Heavy Truck Occupant Safety Analysis using Finite Element Simulation

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1 ISSN: (Print), (Online) doi: /ijvss MechAero Foundation for Technical Research & Education Excellence International Journal of Vehicle Structures & Systems Available online at Heavy Truck Occupant Safety Analysis using Finite Element Simulation R. Asad Ahmed a,b and Abani K. Patra b a Department of Aeronautical Engineering, Tagore Engineering College, Chennai, India b Corresponding Author, asadahmed@tec.net.in c Department of Mechanical and Aerospace Engineering, State University of New York at Buffalo, Buffalo, USA abani@eng.buffalo.edu ABSTRACT: Numerical simulation of automotive crashes plays an important role in reducing the cost and time taken for predicting the results of a collision. The subject of interior crash protection has received much attention for automobiles than commercial vehicles like heavy trucks. This paper presents a Finite Element (FE) simulation of a heavy truck cab for safety analysis of its occupant during frontal collisions. The dashboard of the cab is modelled along with a steering column, floor and a roof top. Two type of acceleration pulses are used to calculate the Head Injury Criteria (HIC), Neck Injury (NIJ) and chest injury levels of the occupant. Three different scenarios of occupant namely - without seatbelt, with seatbelt, and with both airbag and seatbelt are detailed. The results of simulation have demonstrated that the use of safety restraints has reduced the occupant s injury to considerable levels. KEYWORDS: Heavy truck, Finite element analysis, Occupant safety analysis, Head injury criteria, Neck injury criteria CITATION: R.A. Ahmed and A.K. Patra. 21. Heavy truck occupant safety analysis using finite element simulation, Int. J. Vehicle Structures & Systems, 2(2), Introduction FE Methods (FEM) have been successfully used in the field of vehicle crashworthiness. Though the development of computer models is more complex, the computation has become easier and less expensive with the advent of supercomputers. FEM can be used to simulate various crash events and be well-utilized to analyze the occupant behaviour. The injury levels that are difficult to perform in an actual vehicle accident without subjecting the occupant to an extreme amount of danger can be assessed using FE simulations. Each year, too many tractor-trailer drivers are killed in crashes. Others are seriously injured. Tractor-trailer crashes involve such tremendous forces causing serious injuries to the drivers. There is a strong need to improve the driver safety characteristics of cabs in design and development of heavy trucks. Simulation based design methods are widely used in the safety improvements of passenger cars. The subject of interior crash protection has received more attention for automobiles than for commercial vehicles. The experience gained from these studies forms a good foundation for designing improved truck occupant protection systems. Occupant protection systems can be distinguished into the restraints that the occupant requires to actively adopt their use, such as wearing seatbelts etc., and those that are inherently present in the vehicle such as airbags, energy absorbing steering columns, padding of interior structures etc. These are 54 sometimes referred as active and passive systems respectively. Active systems (especially seatbelts) have the disadvantage that their use is not always assured, thus often rendering them as ineffective. Evans [1] compared the effectiveness of airbags and seatbelts in passenger automobiles. reduces the risk of fatality by preventing the occupant ejection and thereby the severity of impacts with interior objects is sustainable. Airbag reduces the chance of injury due to impacts with interior components primarily in frontal collisions. Based on the crash data, Evans [1] has also calculated that the use of seatbelt is effective (77 ± 6%) in reducing the occupant fatality. The use of airbag alone is least effective (18 ± 4%) in curtaining the injury levels. Combined use of seatbelt and airbag is estimated to provide an additional 5% reduction in fatalities over the use of seatbelt alone. Seiff [2] presented the following aspects for reducing the injury toll during truck crashes using crash prevention systems and post-crash occupant protection countermeasures: The use of seatbelt is the most important aspect in preventing injury to the truck occupants. The use seatbelt in heavy trucks has increased. Steering wheel rim and column designs have to be improved to keep the cab interiors free from sharp and hard objects. The cab design has to be improved to provide more crash space and means of evacuation after the crash.

