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34 NASS-GES data do not contain functional class informa- ride or underride of a barrier system. The HVOSM program tion). A more detailed description of the development of has been modified and improved over the years and has been these assigned operating speeds is found in Appendix B. used for studying dynamic behavior of vehicles traversing It should also be noted that since the goal of this project various types of terrain. Development on HVOSM stopped, was to define curb guidelines for higher-speed roads rather however, about 20 years ago as commercial vehicle dynam- than city streets, and as directed by the project panel, a speed ics codes supplanted it. HVOSM is now rarely used and vehi- limit of 40 mph (65 km/h) was used as the lower boundary cle suspension properties for modern passenger vehicles are for most of the analyses conducted; all exceptions are noted. not readily available for HVOSM. VDANL is a comprehensive vehicle dynamics simulation program that runs on a PC in a Windows environment (39). COMPUTER SIMULATION METHODS It was designed for the analysis of passenger cars, light trucks, articulated vehicles and multipurpose vehicles and has been As discussed in Chapter 2, computer simulation has been upgraded over the years to expand and improve its capabili- used to assess the safety effectiveness of curbs since the late ties. It now permits analysis of driver-induced maneuvering 1960s. Many of these analyses were performed using HVOSM, within limit conditions defined by tire saturation characteris- a rigid body vehicle dynamics code. Although early computer tics, as well as driver feedback control features. One of the programs were limited in their abilities (due in large part to significant advantages of using VDANL is that there is a computational constraints), the results of those analyses have large library of vehicle inertial and suspension properties provided a great deal of information regarding the effect of available. Many of those properties have been validated by curb impact on vehicle kinematics. Vehicle dynamics codes NHTSA using full-scale test track results. The one drawback have come a long way since the 1960s and are now able to of VDANL is that it is cannot simulate vehicle impact with provide very accurate results regarding vehicle kinematics. an object and thus terrain must be smooth and continuous. FEA is another computer simulation method that was use- This is because the program only simulates vehicle response ful in the study of curb and curbbarrier combinations. This due to interaction between the bottom of the tires and the method had not been used previously to study vehicle inter- ground. When a tire interacts with a curb that has a steep face, action with curbs, but it has been used extensively in recent the contact will occur at a point higher up on the tire (i.e., not years to study vehicle impacts with roadside hardware. Since on the bottom of the tire), which cannot be accurately simu- the early 1990s FEA has rapidly become a fundamental part lated with VDANL. of the analysis and design of roadside safety hardware sys- tems. In addition to being a reliable and relatively inexpen- sive means of analyzing and simulating impact events, it Nonlinear, Dynamic Finite Element Codes allows the analyst more control over the impact conditions and provides information about the mechanics of the impact For the simple event of vehicles traversing curbs, FEA event (stress, strain, energy, etc.) at specified time increments provides little additional information about the kinematics of during impact. FEA is also capable of dealing with the highly the vehicles than could be obtained through use of today's nonlinear behavior associated with material properties, large vehicle dynamics codes. FEA was, however, invaluable in deformations, and strain rate effects. The advantages and dis- the analysis of impacts with curbbarrier combinations. Vehi- advantages of using vehicle dynamics programs and FEA are cle dynamics codes only provide information regarding vehi- discussed in the following sections. cle kinematics and cannot provide information about the vehicle interaction with the barrier. The performance of traf- fic barriers installed in conjunction with curbs cannot be Vehicle Dynamics Codes directly analyzed using vehicle dynamics codes, because they are not designed to account for deformations of the vehi- The HVOSM is a vehicle dynamics program that has been cle or barrier. Since vehicle dynamics codes only address used extensively in conjunction with full-scale crash testing suspension and inertial forces, they are not appropriate for to study vehicle dynamics during impact with curbs (14). use when a vehicle strikes a barrier. A vehicle striking a bar- Vehicle dynamics codes calculate the motions of the vehicle rier experiences forces arising from the interaction of the by modeling the vehicle as a series of rigid one-dimensional vehicle body and the barrier itself. These forces are highly elements like springs, dampers, and masses. The tire and sus- nonlinear and usually involve large deformations, plastic pension models are the heart of a vehicle dynamics code since behavior, and, often, failure of materials. the only forces acting on the vehicle are presumed to arise In FEA the entire substructure with its many parts and from the tire interaction with the ground and inertia. The type complicated shapes is divided into smaller units (finite ele- of information that can be obtained from such analyses is ments) that are interconnected at discrete points (nodes). The related to the kinematics of the vehicle, such as vehicle tra- stresses, strains, and motions of the model are computed at jectory, roll, pitch, and yaw. The trajectory of the vehicle has the element level and are then combined to obtain the solu- historically been used as a measure of the potential for over- tion of the whole body. The advantage of FEA is that the
