Dynamic Analysis of Child in Misused CRS During Car Accident

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1 Dynamic Analysis of Child in Misused CRS During Car Accident T. KOIZUMI, N. TSUJIUCHI, and R. KAWAMURA Department of Mechanical Engineering Doshisha University Kyotanabe, Kyoto JAPAN Abstract It is widely known that the proper use of the Child Restraint Seats (CRS) can greatly reduce the risk of severe injuries for the infants and children involved in car accidents. However, improper uses or misuses of the CRS are still observed quite often in road surveys, and studies about negative influences on the child s safety based on case studies have been well documented by some authors. In this paper, analytical approach to that issue has been developed using the crash analysis application called MADYMO. The first part of this study was spent on the modeling of the CRS consisting typical geometrical characteristics. The crash simulation was then performed with the numerical model of the three-year-old child dummy restrained in the CRS model. Once the model is validated through the comparison with the actual sled test data, major misuse patterns were taken into account in the model. The effects of improper uses of the CR.9 ware examined by considering body acceleration, injury criteria, and head excursion. The results revealed that even a slightest slack in the installing belt webbings or in the CRS harness considerably reduces the performance of the CRS. Also, combination of misuse modes may lead to even more crucial injury risks. 1. Introduction The statistics show that the proper uses of Child Restraint Seats (CRS) have made a great contribution upon saving the lives of the infants and children involved in traffic accidents t ). Over the years, the rate of restrained children has been growing rapidly, and the knowledge about CRS usage has widely spread in the society. However, improper uses or misuses of the CRS can still be observed quite often in road surveys t* t, In these cases, full performance of the CRS, i.e. child protection and retention, cannot be expected as it is designed. The negative effects of those misuses have been studied and well documented by some authors 14]ts1. These studies are conducted by means of statistical analysis and lab experiments, and are not quite adequate when one wants to examine the occurrence in detail. Creating the numerical model of CRS and performing a crash simulation would allow us to easily investigate crash phenomena in a more thorough manner, not to mention in case of misuse as well. In this paper, a rigid body model of CRS is created and used to perform crash simulations for proper usage and for major misuse patterns. Crash analysis application called MADYMO is widely used to conduct crash simulation and is employed in this paper as well. MADYMO is a useful tool in solving motions of complex rigid body models via equations of motion. 2. Simulation Model Validation The numerical model of the CRS was created using MADYMO entities, i.e. ellipsoidal rigid bodies, facet surfaces, and finite element parts. The three-year-old dummy model, a rigid body model as well, was also employed to perform a crash simulation according to the 252

2 sled test environment regulated in the ECE Regulations No CRS Model The Group I CRS, adaptable for children weigh between 9kg to 18kg, with common geometrical configuration was chosen as a subject for the model. The numerical model was created by interpreting the CRS as a number of ellipsoidal rigid bodies, which are interconnected to each other by rigid joints. The model, as shown in figure 1, consists of 12 ellipsoids or hyper-ellipsoids. Dimensions of those ellipsoids and the connections between them are given so that they can well render the CRS feature. The more detailed surface information is defined for the base of the CRS by means of facet surface. Facet surface consists of 1454 vertices, each placed approximately 1Omm apart, and 1396 facets connecting 3 or 4 vertices. This surface was given in order to obtain precise contact force calculation between the CRS and the environment. Mass of each body is given based on the mass distribution of the actual CRS. The total mass then adds up to 7.7 kg. Inertia properties of each body are taken into account as those of the ellipsoids, due to the difficulty of measuring the actual inertia properties. The impact shield, or the T-shield, and some portions of the CRS harness, which are likely to come in contact with the dummy, are modeled as finite element parts. The remaining portions of the CRS harness are defined as MADYMO seatbelt elements. They are connected to the node at the tip end of each finite element harnesses and routed behind the CRS seat back to the bottom of CRS cushion. The bottom end of the impact shield is provided with a rigid connection between the CRS cushion to model a buckle. 2.2 Simulation Set-ups Crash simulation was performed pursuant to the ECE regulations No sled test conditions. The sled test seat, where the CRS model is to be mounted, is defined simply as a set of rigid planes. The CRS model is installed on the test seat by means of MADYMO seatbelt elements, representing a three-point-belt system with an emergency locking retractor (ELR). The numerical dummy model used for the simulation is the TN0 Hybrid III 3-year-old child dummy database. The dummy model is also a linked rigid body system consisting 28 ellipsoids. Total mass of the system adds up to 15.3 kg. Dimensions, inertia properties of each body, and all joint characteristics are carefully validated by TN0 Automotive. Series of pre-simulation were carried out prior to the aimed crash simulation. In order to obtain a balanced position for the CRS and the dummy, few seconds of simulation was conducted with only the gravity applied to each body of the CRS and the dummy. Another pre-simulation was performed in order to restrain the dummy by eliminating slack of the FEM harness. While the actual sled test employs a rearward-shooting sled, generating a rearward acceleration to the sled, the approach taken in this simulation is to accelerate the CRS and the dummy to the forward direction, relative to the stationary test seat. The acceleration pulse applied in the simulation is an inversed signal of the actual sled acceleration measurement. Note that this method is taken customarily in other crash simulations using MADYMO as well. Integration method taken in this simulation is the 4th order Runge-Kutta method with the time step of 0.01 millisecond. The simulation lasts for 130 milliseconds. Fig.1 A rigid body model of the CRS 2.3 Simulation Results Shown in figure 2 are the trajectories of the dummy s head and the front and the rear most point of the CRS base. It is apparent that the trajectories of all three entities in the simulation well trace those in the experiment. Some deviations occurred at the latter part of simulation can be considered small and as of the acceptable level. Acceleration curves of the head and the thorax in time history were calculated as shown in figure 3 and 4 respectively. Comparison with the experimental data reveals that the simulation gave excellent result for the head acceleration. The curve shape, the peak values and the points of inflection are considerably close to those of 253