2 Clarke [3] and De Coo [4] have dealt efforts to improve the crash protection of truck occupants in USA and EU respectively. Both studies have used a detailed analysis of truck crashes combined with crash tests. Clarke [3] analyzed the crash data from 182 case summaries of fatal heavy truck crashes based on a NTSB study to develop the computational crash simulations and representative crash pulses to evaluate the occupant dynamics and to assess the truck cab crashworthiness. Analysis of the crash data has shown that there are three principal types of crashes namely rollover, collision with a fixed object, and collision with other trucks. In the majority of the cases, the principal impact is a frontal collision. Fatal head on collisions with other trucks or with the fixed object are usually characterized by high closing speeds. Fatalities caused by a collision with the rear end of another truck occur over a wide range of speeds and involve an intrusion of occupant compartment of the striking truck with the frame of the truck in front. Nearly 5% of the cases studied involve rollover crashes. Occupants during 18 o rollovers are generally not survivable due to the crush of the occupant compartment. The 9 o rollovers usually allow sufficient survivable space for the occupant. Approximately 22% of the analyzed crashes are judged to have sufficient occupant survival space. De Coo [4] concluded that a 6% reduction in injury measures is possible by using seatbelt alone. A further 21% reduction in injury is achievable by using airbag and seatbelt. Cheng [5, 6] used crash reconstruction and simulation studies to analyze the effectiveness of occupant restraint systems. Three cases of seatbelt usage are investigated, a three-point seat belt, a lap belt, and an unrestrained occupant. In rear-end collisions the use of three-point seatbelt is shown to be effective in limiting the forward excursion of the upper body limiting the of impact head with the steering wheel and the roof. In rollover crashes, the seatbelt is less effective in preventing the head impact with the roof. Higher impact forces are observed for the cases of lap belt usage and unrestrained occupant. Hollowell [7] presents results from the car crash tests. The principal conclusion of this study is that the use of airbag prevents serious head or chest injuries in the most severe crashes. However, the lower extremity injuries are more common and require improvements in the protection systems. There is a considerable agreement amongst all the studies [3-8] that the restraint systems are the most effective measure in reducing injury severity and fatality rates. Truck occupant protection systems have thus far received less attention than automobile passenger protection systems. A comprehensive look at the truck occupant safety issues is reported in [8]. Some of the studies reported in [8] include a comprehensive data collection effort supported by the National Highway Traffic Safety Administration USA and the High Safety Vehicle (HSV) project in Europe. The studies on the occupant dynamics due to the crash accelerations and the truck cab geometry cover a wide range of issues such as truck crash characteristics, occupant injury modes, and occupant protection countermeasures. 55 In this paper, the safety of a heavy truck occupant during frontal collisions is analysed using detailed FE simulations. The FE model allows us to study the effect of multiple safety systems at a minimal cost. FE modelling of the heavy truck cabin and the simulation of frontal collisions using two crash pulses are presented. The results from these simulations are discussed followed by some concluding remarks. 2. Occupant Safety Analysis Procedure The steps of the truck occupant safety analysis procedure by employing a FE model are shown in Fig. 1. The dashboard, steering column, seat, and a portion of the roof are included in the part geometry of the truck cab. Create a part geometry of typical truck cab Mesh the truck cab part and integrate a dummy of the occupant Apply boundary conditions and export the FE model and dummy to MADYMO Perform occupant safety analysis for a given crash pulse Study the motion of dummy for the use of seatbelt or/and airbag Assess the occupant head, chest, and neck injury levels Fig. 1: Steps of occupant safety analysis procedure The Hybrid III 5 th percentile ellipsoid dummy [9], as shown in Fig. 2, is used to represent the occupant. The meshing of the truck cab and the integration of resulted FE model with the dummy are undertaken using HYPERMESH [1]. The dummy is positioned in its sitting posture on the seat. The hands of dummy are assumed to be on the steering wheel. The legs are angled to the floor of the truck cab. The dummy is rotated by 18 o to the Y-axis representing an adjusted seat position. X Fig. 2: MADYMO Hybrid III 5 th Percentile Dummy Z Y

