2006-01-0246v001安全带,乘员运动学,翻车,MADYMO

2006-01-0246 Modeling the Effects of Seat Belt Pretensioners on Occupant Kinematics During Rollover William Newberry, William Lai, Michael Carhart, Darrin Richards,Jeffrey Brown and Christine RaaschExponent Failure Analysis Associates, Inc.Reprinted From: Side Impact, Rear Impact and Rollover 2006(SP-1997)2006 SAE World CongressDetroit, MichiganApril 3-6, 2006The Engineering Meetings Board has approved this paper for publication. It has successfully completed SAE's peer review process under the supervision of the session organizer. This process requires a minimum of three (3) reviews by industry experts.All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of SAE.For permission and licensing requests contact:SAE Permissions400 Commonwealth DriveWarrendale, PA 15096-0001-USAEmail:permissions@Tel:724-772-4028Fax:724-776-3036For multiple print copies contact:SAE Customer ServiceTel:877-606-7323 (inside USA and Canada)Tel:724-776-4970 (outside USA)Fax:724-776-0790Email:CustomerService@ISSN 0148-7191Copyright © 2006 SAE InternationalPositions and opinions advanced in this paper are those of the author(s) and not necessarily those of SAE. The author is solely responsible for the content of the paper. A process is available by which discussions will be printed with the paper if it is published in SAE Transactions.Persons wishing to submit papers to be considered for presentation or publication by SAE should send the manuscript or a 300 word abstract to Secretary, Engineering Meetings Board, SAE.Printed in USAABSTRACTThe results of a number of previous studies have demonstrated that seat-belted occupants can undergo significant upward and outward excursion during the airborne phase of vehicular rollover, which may place the occupant at risk for injury during subsequent ground contacts. Furthermore, testing using human volunteers, ATDs, and cadavers has shown that increasing tension in the restraint system prior to a rollover event may be of value for reducing occupant displacement. On this basis, it may be argued that pretensioning the restraint system, utilizing technology developed and installed primarily for improving injury outcome in frontal impacts, may modify restrained occupant injury potential during rollover accidents. However, the capacity of current pretensioner designs to positively impact the motion of a restrained occupant during rollover remains unclear. Moreover, the pretensioner characteristics necessary to limit occupant motion and head excursion without exacerbating injury potential associated with restraint contact have not been established. In the present study, we utilized pretensioner testing and computational analysis to evaluate the capacity of pretensioners for altering and reducing occupant head excursion during the early phase of a steer-induced rollover. This study builds upon existing MADYMO models that have been validated for the prediction of occupant head, neck and torso kinematics during the airborne phase of vehicle rollover (Newberry et al., 2005; Lai et al., 2005), as well as experiments which evaluated the occupant motion during the trip phase of steer-induced rollover (Yamaguchi et al., 2005). Although there are many approaches to pretensioner design, most employ pyrotechnic energy that is released when a crash is sensed to reduce the effective length of the restraint system. The capacity of such a pretensioner to retract webbing is likely a function of the seatbelt loads at the time of deployment. In the current study, we conducted bench tests of a typical pyrotechnic retractor-based pretensioner to evaluate its capacity to retract webbing at various loads. It was found that applying a transient force function, as quantified in bench testing, could simulate the pretensioner’s performance. Additional bench tests were conducted to characterize the transient force pulses of several retractor pretensioners. To evaluate the efficacy of these pretensioners in limiting head excursion in the early phase of MADYMO. This simulation focused on the time period from steer initiation until first roof contact, and was validated by comparison of simulated motions of the ATD to those measured during the rollover tip-up tests conducted by Yamaguchi et al. (2005). Vertical and lateral excursion of the head and torso were quantified to evaluate pretensioner performance. Pretensioners with characteristics similar to those found in the existing fleet were found to only modestly affect occupant kinematics and head excursion during the early phase of vehicle rollover.INTRODUCTIONThe rollover automobile accident represents a formidable challenge in occupant protection and safety engineering. Each year in the U.S. there are more than 220,000 rollover accidents resulting in nearly 10,000 fatalities (NHTSA, 2005).The use of the standard 3-point automobile restraint offers substantial protection for an occupant in a rollover, primarily by preventing occupant ejections and projected impacts with the vehicle interior. This benefit is clearly demonstrated by statistical analysis of accident data. James et al. (1997) estimated that unbelted rollover occupants are nearly 5 times more likely to be fatally injured than belted occupants. Similarly, Digges et al. (1998) found that the serious and fatal injury rate for unrestrained occupants is approximately 4.2 times as great as for restrained occupants. Factoring