Study of Frontal Impact of a Passenger Bus Manjunath Rao T S1, Madan J2

1- (Engg) student, 2-Technical Manager (SAS Techno Solutions) Automotive Engineering Centre

M.S.Ramaiah School of Advanced Studies, Bangalore

Abstract In recent times more emphasis has been given to passenger cars and small transport vehicles, but the safety features for

larger vehicles has always been a subject of secondary importance. Hence there exist a tremendous opportunity in improving the safety levels of the passenger bus. There are many analysis done for rollover analysis, but the study of frontal impact behavior of the passenger bus is completely ignored. Hence this project is carried out in this direction. This project work is limited to the study of structural integrity during frontal impact and suggestion of some design changes in order to improve safety.

In the current project work the study of frontal impact of a passenger bus has been done. The bus selected for this purpose is a 54 seater bus from NEKRTC (branch of KSRTC), as the geometrical data of a passenger bus was provide by KSRTC. Complete bus structure was modeled using ProE 2001 from the geometrical data collected from the coach building company. Reverse engineering at certain locations was carried out in order to obtain the geometric data. Further the FE modeling of the entire bus was carried using Hypermesh. Finally the simulation was carried out using commercially available software LS Dyna.

From the analysis it was observed that the peak load acts on structural members, making them vulnerable to irregular load distribution. This led to the implementation of crush initiators, which contributed in reduction of the peak load; thus concluding with the importance of crush initiators in reducing the peak loads. 1. INTRODUCTION

Vehicle crashworthiness and occupant safety are the single most important areas of concerns in today’s automotive industry. In earlier days the importance was driven in the way of safety regulations, but nowadays for an automotive company to survive in the market it is very essential to satisfy the safety expectations of the customers. Hence it is more of building a safer vehicle than that of the competitor’s vehicle. In order to satisfy various safety regulations, it is very important for the safety engineers to perform various tests to evaluate the crashworthiness of the vehicle, this is usually done in the beginning stages of the vehicle design and development. When it comes to safety concerns of vehicle crashworthiness, it is the area which comes under passive safety. Due to advancement in the CAE tools it is far easier in conducting the crash analysis, which gives the results that are closely accurate with the physical test setup. This has got many advantages like cost savings, speed of analysis, accuracy of analysis and the ability of performing various analysis with the same model simultaneously. Thus most of the car manufacturers are able to justify their product’s crashworthiness even before it is actually manufactured.

In automotive world more emphasis has been given to the safety of passenger cars, but seldom is the importance given to the passenger bus. Hence in this project the emphasis is towards the frontal impact behavior of a passenger bus.

Among all the accidents that take place, frontal impact has got the major share of 40%. Again in these conditions the injury caused to drivers or the front passenger is extremely high. Though the damage due to frontal impact of the bus is lesser when compared to other vehicles, but still the consequences of such impact on drivers are fatal. According to the study during frontal impact of bus more

than 80% of drivers die than any other members of the bus. In frontal impact scenario more significance should be given on structural integrity, and hence this project work is carried out in this direction.

The bus selected for crash analysis is a 54 seater bus from NEKRTC as shown in figure 1. North East Karnataka Road Transport Corporation (NEKRTC) is the part of Karnataka State Road Transport Corporation (KSRTC) catering to the transport needs of north eastern region of the Karnataka state. As we have read in many newspapers and articles, where the accident rates due to state run buses are exorbitant, hence the bus selected in this project is NEKRTC bus.

Figure 1. NEKRTC Bus

The coaches built for such buses are not designed with various aspects of safety, neither driver nor passengers are ensured with safety. There exists the tremendous opportunities in improving the safety features of this bus. As far as the safety is considered, only the structural safety can be improved. It requires tremendous amount of

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transformation in change in attitude of the Indian passengers, who are least bothered about safety and following rules & regulations pertaining safety standards. Hence the very first activity will be to improve the structural safety of the vehicle that comes under passive safety, but still there is great scope for improvement in active safety also.

It should also be observed that we will never notice a NEKRTC bus driver wearing a seat belt, not only NEKRTC drivers but any other state transport corporation drivers for that matter. Also it was observed that the coaches designed for these buses are not from any professional automotive designer, most of the design work is carried out using outdated design practices; forget crash analysis, they don’t even use a 3D modeling software for designing purpose. All these discussions just highlights the need for more professional design activities with these corporations, which in turn opens up tremendous opportunity for improved safety features for these buses.