3 The size and weight of the integrated dummy represent an average male adult of USA population. This dummy is widely accepted by several standards such as FMVSS 28 [11], ECE R.94 [12] and New Car Assessment Program (NCAP) [13]. The occupant safety analysis and the assessment of occupant injury levels for the Bus and NCAP crash pulses are undertaken using MADYMO [9]. 3. Results and Discussions Simulations are conducted on the FE model for three cases namely - without seatbelt, with seatbelt, and with seatbelt and airbag. As the first 2 ms of a frontal collision is considered to be the most significant for the observation of occupant dynamics, all the models are run for this duration of the crash pulse to obtain the desired results. For the considered three cases, the occupant motions at 36 ms, 72 ms, 18 ms, and 144 ms of Bus crash pulse are shown in Figs. 3 (a)-(d) to 5 (a)-(d) respectively. If the seatbelt is not used, the occupant has severe injuries as shown in Fig. 3(d). When both seatbelt and airbag are used, the occupant is able to survive the crash with reduced neck injuries as shown in Fig. 5(d) Head Acceleration The X-component and Z-component of the head acceleration resembles the forward and upward directions respectively with respect to the occupant s movement. The Z-component does not vary much for the considered three cases. The response of resultant head acceleration is shown in Fig. 6. For the case of without seatbelt, the resultant head acceleration is higher than the other two considered cases. Combined use of airbag and seatbelt has helped to reduce the head acceleration levels to some extent Chest Deflection There is a significant chest deflection is observed (see Fig. 7) when seatbelt is not used. This is due to the impact of the dummy over the steering wheel. The chest deflection is reduced with the use of seatbelt. A further reduction in the chest deflection is observed when both the seatbelt and airbag are used Neck Forces and Moments The moment, axial and shear forces for the upper neck are observed. For all the cases, the neck moments have not reached the tolerable limits of -57 Nm (-My: Neck extension) and 19 Nm (+My: Neck flexion). When the seatbelt is not used, the neck moments are relatively higher than that of the other two cases. The neck axial force, F z, is observed to be within the tolerable limits (4 N in compression and 33 N in tension). The neck axial force in tension for the case of with seatbelt is found to be more than that of the other two cases. This could be due the interaction of forces on the neck with the seatbelt which is not included in the simulation. The response of neck shear force, F x, for a bus crash pulse is shown in Fig. 8. In the case without seatbelt, the neck shear force for the case of without seatbelt is observed to be higher than that of other two cases. But, the peaks in the neck shear force response are below its allowable limit of 31 N. 56 Fig. 3(a): Motion of Dummy without seatbelt after 36ms Fig. 3(b): Motion of Dummy without seatbelt after 72ms Fig. 3(c): Motion of Dummy without seatbelt after 18ms Fig. 3(d): Motion of Dummy without seatbelt after 144ms

4 Fig. 4(a): Motion of Dummy with seatbelt after 36ms Fig. 5(a): Motion of Dummy with seatbelt and airbag after 36ms Fig. 4(b): Motion of Dummy with seatbelt after 72ms Fig. 5(b): Motion of Dummy with seatbelt and airbag after 72ms Fig. 4(c): Motion of Dummy with seatbelt after 18ms Fig. 5(c): Motion of Dummy with seatbelt and airbag after 18ms Fig. 4(d): Motion of Dummy with seatbelt after 144ms Fig. 5(d): Motion of Dummy with seatbelt and airbag after 144ms 57

5 Acceleration (m/s 2 ) 5 1 Fig. 6: Head resultant acceleration response for Bus crash pulse ( out seat belt; seat belt; seat belt & airbag) 6 Distance (mm) 2 4 Fig. 7: Chest deflection response for Bus crash pulse ( out seat belt; seat belt; seat belt & airbag) Force (N) Time (sec) Time (sec) Time (sec) Fig. 8: Neck shear force response for Bus crash pulse ( out seat belt; seat belt; seat belt & airbag) Assessment of Injury Levels The analysis of the occupant s injury criteria are performed using MADYMO. The HIC is given [9] by: 2.5 t2 1.5 HIC = max ( t2 t1) a( t) dt (1) t1 where t 1 and t 2 are the initial and final times (in seconds) of the interval during which the HIC attains a maximum. 