in the economic cost of injuries, Digges et al. determined that unrestrained occupants sustained 84% of the “total harm” associated with rollovers, while their restrained counterparts sustained just 16%.Restrained occupants may still suffer severe and fatal injuries (Digges and Malliaris 1998). This finding can be attributed, in part, to the nature of typical occupant motion during the airborne phase of a rollover. Occupants tend to be held in an upright position by centrifugal motion, which projects them upward and outward relative to the compartment. Under these conditions, typical 3-point restraints may not prevent the head of the occupant from possibly contacting the roof rail. When the vehicle subsequently strikes the ground, injurious head and neck loading may result.2006-01-0246Modeling the Effects of Seat Belt Pretensioners onOccupant Kinematics During RolloverWilliam Newberry, William Lai, Michael Carhart, Darrin Richards,Jeffrey Brown and Christine RaaschExponent Failure Analysis Associates, Inc. Copyright © 2006 SAE InternationalThere are indications that limiting the upward and outward excursion experienced by an occupant during the airborne phase of a rollover may reduce injuries to restrained occupants by mitigating the injury mechanism described by Bahling et al. (1990). This injury mechanism involves the head stopping as a result of being in contact with, or in close proximity to, the roof at the time of a significant roof-to-ground impact and the torso continuing to move downward, loading the neck in a “diving-type” fashion. One potential mechanism to limit occupant excursion involves the use of seatbelt pretensioners. It is well accepted that seatbelt pretensioners improve frontal impact safety by enhancing the coupling between the occupant and vehicle in the early stages of a collision. Maximizing the over-ground distance and duration of occupant deceleration in this manner adheres to basic principles in restraint design (Eppinger, 2001), and has been shown to reduce maximum loads, accelerations, and injury parameters experienced by occupants in frontal collisions (Miller 1996; Müller and Linn 1998; Bohman and Boström, 2000).Although there are many approaches to pretensioner design, most employ pyrotechnic energy that is released when a crash is sensed to reduce the effective length of the restraint system. Pretensioners have been developed for both the retractor and for the buckle. When a crash is sensed, the pyrotechnic material is ignited and the expanding gas pushes a rack gear or piston, thus taking up the slack in the seatbelt webbing.As described above, pretensioners have been shown to improve occupant restraint and reduce injury potential in frontal impacts. Although pretensioners are standard equipment on many new vehicles, they are generally deployed only in response to frontal impact conditions. The efficacy of these devices in controlling occupant motion during dynamic rollover events remains unclear.Previous studies have demonstrated that increasing the tension in the restraint system (the lap belt in particular) prior to rollover may be of value in controlling occupant displacement and flail (Moffatt et al., 1997; Arndt 1998). Recently, Hare et al. (2002) conducted full-scale vehicle testing to investigate pretensioner performance during rollovers. In this study, eight full-scale dolly tests were performed using a late model SUV (2001 Pathfinder) with restrained Hybrid III 50th-percentile male anthropomorphic test devices (ATDs) in both front-seated positions. In a subgroup of these tests, the retractor pretensioners standard on this vehicle were electronically fired at a fixed time relative to the dolly release, at a point in the trip event when the vehicle roll angle measured approximately 10 degrees. Results of this study demonstrated that the standard vehicle pretensioner was able to remove modest amounts of webbing from the shoulder portion of the restraint system. This effect was influenced by the seated position of the occupant relative to the roll direction, with near- and far-side pretensioner removing an average of 25 mm and 50 mm, respectively. Despite this favorable result, the determination of the effectiveness of the pretensioners in improving occupant injury exposure was inconclusive, as the test-to-test variation in roll dynamics was large. The difficulties experienced by Hare et al. (2002) demonstrate a fundamental difficulty with full-scale rollover testing. That is, vehicle roll dynamics are notoriously variable (Bahling et al, 1990), even when elaborate test fixtures are utilized to setup roll initial conditions (Croteau and Carter, 2002). As the kinematics and dynamics experienced by the occupant are intimately tied to the vehicle dynamics, variability in roll conditions make it difficult to assess the influence of various countermeasures on occupant motion and injury potential.An alternative is that offered by the simulation environment. In fact, a number of studies have utilized MADYMO for rollover simulation, and a review of applications in this area is provided by Prasad and Chou (2002). These authors concur that MADYMO is a viable tool for modeling the airborne phase of a rollover. A number of recent studies have also utilized MADYMO in rollover occupant simulation (Newberry et al., 2005; Lai et al., 2005; Bardini and Hiller 1999; Bohman and Boström 2000). Recent studies