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35 body of the vehicle is not rigid, and thus it can deform in a collision. Parametric analyses are particularly straightforward, realistic manner during impact, whether it be the simple elas- using simulation so that the variation of speeds and angles tic deformations involved in transferring the load through the can be examined to find the critical impact conditions at framework of the vehicle when crossing curbs or the large, which poor performance might occur. Simulation provides a plastic deformations involved in vehicle impacts with road- method to explore a wide variety of curbbarrier combina- side safety barriers. tions that would provide the broadest type of information for Vehicle dynamics codes have been used in previous stud- development of guidelines for the use of curb or curb ies to determine the potential for vaulting over or underriding barrier combinations. The primary drawback of finite element barriers. In those studies, however, such potential was only simulations is that they must be validated to make sure that speculated based on the vehicle's trajectory after crossing a the predictions are realistic. curb; an actual impact event is much more complicated. FEA There are several public domain vehicle models available can provide detailed information about the impact event, from the FHWA/NHTSA National Crash Analysis Center at including vehicle kinematics prior to and during interaction George Washington University that have been validated for with the barrier, as well as damage sustained by both the various impact conditions. A list of currently available vehi- vehicle and the barrier. FEA can also provide vehicle accel- cle models appears in Table 8. eration data that can be used for measuring injury risk factors Of the vehicle models listed in the table, the 1994 Chevro- of vehicle occupants. let C-1500 reduced model has been used most widely by WPI For many years, full-scale crash testing was the primary researchers in particular and the Centers of Excellence com- method of determining the effectiveness of roadside safety munity in general. While any of the models listed in Table 8 hardware. More recently, there has been a great deal of could have been used in this project, there is often consider- advancement in computation power and in code develop- able work needed to make a model useable in a particular ment (40). As a result the use of FEA for simulating collision impact scenario. The 1994 reduced model of the Chevrolet events has become a reliable and widespread tool for inves- C-1500 was the easiest model to use since it had been widely tigating crashworthiness of roadside safety structures. used and debugged. The 1994 Chevrolet C-1500 (detailed In 1998, the FHWA began the Centers of Excellence Pro- model) and the 1993 Ford Taurus were also reasonably gram, in which it funds leading research organizations, debugged but most of the other models had not been widely including Worcester Polytechnic Institute (WPI), to investi- used outside of the NCAC and might have required signifi- gate the impact performance of various roadside safety hard- cant debugging to be useful in this research. ware. LS-DYNA was chosen by the FHWA to serve as the The basic procedure used by the researchers at WPI in pre- primary analysis tool to be used by the centers. LS-DYNA is vious projects using FEA to examine roadside hardware has a nonlinear, dynamic, explicit finite element code that is very three steps: (1) build the finite element models, (2) validate efficient for the analysis of vehicular impact and is used them using crash tests found in the literature, and then (3) use extensively by the automotive industry to analyze vehicle the validated models to develop alternative designs. This pro- crashworthiness (41). It evolved from DYNA3D, public cedure was followed in this project to ensure that the guide- domain software developed in the mid- to late 1970s by John lines were based on models that had been validated against Hallquist at Lawrence Livermore National Laboratory. LS- observable physical phenomena (e.g., crash tests). DYNA's efficiency in simulating contact between various parts in a finite element model, along with its ability to effec- tively use underintegrated elements, has put LS-DYNA at Validation of Computer Models the forefront of the nonlinear dynamic finite element soft- ware industry. Computer simulations were validated by comparing the One advantage of FEA is that it is easy to vary parameters simulated results to those obtained from full-scale crash tests. and assess exactly the structural and dynamic context of the The accelerations at the center of gravity of the vehicle in the TABLE 8 Public domain vehicle models available from the National Crash Analysis Center Vehicle model type 1998 Oldsmobile Cutlas Ciera 1996 Ford F-Series Truck 1994 Chevrolet C-1500 (detailed model) 1997 Geo Metro 1994 Chevrolet C-1500 (reduced model) 1993 Ford Taurus 1996 Plymouth Neon Honda Accord Chevrolet Lumina Dodge Intrepid Ford Crown Victoria Ford Explorer