As discussed in earlier sections, frontal impact behavior is not a subject that has been extensively studied. Hence not many people have published the literatures based on this study. Vincze-Pap Sándor and Csiszár András [1] have carried out frontal impact test using FEM simulation and comparing the simulation results with the physical test carried out on a Hungarian Ikarus bus. They have presented very clearly about the emphasis on the under-structure energy absorption during frontal impact of a bus. Though there were no design changes, but their comparison of both the test results clearly focuses on more importance of good design of structural elements. They have noted that the maximum deformation takes place on the left side of the bus; this is mainly due to the availability of the doors on that side. The impact force distribution should also be noted, where the impact force distribution is not uniform on the both sides of bus. Hence it is noted that the load carrying capabilities of

the structures that are located on door side has to be very good. It was also observed that the floor deceleration is bit higher than with that of the prescribed trolley deceleration by ECE R80. (Where ECE R80 prescribes 8-12 g of deceleration for trolley at 30 km/h speed standardized impact)

Yoshiriro Sukegawa et al [2] have extensively worked on defining the crash testing methods that has to be carried out on heavy duty vehicles. In their work they have analyzed the crash survey reports from 9000 heavy vehicles (buses & trucks), from which they have confirmed that more than 90% of the crashes occur at the velocity of 30-35km/h for buses & 35-40km/h for trucks. It was also found from the analysis of accident data that frontal impact against the other vehicle shows the highest rate. As the bus studied in this project does not comes under large bus category, hence the speed that will be considered in our project is 30km/h.

Jeffrey C. Elias et al [3] have worked on a similar project, where the analysis is carried out on a school bus. They have highlighted the fact that a rigid barrier impact is equivalent to two vehicles of similar size impacting at a closing speed of approximately 96 km/h. Thus based on this fact the analysis carried out in our project will approximately equal to the collision between two vehicles with a closing speed of 60 km/h. One design feature that is similar to that of our bus (NEKRTC bus) is the mounting of body to the rails of chassis by a series of clips or clamps as shown in figure 2. The crash analysis carried out on this bus has revealed the instabilities that occur with such kind of mounting of the body, where this non-rigid mounting feature allowed the bus body to slide forward approximately 92 cm (36 in.) during impact as shown in figure 3. Hence a rigid type of mounting is desired, thus the type of constraint used in the finite element model (FEM) of the bus is rigid type.

Figure 2. Clamp design used in NEKRTC bus

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Figure 3. Coach separated from chassis [3] The literature reveals the need for full scale frontal

impact analysis to better understand the behaviour of the structures. Though the fatalities due to the frontal impact of passenger bus are very high, the scale of such incidents has not prompted many automotive manufacturers to study such behaviour. Hence the work here is carried out in order to study the frontal impact of a passenger bus; also efforts have been made in order to improve the structural safety.

2. GEOMETRIC MODEL As discussed in earlier sections, the bus that has been

considered for the analysis is a passenger bus plying for “North East Karnataka Road Transport Corporation” (NEKRTC), which is the part of Karnataka State Road Transport Corporation (KSRTC) catering to the transport needs of north eastern region of the Karnataka state. All the geometric details were obtained from the drawings provided by KSRTC; other dimensional details unavailable in the drawings were directly measured. The snapshots of the bus under construction are as shown in figure 4 and figure 5.

Figure 4. Bus Chassis

Figure 5. Bus Structure

The geometric modeling of the bus was carried out using ProE 2001 software. In the beginning stages of the model construction, solid modeling approach was adopted. After which the mid-surfaces of these solid sections were extracted, as most of the geometrical features of the bus can be defined with thickness, which paves the way for shell meshing in the FE model. Most of the body structural members are steel tubes of square and rectangular cross sections as shown is figure 8.

Figure 6. Isometric view of 3D model

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Figure 7. Right view of 3D model

The geometric modeling was carried out with the considerations of following assumptions:

Parts which are not directly related to the frontal impact or which have no significant effect on the final output have not been considered.