by Newberry et al. (2005) and Lai et al. (2005) have specifically demonstrated the utility of MADYMO for the study of occupant kinematics during the airborne phase of vehicle rollover.In the present study, we utilized pretensioner testing and computational analysis to evaluate the capacity of pretensioners for altering and reducing occupant head excursion during the early phase of a steer-induced rollover. This study builds upon existing MADYMO models that have been validated for the prediction of occupant head, neck and torso kinematics during the airborne phase of vehicle rollover (Newberry et al., 2005; Lai et al., 2005). While the MADYMO seatbelt retractor model has the ability to simulate a pretensioner, the pretensioner feature requires a retracted length versus time input, which is likely a function of the seatbelt loads at the time of deployment. In the current study we conducted bench tests of a typical retractor-based pyrotechnic pretensioner to evaluate its capacity to retract webbing at various loads, then modeled the pretensioner’s performance using a transient force function. Additional bench tests were conducted to characterize the transient force pulses of several different pretensioners. Finally, to evaluate the efficacy of these pretensioners in limiting head excursion in the early phase of rollover, we simulated a steer-induced rollover event using MADYMO. This simulation focused on the time period from steer initiation until first roof contact, and was validated by comparison of simulated motions of the ATD to those measured during the rollover tip-up tests conducted by Yamaguchi et al. (2005).METHODSBench Testing of PretensionersA series of five bench-top tests was conducted on a typical pyrotechnic retractor-based pretensioner (Toyota 4-Runner) (Table 1). Each pretensioner was deployed with approximately 25% of the webbing on the spool (total web length = 136", web on spool = 32-33"). The webbing was routed from the retractor up to the D-ring and down to aweight that was initially supported by a scale (Test 1-3, no preload) (Figure 1).Table 1. Bench Testing Matrix.Number (lb)Comments1 512 903 186 supported with webbing slack4 zero --5186 Load hanging without slackThe results of the bench testing are shown in Figures 2-4.The time history of the webbing loads, as measured betweenthe pretensioning retractor and the D-ring, is shown in Figure 3. The results were similar for all trials. The mean, peakwebbing load was 320.0 pounds, with a mean time-to-peak of0.0053 s.As shown in Figure 2, the amount of webbing retracted by thepretensioner under a “zero” load condition (Test 4) is 4.5 inches. The results for Tests 1-3 show that the pretensioner retracts 1.4 inches of webbing. However, the weight was not completely lifted off of its supporting structure. In these tests the pretensioner appeared to only retract the available slack in the system and the additional webbing elongation generated by stretch during the initial force pulse. This is evident in the force time histories, which show that the weight was only partially supported by the webbing at the end of the test (Figure 3). Deployment of the pretensioner against a preload of 186 pounds (Test 5) did not result in measurable net motion of the webbing at the retractor opening (as determined from high-speed video).MADYMO Model of PretensionerA series of linear belt segments were routed so as to model the experimental configuration. We modeled the pretensioner by applying a force actuator to a massless pretensioner body that was attached to a linear belt segment between the D-ring and the retractor. A simplified transient force function was applied to that actuator in a direction towards the retractor and parallel to the webbing such that as the force was applied the retractor spooled in webbing.The amplitude (320 lb),time-to-peak(0.0053 s), and pulse duration (0.013 s) of the triangular force function was determined based upon the mean values measured during bench testing (Figure 4). Based on the results ofthe bench testing, it was determined that the pretensioner stopped retracting at the time of the peak force developed in the belt. In the MADYMO model, the pretensioner body was locked in place at the time corresponding to peak force development. Figure 1. Bench tests of pretensioner.Figure 2. Retracted webbing as a result of pretensioner deployment measured at retractor opening by high-speed video analysis.Figure 3. Webbing forces measured between the D-Ring and load as a result of pretensioner deployment.Figure 4. Webbing forces measured between the D-Ring and load as a result of pretensioner deployment, and idealized MADYMO input for pretensioner model.The efficiency of the pretensioner to transmit load across the D-ring was dependent on the loading scenario. The efficiencies for Tests 1-3 were 88% (278 lb to 316 lb), 85% (268 lb to 314 lb), and 85% (280 lb to 330 lb), respectively. The efficiency for Test 5, in which the 186-pound load was present at the deployment time, was approximately 68% (204 lb to 302 lb).A MADYMO simulation of this testing configuration yielded similar results. Peak forces for the experiment and for the simulation were 302 lb and 289 lb, respectively. The simulation yielded an insignificant amount of net displacement (0.02 inches), which was similar to the experiment (not measurable).Additional bench tests were conducted to characterize the