3 the experimental result. Larger difference can be found after 110 milliseconds, but the simulation has gone on the rebound phase by that time. Hence its effect to the total simulation is considered negligible. The acceleration curve of the thorax from the start up to around 60 milliseconds is experiencing a precise match. Although the peak values do show a deviance not quite small, their point of occurrence is only a few milliseconds apart. Also the overall curve shapes are modally similar. Thus, it is quite fair to say that the simulation model used here well represents the actual phenomena, though some future improvements should be expected X-Displacement Cm) 900, T. & Fig.2 Trajectories of CRS and dummy s head in experiment and simulation. Fig EXP.head - ---SML hea,, L--m Time (ms) head. Resultant acceleration acting on the dummy s 900 I I Fig Time (ms) head. Resultant acceleration acting on the dummy s I 3. Application of Misuse Modes in the CRS Model Misuse of the CRS observed in the field can be categorized in two major modes, i.e. misinstallation of CRS itself and misuse of CRS harness CRS Misinstallation Major misinstallation results from failure to set a locking clip correctly or not using it at all t. A locking clip is a metal plate with slits for the webbing to run through and firmly hold its position. It is essential for a secure mount in CRS installation using three-point-belt system with an ELR. It is very probable that insufficient use of locking clip causes slacks in belt webbings. In the simulation model, misinstallation is taken into account in terms of slack belt webbings. For each segment of the seatbelt system, some portion of initially added belt length was given in order to model a slack. Total slack of the belt system was set to 20mm to 2OOmm with a 20mm increment Harness Misuse Misuse of CRS harness is also a frequent occurrence. Most common case is a misuse or a non-use of harness retainer clip, which is intended to keep the harness constantly over the dummy s chest, with certain types of CRS requiring its use. For other types of CRS not requiring a retainer clip, slack of a harness or non-use of a harness at all becomes the major problem I*]. The model used for simulation comes under the latter kind. Thus, the consideration of harness misuse with this model shall be in forms of application of slack in the harness, in the same way as mentioned in previous section. Initially added length for each harness was set to 1Omm to 1COmm with an increment of 10mm. 254