All the sub-systems that were discarded in design process have been considered as lumped mass at appropriate locations.

All structural designs are as per the documents obtained from KSRTC.

Figure 8. Structure Cross section

2.1 Finite Element Modeling The FE modeling of the bus is carried out using Ansa,

as Ansa is very good software for shell meshing. Then the model is imported into Hypermesh for mesh refinement and creation of 1d Elements as shown in figure 9. The total elements of the model obtained from Ansa was too high, hence the areas that don’t have much significance to the final results have been meshed with 1d elements, for which the cross sectional properties have been assigned. This helped in reducing the total elements from 486350 elements to 264139.

The next activity was to define the contacts between various parts and sub-systems. To reduce the total modeling

time, the type of contact between various parts was node to node type of contact. For rest of the parts rigid contacts were defined, which was done by using 1d rigid element.

As discussed in assumptions section, the parts and sub-systems that don’t have much significance in frontal impact have been replaced with mass elements at appropriate places. This is done by connecting the mass elements with the nodes of the parts by using rigid elements.

Figure 9. Finite Element Model

Once the creation of finite element model is completed, the next activity was to create boundary conditions. It is very important to define the type of contacts between the bus model and the rigid wall. Hence a contact block was created wherever the significance of the impact is high as shown in figure 10 by using “*CONTACT_AUTOMATIC_SINGLE_ SURFACE”. The type of contacts used in crashworthiness analysis is “*CONTACT_ AUTOMATIC_SINGLE_ SURFACE”, hence this contact type is selected.

Figure 10. Contact block

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2.2 Units and Inputs The simulation has been carried out using various

inputs, and units used are as shown in table 1.

Table 1. Units and Inputs

Parameters Units / values used Length unit Millimeter (mm) Time 0.2 Seconds Mass unit Tonnes Acceleration due to gravity 9.81e+03 mm/sec2

Velocity equivalent at 30 kmph 8.33e+03 mm/sec

3. SIMULATION AND RESULTS The simulation of the bus is as shown in figure 11 at

different time zones, which was obtained from LS-Post post processing tool.

Figure 11. Simulation without modification The load plot is as shown in figure 12.The important

observation that has been made here is the peak load of 16.6 x 105 kN occurs at 0.06 seconds. The curve trend witnessed here clearly shows the time difference between buckling of various parts. The peak deceleration is around 12.5g as shown in figure 13. It can be observed that there is a spike in deceleration between 0.04-0.06 sec, this can be attributed to peak load acting on the body structural members. From the energy plot as shown in figure 14 it can be observed that the kinetic energy drops drastically the moment when the bus hits the rigid wall. The total energy obtained from the simulation is well in congruent with calculated values from the equations as shown below.

Figure 12. Force vs Time Plot

Figure 13. Deceleration vs Time Plot

Figure 14. Energy Plot

Impact force on right structural members was higher when compared to the left structure. This difference in impact force is mainly due to the presence of the door opening on the left side of the bus, this difference can be observed from the simulation as shown in figure 15. As shown in figure 5.8 the load on right structural member is twice as much as load on left side, which is around 14000 kN. Hence the load carrying capability on the right structures has to be given more importance in design phase

Figure 15. Comparison of right and left structure

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Figure 16. Impact force comparison on right and

left structure When we study the load vs time plot as shown in figure

12, it is clearly visible that the peak load is occurring at 0.06 secs. As discussed in earlier sections, the trend of curve where the spike occurs is mainly due to the sudden increase in load on one part or sub system. When we closely study the phenomenon with respect to simulation, it is observed that the frontal structural members starts carrying this high load and at the moment these members start buckling under load the peak load drops drastically. The parts subjected to peak load are as shown in figure 5.9 (parts highlighted with cursor arrow marks). The peak load can be reduced by providing crush initiators, which absorbs some amount of load during crumpling. These design enhancements and its study is discussed in detail in the following sections.

Figure 17. Parts subjected to peak load

4. DESIGN AND IMPLEMENTATION OF CRUSH INITIATORS

There are many different types of crush initiators, which help in reducing the peak loads. After comparing to the various types of crush initiators from [5], bead type crush initiator was selected for implementation. Bead type of crush initiators is provided at multiple locations, which in turn helps in reducing the peak load. Hence a comparison of this was made to justify the selection of this type of crush initiators. The comparisons is made by analyzing a steel tube without crush initiator, with single bead type crush initiator and with 3 bead type crush initiator as shown in figure 5.12. From figure 5.12 it is clearly evident that the 3 bead design crush initiator is more stable during crush.