transient force pulses of several other pretensioners. Pretensioners were chosen from four 1990’s vintage SUVs: a Honda CRV, a Toyota Land Cruiser, a Mercedes Benz ML320, and a Toyota Rav4. Each retractor had 75 percent of its webbing cut such that approximately 10 inches of webbing extended beyond the retractor opening. The pretensioning retractor was rigidly mounted to a steel test bed. The terminal end of the webbing was mounted above the retractor opening to a load cell that was attached to a frame (Figure 5). The pretensioner was deployed and the resulting time history of the force was recorded.The results of that testing are shown in Figure 6 and Table 2. The mean, peak webbing load produced was 490 pounds witha range of 260 to 650 pounds. The mean time-to-peak was0.006 s.Figure 5. Static deployment of pretensioners with fixed-length of webbing.Figure 6. Webbing loads developed as a result of deployment of pretensioner with a fixed-length of extracted webbing (25% remaining on spool).Table 2. Peak pretensioner force and time-to-peak in static deployment tests.Pretensioner(lb) (s)Rav4 650 0.0063ML320 510 0.0058CRV 260 0.0059 Land Cruiser 540 0.0060 Mean 490 0.0060ROLLOVER SIMULATIONSA MADYMO model was used to evaluate the capacity of pretensioners for altering and reducing occupant head excursion during the early phase of a steer-induced rollover. This study builds upon existing MADYMO models that have been validated for the prediction of occupant head, neck and torso kinematics during the airborne phase of vehicle rollover (Newberry et al., 2005; Lai et al., 2005).MADYMO version 6.1, published by TNO Automotive, was used in the production of the rollover simulations. MADYMO is a mathematical dynamic modeling software package that has been widely used in automotive crash engineering applications.Model OverviewThe model consists of a seated ellipsoid model of the 50th-percentile-male Hybrid III ATD positioned in the driver’s seat surrounded by a simplified vehicle interior modeled in ellipsoid surfaces and restrained by a finite element belt model to match the geometry of the Isuzu Rodeo used in the tip-up studies of Yamaguchi et al., 2005 (Figure 7).Figure 7. Frame captures from MADYMO model and volunteer tip-up testing (Yamaguchi, et al., 2005) at 2.25 s.Interior ContactsA representative force-deflection curve was chosen for the ATD-to-seat contact interaction and a friction coefficient of 0.6 was used. A stiffer force-deflection curve was used for the rest of the vehicle interior contacts, which included the dash, knee bolster, door and floor, with a corresponding friction coefficient of 0.4.SeatbeltThe vehicle seatbelt system was measured to model the attachment locations for the retractor, D-ring, buckle, and lap belt anchor. The belt system was implemented using the MADYMO recommended combination of linear belt segments attached to a finite element belt system consisting of triangular membrane elements. The contact between the ATD and finite element belt was defined using a kinematic characteristic with a friction coefficient of 0.4. The friction for the seatbelt passing through the D-ring and the buckle was modeled with a friction coefficient of 0.1. The belt material property was modeled with a 10% elongation for a 2,500-pound load. ValidationThe MADYMO model showed good fit with the experimental data in temporal behavior and in magnitude. Figure 7 shows frame captures from the MADYMO simulation and the tip-up testing of Yamaguchi, et al. (2005) at 2.25 s. Figure 8 shows the lateral (Y) and vertical (Z) head excursions for both the model and experiment at 2.25 seconds, which is the last comparable point before outrigger strike occurred in theexperiment.PretensionerTransient force input from the four fixed-length staticdeployment tests described above was used to model thepretensioners. Each pretensioner was deployed at 2 seconds orvehicle roll angle of 10 degrees.Simulation ProcedureA vehicle dynamics simulation program (PC-Crash, version6.2, published by Dr. Steffen Datentechnik, Linz, Austria) wasused to extend the volunteer tip-up testing of Yamaguchi et al.(2005) beyond the point of outrigger strike. The vehicle wasmodeled and validated by matching vehicle dynamics including the following: yaw angle, yaw rate, roll angle, rollrate, and lateral acceleration. The simulation showed good fitin both temporal behavior and magnitude up to the point ofoutrigger contact, t=2.25 s (Figure 9). The simulation wasextended out to 3.64 s or a roll angle of 99 degrees. Thispoint coincided with the first hypothetical roof-to-ground contact.In the rollover simulation, the belt load at the time of pretensioner firing was 19 pounds. A corresponding force function was calculated so the total belt load at the time of firing did not exceed the experimental measurements (i.e. the peak of the force function was the difference between maximum force measured and the 19 pound belt load).Figure 8. Lateral (Y) and vertical (Z) head excursions for both the model and experiment at 2.25 seconds, which is the last comparable point before outrigger strike occurred in the experiment.Figure 9. Comparison of vehicle dynamics between PC-Crash model and experimental testing of Yamaguchi, et al. (2005).RESULTSThe simulation results showing the effect of pretensioner deployment can be seen in Figures 10-12. The four pretensioners studied showed only a modest effect