Bus Impact
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ARTICLE IN PRESS
International Journal of Impact Engineering 32 (2006) 883–888 www.elsevier.com/locate/ijimpeng
Short communication
Crashworthiness assessment of a paratransit bus Leslaw Kwasniewski, Hongyi Li, Ravi Nimbalkar, Jerry Wekezer FAMU-FSU College of Engineering, Tallahassee, FL, USA Received 7 August 2004; accepted 18 March 2005 Available online 13 June 2005
Abstract Most of the bus safety standards in the USA are not applicable to cutaway buses for which a production process is split into two stages. First, the chassis and cab section are assembled by automobile manufactures. Then the vehicle is shipped to another company, where bus body and additional equipment are installed. Lack of strict structural standards for transit bus body builders stimulates the need for crashworthiness and safety evaluation for this category of vehicles. This study focused on a selected transit bus, the Ford Eldorado Aerotech 240. Due to the lack of design data the reverse engineering process was used to acquire the geometric data of the bus. The finite element (FE) model was developed based on the geometry obtained by disassembling and digitizing all major parts of the actual bus. The FE model consists of 73,600 finite elements, has 174 defined properties (groups of elements with the same features) and 23 material models. LS-DYNA non-linear, explicit, 3-D, dynamic FE computer code was used to simulate behavior of the FE model under different impact scenarios, such as front impact and side impact of two buses at various velocities. r 2005 Elsevier Ltd. All rights reserved. Keywords: Impact; Transportation safety; Vehicle-crash worthiness; Finite element analysis; LS-DYNA
1. Introduction Paratransit buses are often made by at least two major auto manufacturers. While the first makes the chassis and cab section, the second installs bus body and assembles additional Corresponding author.
E-mail addresses: [email protected] (L. Kwasniewski), [email protected] (J. Wekezer). 0734-743X/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijimpeng.2005.03.007
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equipment. This process lacks a comprehensive safety assessment and is not regulated (in contrast with school buses) by strict federal safety standards. Two methods are available for crashworthiness and safety evaluation: full scale testing and finite element analysis (FEA). Although a full scale test is considered the most reliable source of information regarding crashworthiness and safety of vehicles, high cost of such tests and difficulties of collecting sufficient data result in increasing interest in finite element method. This efficient and cost effective tool [1] enables extensive investigations once the finite element model is validated. This paper presents some experiences, findings and conclusions from the research conducted to evaluate the crashworthiness of a selected paratransit bus through computer impact analysis (virtual crash testing).
2. Vehicle tear down and digitizing Since blueprints and design data of the bus are usually not available, the process called reverse engineering [2] had to be adopted to acquire geometric data needed for development of FE models. A FaroArm digitizing equipment and the accompanying software AnthroCAM were the major tools used to obtain the geometric data of the bus components. It allowed for digitizing of points, scanning curves and construction of polylines [3]. Before tear down process began, the global coordinate system was established by setting up about 750 reference points and recording their coordinates using FaroArm (Fig. 1). Three to ten reference points were digitized for each structural part. These points were used to establish the position of the part in the global coordinate system after it was removed from the bus.
Fig. 1. Defined coordinate system and reference points.
Fig. 2. Front part of the bus and the scanned curves.
ARTICLE IN PRESS L. Kwasniewski et al. / International Journal of Impact Engineering 32 (2006) 883–888
885
All structural components were identified, labeled and removed from the bus and subsequently scanned. Fig. 2 presents the disassembled front part of the bus and the scanned curves.
3. Development of FE model Disassembly resulted in full exposure of the connections of the major components. Joints among the structural parts, such as hinges, rivets, welds, bolts, and rubber pads were identified and were appropriately modeled on the computer, using multipoint constraints (MPCs), spot welds, node merging and node tying. Some deterioration of components and manufacture flaws were found after the disassembly. These defects could adversely affect the performance of the bus, and in some cases, should be accounted for in the FE model, by reducing material properties. The models of components were assembled together by several methods, one of which was MPCs. By defining the relationship between dependent degrees-of-freedom and the response of independent degrees-of-freedom, MPC provided the opportunity to model bolts, screws and welds with failure [4]. Contact describing the interaction between the adjoining parts occurs frequently in transient dynamics systems [5]. Different contact algorithms, such as self contact, master-slave contact, were applied to the model. 23 material types were identified for the structural components of the actual bus structure. The bus body was built using two layers of composite material with additional honeycomb layer placed in between them. Composite layers were modeled using shell elements and honeycomb was represented by solid elements. The final FE model of the bus consisted of 174 parts, 23 material models, and 73,595 elements. A fully integrated quad element number 16, available in LS-DYNA, was selected for analysis as the most reliable element formulation, based on several numerical tests [6]. The final FE model of the bus is presented in Fig. 3.