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36 simulation and the full-scale test were compared using four viously. There was good agreement between the test and the quantitative techniques: simulation with respect to velocity histories, event timing, exit conditions, guardrail damage, and guardrail deflections, 1. the Numerical Analysis of Roadside Design (NARD) as well as the TRAP, NARD, Geers, and ANOVA evaluation validation parameters, parameters. A summary of major impact events, the time at 2. the analysis of variance (ANOVA) method, which they occurred, and the corresponding velocity of the 3. the Geers parameters, and vehicle is presented in Table 9. Both the qualitative and 4. the Test Risk Assessment Program (TRAP). quantitative comparisons of the finite element simulation to the physical crash test indicate that the simulation results rea- The NARD validation procedures are based on concepts of sonably replicate the guardrail performance in the test. signal analysis and are used for comparing the acceleration- As an example of the use of FEA in this project, the vali- time histories of finite element simulations and full-scale tests dated model of the G4(2W) was used to simulate a Test Level (42). The ANOVA method is a statistical test of the residual 3 impact event involving the G4(2W) with a 150-mm-high error between two signals (43). Geers' method compares the AASHTO Type B mountable curb located just behind the magnitude, phase, and correlation of two signals to arrive face of the W-beam. The results are shown in Figure 23. at a quantitative measure of the similarity of two acceleration- The impact conditions were the same as those in TTI Test time histories (44). TRAP is a software program that was 471470-26. A rear view of both of the simulations (i.e., with developed to evaluate actual full-scale crash tests and gener- and without a curb) is compared in Figure 24. From the ate important evaluation parameters like the occupant impact results of the simulations it appears that the 150-mm-high velocities (OIVs), ride down accelerations, 50 msec average AASHTO Type B curb placed behind the face of the G4(2W) acceleration, and so forth. The program calculates standard- guardrail system will likely cause serious instability when ized occupant risk factors from vehicle crash data in accor- the vehicle exits the system. It is commonly observed in full- dance with the NCHRP guidelines and the European Commit- scale tests involving the 2000-kg pickup truck impacting var- tee for Standardization (CEN) (45). Using the same evaluation ious roadside barriers that when the rear tire contacts the bar- rier, the rotation of the tire tends to pitch the rear of the software for finite element simulations and full-scale tests vehicle upwards, as shown in Figure 21. This phenomenon is further simplified the comparisons between actual physical further amplified when a curb is placed in combination with tests and mathematical simulations. the guardrail. When the rear wheel hits the curb, an initial vertical displacement of the wheel prior to tire interaction Applicability of FEA to Roadside Barrier with the barrier results, as demonstrated in Figures 23 and 24. Impact Studies The high pitch and exit angle of the vehicle during impact with the curbguardrail combination make the post-impact Researchers at WPI had considerable experience using behavior of the pickup very unpredictable. Rollover would the LS-DYNA program for simulating vehicle impacts into be very likely given the exit conditions shown in Figure 23. roadside hardware (46). As part of previous FHWA projects, Typically, during impact with strong-post guardrail sys- Plaxico and Ray had developed finite element models of vari- tems without a curb present, the front wheels of the pickup ous roadside structures that were used to assess the impact per- truck remain in contact with the ground over much of the formance of the systems. All the models were validated with event, which in effect reduces the lateral deflection of the the results of full-scale crash tests (31). These models included system during impact and also decreases the redirection the breakaway cable terminal; the MELT terminal; a weak- angle of the vehicle as it exits the system. In this finite ele- post guardrail system; and two strong-post guardrail systems, ment simulation of the curbguardrail combination the vehi- the G4(1W) and G4(2W) (4750). The G4(1W) and G4(2W) cle was completely airborne during the time that it was in are both blocked-out strong-post W-beam guardrails; the contact with the barrier, resulting in increased lateral deflec- G4(1W) uses 200 x 200mm wood posts; and the G4(2W) tion of the barrier and a much higher angle of redirection of uses 150 x 200 mm wood posts. The G4(1W) is used in Iowa, the vehicle. The total deflection of the system in the simula- and the G4(2W) is used in a number of other states. tions with and without a curb was 0.79 m and 0.71 m, respec- A finite element model of the G4(2W) guardrail had been tively (i.e., the deflection in curbbarrier combination was developed by researchers at WPI as part of a study sponsored 11.2% greater). The redirection angles of the vehicle in the by the Iowa Department of Transportation and the FHWA simulations with and without a curb were 14 and 21 degrees, (46). Simulations of Report 350 Test 3-11 impact conditions respectively. The redirection angle of the vehicle in the curb were performed with the model, and the results were com- guardrail simulation exceeded the allowable exit angle spec- pared to a full-scale crash test performed by TTI that estab- ified in NCHRP Report 350. According to criteria M of lished that the guardrail system successfully met the stan- Report 350, the exit angle from the test article should be less dards set in NCHRP Report 350 (31). Figures 21 and 22 than 60% of the test impact angle, measured at time of vehi- compare the FEA to the results of the full-scale crash test. cle loss of contact with test device. The exit angle in the This model was validated using the methods described pre- curbguardrail simulation was 84% of the impact angle.
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37 Figure 21. Sequential photographs for TTI Test 471470-26 (left) and G4(2W) finite element simulation (right).