4 4. Injury Criteria In order to quantitatively evaluate and compare the occupant s injury severity in various cases, Gadd Severity Index (GSI) and 3 ms Criterion is introduced. These criteria are derived from acceleration measurements at the center of gravity of the head and the thorax, respectively, and are commonly used in automobile and other industry for evaluation and optimization of safety design. Injury criterion for the head, GSI, is defined as follows. GSZ = rr(t)*% 111 f R(t) is the resultant head acceleration in g s, and to and ts are the initial and final times of the simulation [71. The benefit of employing GSI as an evaluation value is that GSI is applicable for non-contact impact condition, in which this simulation lies, as well as for the impact involving head contact. The calculated GSI of the original simulation model is Total Slack in the Webbings (mm) Fig. 5 Variance of injury criteria with slack given in installing belt webbings. A thoracic injury criterion called 3msG is the highest acceleration level in g s, which sustained for at least 3 milliseconds t71. 3msG is calculated simply by tracing the resultant acceleration signal of the thorax with a time window of 3milliseconds. The 3msG value for the original model is g. 5. Discussion Shown in figure 5 and 6 is a variance of injury criteria with slack in the belt webbings and in the CRS harness, respectively. For each value, a scaling was done with the criteria of the original model, with an intention to show its increase rates rather than the specific values. 3msG value is constantly increasing when more slack was given for the belt webbings. Most severe slacks, namely 160 mm to 200 mm, generate as much as 20 % of rise in 3msG. GSI, on the other hand, is showing a fluctuant behavior. However, more slack applied in belt webbings tend to result in higher GSI values. More apparent tendency can be found in injury criteria with slack applied in CRS harness. While 3msG remains almost constant, GSI shows a rapid growth with respect to the increasing slack length. Only 100 mm of slack can actually double the injury severity of the child. The graphs indicate that even a slightest laxation in the belt webbings or in the harness can greatly reduce the CRS performance. Figure 7 and 8 illustrate the trajectories of the center of the dummy s head with slack in belt webbings and in CRS harness, respectively. These figures give a clear image Total Slack in the Harness (mm) Fig. 6 Variance of injury criteria with slack given in CRS harness. about how the slack belt and harness can influence the dummy s motion. These trajectories reveal that in both cases, the dummy s head gains its maximum forward displacement by a considerable level. Notice that a large slack in the harness also causes quite a large upward movement to the dummy s head. With a limited space provided inside an actual automobile, much higher risk for the child to strike the car interior will arise by a loose installation of the CRS or by not adjusting the harness sufficiently. These misuse cases may then lead to a critical damage of the children. The results shown here warn that an occupant placed in misused CRS may be forced to expose their lives in danger even m a survivable accident. Note that the influences of 255

5 N each misuse mode were studied separately. There is much probability that these misuse modes appear combined. It is easily conceivable that any serious multiplier effect may occur in those cases, considering the results stated above. It is very important that the users be informed about the significance of proper use of the CRS and about how misuse can lead to crucial consequences O.f5 i(- 0.0 t I-thNct -4OlWll --1OOmm Nhnm 1 2 I a X - Displacement Fig. 7 Variance of head trajectory with slack given in installing belt webbings. (m) -2Omm -- 50mm i 1OOllWl 3, X - Displacement Cm) A Fig. 8 Variance of head trajectory with slack given in CRS harness. 6. Conclusion This study was an attempt to perform crash simulations with a simplified model of the CRS and to investigate the effect of the misuse. Following were achieved through this study. 1. The dynamic behavior of the CRS and the child dummy during car accident can be well simulated using simple rigid body models. 2. Slack in vehicle belt webbings or in the CRS harness can cause a significant rise of the injury criteria of the child dummy. 3. Slack in vehicle belt webbings or in the harness can greatly affect the maximum forward and upward displacement of the child dummy s head, consequently increasing its risk to collide against the car interior. 4. As a combination of misuse modes is of a probable occurrence in an actual field, there may be even greater injury risk for the children if parents did not carefully utilize the CRS. Acknowledgments This work was partially supported by a grant to RCAST at Doshisha University from the Ministry of Education, Japan, and also by the Doshisha University s research promotion fund. References [l] Gail E. McCarthy; Rose M. Ray; Roger L. McCarthy, In-Use Effectiveness of Child Restraints and Seat Belts for Children from Birth to Age Four, SERA-Vol.2, ~~85-91, (1994) [2] Lawrence E. Decina; Kathleen Y. Knoebel, Child Safety Seat Misuse Patterns in Four States, Accident Analysis and Prevention 29-1, ~~ , (1997) [3] Annemarie Shelness; Jean Jewett, Observed Misuse of Child Restraints, SAE Technical Paper No , (1983) [4] Noppom Khaewpong; Thuvan T. Nguyen; Francis D. Bents, Injury Severity in Restrained Children in Motor Vehicle Crashes, SAE Technical Paper No , (1995) [5] K. Weber; J. W. Melvin, Injury Potential with Misused Child Restraining Systems, SAE Technical Paper No , (1983) [6] TN0 Automotive, MADYMO Utilities Manual Version 5.4 Part V : MADYSCALE User s Manual, pp3-8, (1999) [7] TN0 Automotive, MADYMO Theory Manual Version 5.4, ~~ , (1999) 256