Fig. 3. FE model of the transit bus.
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Fig. 4. Initial position of buses.
Fig. 5. Sequence of pictures showing both buses at characteristic time points.
4. Numerical results The most common type of accident events such as: frontal impact at 30 mph, side impact at 30 mph, and side impact at 50 mph velocity were studied in detail. As an example, the side impact of two identical Ford Eldorado buses, at a velocity of 30 mph, is presented below. The initial position of vehicles is shown in Fig. 4. The initial velocity was applied to the right bus, which served as an impacting vehicle. The simulation started 0.33 s before the vehicles came to a contact, to allow them to settle down under gravity, and to induce initial deformations and forces in springs and dampers of the suspension system. The initial distance between the front bumper of the right vehicle and the street wall of the left bus was determined as 4.2 m (130 900 ). Fig. 5 shows an animation sequence for side impact simulation. Most of the kinetic energy was dissipated due to inelastic deformation in the impacted zone (street wall) in the left bus. Fig. 6 shows deformation and damage in the street wall at the end of simulation at the time 1.2 s. The impacting bus was erased from the view. Fig. 7 presents contours of effective stress (von Misses stress) for steel cage and the floor at the moment when they were the largest, at the time t ¼ 0:37 s.
ARTICLE IN PRESS L. Kwasniewski et al. / International Journal of Impact Engineering 32 (2006) 883–888
887
Fig. 6. Inelastic deformations in the impacted bus, t ¼ 1:2 s.
Fig. 7. Contours of effective stress, t ¼ 0:37 s.
5. Summary and conclusions Presented research was carried out to investigate the crashworthiness and passenger safety of modified body-on-chassis buses. The study was focused on a Ford Eldorado Aerotech 240 transit bus, which served as a sample representing a wider range of similar transit buses. The major goal of this research was to develop an effective tool for collecting data useful for future development of recommendations for bus body manufactures and operators. High cost of actual crash test experiments and difficulties with collecting extensive data from full-scale crash tests result in increasing interest in analytical and computational methods. Also, damage and failure mechanisms, especially those with sequential, complex and progressive failure could only be fully understood from computational mechanics and crash analysis. A full-scale crash test of Ford Eldorado bus is necessary to validate the entire finite element model. Although the finite element model was not validated, it is complete in the sense that it represents all the structural parts of the actual bus, their geometry and mechanical properties and connections between them. Numerical data presented shows that the FE model yields realistic results and, with possible modifications and improvements (depending on laboratory test data), can accurately represent the behavior of the actual bus.
ARTICLE IN PRESS 888
L. Kwasniewski et al. / International Journal of Impact Engineering 32 (2006) 883–888
The results show the potential for thorough evaluation of the bus crashworthiness and safety of its passengers. The existing model can be easily converted and modified to reflect potential changes in the body structure of new buses for detailed parametric studies. Acknowledgements Funding for this project was provided by the Florida Department of Transportation (FDOT). The development of the finite element model of the bus was performed within the research sponsored by the FDOT. The authors wish to acknowledge the assistance and support from the Project Managers: Mr. Robert Westbrook and Mr. Paul Johnson, Jr. The tear down of the bus was professionally performed by the QMG Consulting Inc. Thanks are due to Mr. Paul Queen and Mr. Cecil Carter for their excellent work and technical support.
References [1] Bathe K-J. Crush simulation of cars with FEA. Mechanical Engineering, ASME, 1998, http://www.asme.org/pdf/. [2] Chenga ZQ, Thackera JG, Pilkeya WD, Hollowell WT, Reagana SW, Sieveka EM. Experiences in reverseengineering of a finite element automobile crash model. Finite Element Anal 2001;37:843–60. [3] FARO ARM ANTHROCAM. Design Basic Training Workbook, Version 2.3/2.5, Faro Technologies Inc: Lake Mary, Florida; July, 1999. [4] LS-DYNA Keyword user’s Manual (Nonlinear Analysis of Structures), Livermore Software Technology Corporation: Livermore, California; May, 1999. [5] LS-DYNA Theoretical Manual, Livermore Software Technology Corporation: Livermore, California; May, 1998. [6] Kwasniewski L, Wekezer JW, Li H. Development Of Finite Element Model For Ford Eldorado Transit Bus, Theoretical Foundations of Civil Engineering, 10th Polish-Ukrainian Transactions, Warszawa; June, 2002.