Figure 18. Comparison of crush pattern

As shown in figure 19, the load vs time plot for all three designs have been obtained. The peak load in steel tube without crush initiator is around 190kN and the crush pattern is irregular. The peak load with the single bead design is around 160kN and the crush pattern in the beginning stages of crush is uniform, and at later stages the part starts bending. The peak load with the 3 bead design is around 150kN and the crush pattern is very much uniform with this design. With the 3 bead design there is a drop in 20% of the peak load when compared to with the normal tube. But we cannot expect similar percentage of drop in peak load when implemented in actual bus; this is because of the absence of ideal conditions during the frontal crash of the bus. Hence the 3 bead crush initiator design has been implemented at key locations of the bus structure where the peak load was occurring as shown in figure 20.

Figure 19. Load comparison of crush initiators

Figure 20. Implementation of crush initiators

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5. RESULTS AND DISCUSSION OF MODIFIED DESIGN

The simulation of modified structure design has been carried out with the same input details as the one which was carried out earlier. All the input data are same, only the model has been changed to accommodate 3 bead crush initiators. The simulation of the modified bus design is as shown in figure 21.

Figure 21. Simulation of modified bus with crush

initiators The load plot which shows the force vs time is as

shown in figure 22. The important observation that has been made here is the peak load of 15.9 x 105 kN occurs at 0.06 seconds. The curve trend witnessed here clearly shows the time difference between buckling of various parts. It is also evident that the curve trend remains the same from the original design; this is because of the design change being done for only one structural member. The curve trend can change with more design modifications.

Figure 22. Force vs Time plot

The comparison of load plot for both old design and the modified design is as shown in figure 23. It can be noted that there is a reduction in peak load; the total reduction is around 62000 kN which is around 4% reduction in the peak load. As discussed earlier the load curve trend is very much similar to the previous design, this can be attributed to the non-modification of any other design aspect of the bus.

Figure 23. Comparison of load plots

6. CONCLUSIONS The study of frontal impact of a passenger bus has

suggested many key areas that have to be considered while designing the structures of the bus. It was also studied for some of the design changes that could help in improving structure safety, although these suggestions are at very nascent stage which needs much finer design refinement in order to have an optimized solution. It was also evident that the lack of professional approach towards bus body design has contributed in unacceptable results during analysis, which opens up the road towards consideration of many design improvements. By non-linear analysis conducted on bus, the following conclusions have been drawn up. • It has been understood that the load distribution on the

structures are not uniform, which lays down the road to improvement in buckling characteristics of the structures.

• A redesign of left structural members is very essential, because of the presence of door opening on this side the structure is very much weak and it requires proper design in order to improve strength on this side. By doing this the load distribution can uniform on both the sides and it also improves the overall behavior of the bus during the frontal impact

• By having crush initiators, the peak load can be reduced. This has been achieved by implementing such designs to some of the structural members, which is around 4% reduction in peak load. Although there is an improvement in reducing the peak load, but a design with much better analysis can improve the situation.

• Absence of structural members that are specifically designed for absorbing high loads is evidently felt from the analysis. By having such members the damage caused to many other parts can be reduced and load management can be in a much controlled manner.

• The design improvement that has been achieved is just for few structural elements, if this approach is followed

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for many other key structural members then the design could be far superior.

• The floor deceleration is around 12g, which is well in agreement with ECE R80 regulation that specifies the floor deceleration to be around 8-12g at 30km/hr.

• From the analysis it is very much evident that a holistic approach in designing of various parts is required. It can be hereby concluded that there is tremendous amount of options available for the redesigning of the bus structures that can significantly improve safety.

REFERENCES [1] Vincze-Pap Sándor, Csiszár András, “Real and

Simulated Crashworthiness Tests on Buses” ESV 19th Conference, NHTSA, Paper Number 05-023, 2005

[2] Yoshiriro Sukegawa, Fujio Matsukawa, