short- and long-fiber composite materials, along with different geometry alternatives, were considered. The femur, tibia, and knee caps were modeled along with the knee bolster to capture the kinematics of the occupant's lower body. The study resulted in a knee-bolster design that meets government standards for femur loads. Static Strength

Scat performance in a crash is another area that is becoming more important. improving the structural characteristics of the seat is one approach to better crash performance. Another trend is to mount the seat belt anchor directly on the seat; however, this imposes further structural demands on the seat. Present regulations require that the belt anchor be capable of withstanding a 6000-pound pull. This parameter can usually be studied with conventional static finite element analysis. Portable seats used in minivans and recreational vehicles add more complications: they must be light enough to be easily carried and strong enough to resist damage if accidentally dropped.

To date, the only requirement for automobile doors has been for static strength. This has generally been met by placing inside the door frame a beam whose strength can be easily measured with static finite element analysis. Because a new side-impact crash test requirement is expected to be released soon, automobile manufacturers and suppliers are scrambling to improve door crashworthiness. One idea being explored is door-mounted air bags. These will present tremendous design and analysis problems. The key difficulty is that door-mounted air bags will have to deploy in one-tenth the time allowable for front-mounted models. This is because in side impacts, the crush space between the door and the occupant is much smaller than the crush space in the engine compartment that helps absorb front crashes. Another approach to side-impact

protection involves the use of protection devices that include energy-absorbing materials such as foam padding on the door. Here, recently developed tools include Engineering Systems International's PAM-Safe, which offers good potential for simulating the deployment of the air bag and its subsequent interaction with the occupant.

Another vehicle subsystem involved in crash analysis is the hood. Currently, there is no federal requirement for hoods, but windshield intrusion regulations affect how hoods are designed. In order to prevent the hood from moving into the windshield, designers try to construct them so that they will fold up like a tent upon impact. Here, the most difficult problem is making the center of the hood fold as crisply as the sides. The standard technique is to strategically locate "weak spots" on the hood inner panel to initiate folding during a crash, without compromising the hood's static strength and stiffness. The size, location, and number of these initiators can be evaluated with crash analysis software.

Complementary Process

Analysis, design, testing, and manufacturing must be carefully coordinated so that both the assumptions and recommendations made by the FE analyst are acceptable to all parties. The manufacturer should be willing to openly discuss all pertinent issues relating to the design so that the analyst can, in turn, generate innovative ideas born out of analytical insight.

The large suppliers like Gencorp Automotive, Rockwell International, Dow Chemical, and GE Plastics are aware of computerized crash analysis and some of them are already actively using it in their development cycles. But the vast majority of suppliers have had only a casual exposure to computerized analysis.

Extracting the maxim from crash analysis during the product development cycle requires more than simply hiring analysts and procuring computer hardware and software. Managing crash analysis necessitates a thorough understanding of the tools, their scope and limitations, and how crash analysis fits in with design and test activities. Perhaps most important, it means establishing a corporate culture that promotes the use of engineering analysis and analysis tools.
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Title Annotation:computer programs that simulate vehicle crashes
Author:Krishnaswamy, Prakash; Mani, Ayyakannu
Publication:Mechanical Engineering-CIME
Date:Apr 1, 1991
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