International Journal of Impact Engineering 32 (2006) 883–888 www.elsevier.com/locate/ijimpeng
Short communication
Crashworthiness assessment of a paratransit bus Leslaw Kwasniewski, Hongyi Li, Ravi Nimbalkar, Jerry Wekezer FAMU-FSU College of Engineering, Tallahassee, FL, USA Received 7 August 2004; accepted 18 March 2005 Available online 13 June 2005
Abstract Most of the bus safety standards in the USA are not applicable to cutaway buses for which a production process is split into two stages. First, the chassis and cab section are assembled by automobile manufactures. Then the vehicle is shipped to another company, where bus body and additional equipment are installed. Lack of strict structural standards for transit bus body builders stimulates the need for crashworthiness and safety evaluation for this category of vehicles. This study focused on a selected transit bus, the Ford Eldorado Aerotech 240. Due to the lack of design data the reverse engineering process was used to acquire the geometric data of the bus. The finite element (FE) model was developed based on the geometry obtained by disassembling and digitizing all major parts of the actual bus. The FE model consists of 73,600 finite elements, has 174 defined properties (groups of elements with the same features) and 23 material models. LS-DYNA non-linear, explicit, 3-D, dynamic FE computer code was used to simulate behavior of the FE model under different impact scenarios, such as front impact and side impact of two buses at various velocities. r 2005 Elsevier Ltd. All rights reserved. Keywords: Impact; Transportation safety; Vehicle-crash worthiness; Finite element analysis; LS-DYNA
1. Introduction Paratransit buses are often made by at least two major auto manufacturers. While the first makes the chassis and cab section, the second installs bus body and assembles additional Corresponding author.
E-mail addresses: [email protected] (L. Kwasniewski), [email protected] (J. Wekezer). 0734-743X/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijimpeng.2005.03.007
ARTICLE IN PRESS 884
L. Kwasniewski et al. / International Journal of Impact Engineering 32 (2006) 883–888
equipment. This process lacks a comprehensive safety assessment and is not regulated (in contrast with school buses) by strict federal safety standards. Two methods are available for crashworthiness and safety evaluation: full scale testing and finite element analysis (FEA). Although a full scale test is considered the most reliable source of information regarding crashworthiness and safety of vehicles, high cost of such tests and difficulties of collecting sufficient data result in increasing interest in finite element method. This efficient and cost effective tool [1] enables extensive investigations once the finite element model is validated. This paper presents some experiences, findings and conclusions from the research conducted to evaluate the crashworthiness of a selected paratransit bus through computer impact analysis (virtual crash testing).
2. Vehicle tear down and digitizing Since blueprints and design data of the bus are usually not available, the process called reverse engineering [2] had to be adopted to acquire geometric data needed for development of FE models. A FaroArm digitizing equipment and the accompanying software AnthroCAM were the major tools used to obtain the geometric data of the bus components. It allowed for digitizing of points, scanning curves and construction of polylines [3]. Before tear down process began, the global coordinate system was established by setting up about 750 reference points and recording their coordinates using FaroArm (Fig. 1). Three to ten reference points were digitized for each structural part. These points were used to establish the position of the part in the global coordinate system after it was removed from the bus.
Fig. 1. Defined coordinate system and reference points.
Fig. 2. Front part of the bus and the scanned curves.
ARTICLE IN PRESS L. Kwasniewski et al. / International Journal of Impact Engineering 32 (2006) 883–888
885
All structural components were identified, labeled and removed from the bus and subsequently scanned. Fig. 2 presents the disassembled front part of the bus and the scanned curves.
3. Development of FE model Disassembly resulted in full exposure of the connections of the major components. Joints among the structural parts, such as hinges, rivets, welds, bolts, and rubber pads were identified and were appropriately modeled on the computer, using multipoint constraints (MPCs), spot welds, node merging and node tying. Some deterioration of components and manufacture flaws were found after the disassembly. These defects could adversely affect the performance of the bus, and in some cases, should be accounted for in the FE model, by reducing material properties. The models of components were assembled together by several methods, one of which was MPCs. By defining the relationship between dependent degrees-of-freedom and the response of independent degrees-of-freedom, MPC provided the opportunity to model bolts, screws and welds with failure [4]. Contact describing the interaction between the adjoining parts occurs frequently in transient dynamics systems [5]. Different contact algorithms, such as self contact, master-slave contact, were applied to the model. 23 material types were identified for the structural components of the actual bus structure. The bus body was built using two layers of composite material with additional honeycomb layer placed in between them. Composite layers were modeled using shell elements and honeycomb was represented by solid elements. The final FE model of the bus consisted of 174 parts, 23 material models, and 73,595 elements. A fully integrated quad element number 16, available in LS-DYNA, was selected for analysis as the most reliable element formulation, based on several numerical tests [6]. The final FE model of the bus is presented in Fig. 3.