58 The resultant acceleration, a(t), is measured at the centre of gravity of occupant s head. The time duration, (t 1 -t 2 ), is taken as the contact time of the impact. For the considered three cases, HIC is calculated for 15 and 36 ms windows of crash pulse and are given in Table 1. HIC value of greater than 1 is considered to be a fatal accident. HIC levels for the case of using both seatbelt and airbag are less than the other two cases. Thorax resultant acceleration does not seem to have much variation for all the cases. Combined thoracic index of more than one relates to a serious injury to the thorax. For the considered cases, the combined thoracic index (see Table 1) is found to be less than one. Contiguous_3ms injury criterion measures the linear acceleration over 3 ms and relates to the organ injury in thorax. For the case of using both seatbelt and airbag, the contiguous_3ms index is less than that of the other two cases. A chest compression of more than 76 mm is observed to be a serious injury. For Bush crash pulse, a maximum chest compression of 58 mm (see Table 1) is observed in the without seatbelt case. NIJ is a biomechanical neck injury predictor that measures the injury levels such as NIJ_N TE, NIJ_N TF, NIJ_N CE, and NIJ_N CF due to load transferred through the occipital condyles. A maximum limit of 1 is allowed for the NIJ. For all the considered cases, the NIJ is below 1 and hence the occupant is considered to be safe. Table 1: Comparison of injury criteria for Bus pulse Injury Criteria out +Airbag HIC_15ms HIC_36ms Chest compression (m) Comb d Thoracic Index Contiguous_3smax Viscous Injury Criterion NIJ_N TE NIJ_N TF NIJ_N CE NIJ_N CF NCAP pulse is used to demonstrate the use of the FE model for other pulses. The injury criteria results are summarized in Table 2. A similar trend in the injury levels of Bus pulse is observed. The injury levels for the case of occupant using both the seat belt and airbag restraints are lesser than the other two cases. Table 2: Comparison of injury criteria for NCAP pulse Injury Criteria out +Airbag HIC_15ms HIC_36ms Chest compression (m) Comb d Thoracic Index Contiguous_3smax Viscous Injury Criterion NIJ_N TE NIJ_N TF NIJ_N CE NIJ_N CF

6 4. Conclusions In this paper, the occupant safety analysis and the assessment of injury levels are detailed using FE simulations of the truck cab and a dummy. Bus and NCAP crash pulses are tested on the FE model to evaluate the occupant injury levels for three cases namely without seatbelt, with seatbelt, with seatbelt and airbag. The use of seatbelt and airbag has helped to reduce the head, neck, and chest injuries to considerable levels. The lower extremity injuries are severe for the case of occupant with no seatbelt. Thus, the importance of wearing a seatbelt and having a fully deployable airbag in a vehicle is emphasized to a great extent in safeguarding the truck occupant safety. REFERENCES: [1] L. Evans Passive compared to active approaches to reducing occupant fatalities, SAE , Proc. 12 th Int. Tech. Conf. Enhanced Safety of Vehicles, Goteborg, Sweden. [2] H. Seiff Status report on large truck safety in the United States, SAE , Truck and Bus Meeting and Exposition, Charlotte, North Carolina. [3] R.M. Clarke U.S. efforts to improve heavy truck occupant crash protection and reduce aggressivity in frontal truck/car collisions, SAE , Proc. 14 th Int. Tech. Conf. Enhanced Safety of Vehicles, Munich, Germany. [4] P.J.A. de Coo Heavy good vehicles safety considerations, SAE , Proc. 14 th Int. Tech. Conf. Enhanced Safety of Vehicles, Munich, Germany. [5] L.Y. Cheng Heavy truck crashworthiness - Collision crashes, SAE , Proc. 15 th Int. Tech. Conf. Enhanced Safety of Vehicles, Melbourne, Australia. [6] L.Y. Cheng Heavy truck crashworthiness - 9 o Rollover crashes, SAE , Proc. 15 th Int. Tech. Conf. Enhanced Safety of Vehicles, Melbourne, Australia. [7] W.T. Hollowell Improving occupant protection systems in frontal crashes, SAE 96665, Occupant Protection Technologies for Frontal Impacts, Detroit, Michigan. [8] V. Krishnaswami. 23. Feasibility of heavy truck occupant protection measures, The University of Michigan Transportation Research Institute. [9] MAthematical DYnamic MOdels - MADYMO Theory Manual. 25. Version 6.3. [1] HYPERMESH User's Manual. 24. Altair Engineering. [11] Federal Motor Vehicle Safety Standards 28: Occupant crash protection U.S. Dept. of Transportation. [12] Protection of occupants against frontal collision. 27. UNECE Vehicle Regulations 91. [13] Airbag safety - New Car Assessment Program. (visited on June 21). National Highway Traffic Safety Administration, U.S. Dept. of Transportation. 59