on limiting head excursion or altering the kinematics of the dummy. As expected, the Rav4 pretensioner, which had the largest peak force during deployment, had the greatest effect on head excursion with a reduction of 0.83 inches in outboard excursion, and only 0.27 inches in vertical excursion (Figure 10). At the time of roof-to-ground contact (3.64 s), the Rav4 pretensioner reduced the lateral and vertical head excursions by 0.88 inches and 0.35 inches, respectively. Similarly, the pretensioners had little effect on the motion of the occupant’s torso, as measured by the sternum position (Figure 11). The Rav4 pretensioner reduced the lateral excursion of the sternum by less than 1 inch. The head angle was reduced by less than 4 degrees (Figure 12).Figure 10. Vertical (Z, +upward) and Lateral (Y, +outboard) Head CG position in MADYMO simulations of steer-induced rollover.Figure 11. Vertical (Z, +upward) and Lateral (Y, +outboard) Sternum position in MADYMO simulations of steer-induced rollover.Figure 12. Head angle (+ATD right) in MADYMO simulations of steer-induced rollover.The amount of webbing the pretensioner model retracted for the Rav4, Land Cruiser, ML320, and CRV were: 0.63 inches, 0.60 inches, 0.47 inches, and 0.28 inches, respectively. DISCUSSIONPrevious studies have demonstrated that increasing the tension in the restraint system (the lap belt in particular) prior to rollover may be of value in controlling occupant displacement and flail. Moffatt et al. (1997) conducted a series of static and dynamic mechanical tests to investigate occupant head excursion during the airborne phase of a rollover. Using a combination of human volunteers, a Hybrid III 50th-percentile device, and a human cadaver, this study demonstrated that increasing seatbelt pre-tension and lap belt angle reduce head vertical and lateral excursions. Similar conclusions have been offered by Arndt (1998), who performed mechanical tests using volunteers and a Hybrid III mannequin in an effort to simulate rollover centrifugal loading. Increasing webbing tension may be of value in controlling occupant displacement, and there are many design approaches to pretensioning devices. Our bench testing of pyrotechnic retractor-based pretensioners has shown that they have limited capability to retract belt webbing in the presence of a seatbelt preload.In modeling pretensioner behavior during a steer-induced rollover, we observed limited webbing retraction, which resulted in an insignificant reduction of upward and outward motion of the head. This conclusion is consistent with the studies of others that have shown a limited ability of the retractor-based pretensioner to retract enough webbing to increase the ability of the lap belt to significantly affect head excursion that occurs during the trip phase (McCoy et al, 2005). The steer-induced rollover examined in the current study may be considered less aggressive than other possible trip events. However, more aggressive trip and rollover events would likely include higher lateral and roll accelerations and consequently higher belt loads. The higher belt loads would necessarily result in less effective webbing retraction by these pyrotechnic pretensioners.The pretensioner model available in MADYMO is appropriate for well-defined systems but may be of limited use without appropriate testing to determine the amount of webbing that is retracted for an uncertain loading scenario. The bench-top model described herein presents an incremental improvement that reflects experimental measurements. In the presented pretensioner model, a transient force function was implemented, which characterizes the maximum force that can be generated by a particular pretensioning retractor. Because the pretensioner cannot exert more than its inherent pyrotechnic power (explosive potential), the peak of the pretensioner force function was calculated to include the belt load at the time of pretensioner ignition. This method could be readily implemented into the code such that the nominal force function, as characterized by a bench test as we have shown here, could be altered based on the value of seatbelt load at the time of deployment. We used this approach in a validated rollover model to examine the effects of this type of pretensioner on head excursion and occupant kinematics during a realistic steer-induced rollover. We found that this pretensioner had only a modest effect on head excursion. In addition, our testing shows at higher belt loads, the pyrotechnic pretensioner becomes less effective because of its force limiting design. In theory, the explosive potential of pretensioners could be increased, possibly increasing the amount of webbing retracted and further reducing head excursion. However, the injury potential of more aggressive pretensioners is not known. In fact, seat belt systems incorporating pretensioners typically include an energy-management feature, such as a constant-force retractor, to limit torso belt loading on the occupant. Such a device would tend to counteract higher pretensioner forces by allowing webbing extension under high loads.Although the reduction of head excursion may reduce the injury potential, others have found contradictory field data that suggest head-to-roof contact may not be the primary factor in predicting injury potential (Moffatt and Padmanaban, 2005). 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