Fig. 3. FE model of the transit bus.
ARTICLE IN PRESS 886
L. Kwasniewski et al. / International Journal of Impact Engineering 32 (2006) 883–888
Fig. 4. Initial position of buses.
Fig. 5. Sequence of pictures showing both buses at characteristic time points.
4. Numerical results The most common type of accident events such as: frontal impact at 30 mph, side impact at 30 mph, and side impact at 50 mph velocity were studied in detail. As an example, the side impact of two identical Ford Eldorado buses, at a velocity of 30 mph, is presented below. The initial position of vehicles is shown in Fig. 4. The initial velocity was applied to the right bus, which served as an impacting vehicle. The simulation started 0.33 s before the vehicles came to a contact, to allow them to settle down under gravity, and to induce initial deformations and forces in springs and dampers of the suspension system. The initial distance between the front bumper of the right vehicle and the street wall of the left bus was determined as 4.2 m (130 900 ). Fig. 5 shows an animation sequence for side impact simulation. Most of the kinetic energy was dissipated due to inelastic deformation in the impacted zone (street wall) in the left bus. Fig. 6 shows deformation and damage in the street wall at the end of simulation at the time 1.2 s. The impacting bus was erased from the view. Fig. 7 presents contours of effective stress (von Misses stress) for steel cage and the floor at the moment when they were the largest, at the time t ¼ 0:37 s.
ARTICLE IN PRESS L. Kwasniewski et al. / International Journal of Impact Engineering 32 (2006) 883–888
887
Fig. 6. Inelastic deformations in the impacted bus, t ¼ 1:2 s.
Fig. 7. Contours of effective stress, t ¼ 0:37 s.
5. Summary and conclusions Presented research was carried out to investigate the crashworthiness and passenger safety of modified body-on-chassis buses. The study was focused on a Ford Eldorado Aerotech 240 transit bus, which served as a sample representing a wider range of similar transit buses. The major goal of this research was to develop an effective tool for collecting data useful for future development of recommendations for bus body manufactures and operators. High cost of actual crash test experiments and difficulties with collecting extensive data from full-scale crash tests result in increasing interest in analytical and computational methods. Also, damage and failure mechanisms, especially those with sequential, complex and progressive failure could only be fully understood from computational mechanics and crash analysis. A full-scale crash test of Ford Eldorado bus is necessary to validate the entire finite element model. Although the finite element model was not validated, it is complete in the sense that it represents all the structural parts of the actual bus, their geometry and mechanical properties and connections between them. Numerical data presented shows that the FE model yields realistic results and, with possible modifications and improvements (depending on laboratory test data), can accurately represent the behavior of the actual bus.
ARTICLE IN PRESS 888
L. Kwasniewski et al. / International Journal of Impact Engineering 32 (2006) 883–888
The results show the potential for thorough evaluation of the bus crashworthiness and safety of its passengers. The existing model can be easily converted and modified to reflect potential changes in the body structure of new buses for detailed parametric studies. Acknowledgements Funding for this project was provided by the Florida Department of Transportation (FDOT). The development of the finite element model of the bus was performed within the research sponsored by the FDOT. The authors wish to acknowledge the assistance and support from the Project Managers: Mr. Robert Westbrook and Mr. Paul Johnson, Jr. The tear down of the bus was professionally performed by the QMG Consulting Inc. Thanks are due to Mr. Paul Queen and Mr. Cecil Carter for their excellent work and technical support.
References [1] Bathe K-J. Crush simulation of cars with FEA. Mechanical Engineering, ASME, 1998, http://www.asme.org/pdf/. [2] Chenga ZQ, Thackera JG, Pilkeya WD, Hollowell WT, Reagana SW, Sieveka EM. Experiences in reverseengineering of a finite element automobile crash model. Finite Element Anal 2001;37:843–60. [3] FARO ARM ANTHROCAM. Design Basic Training Workbook, Version 2.3/2.5, Faro Technologies Inc: Lake Mary, Florida; July, 1999. [4] LS-DYNA Keyword user’s Manual (Nonlinear Analysis of Structures), Livermore Software Technology Corporation: Livermore, California; May, 1999. [5] LS-DYNA Theoretical Manual, Livermore Software Technology Corporation: Livermore, California; May, 1998. [6] Kwasniewski L, Wekezer JW, Li H. Development Of Finite Element Model For Ford Eldorado Transit Bus, Theoretical Foundations of Civil Engineering, 10th Polish-Ukrainian Transactions, Warszawa; June, 2002.
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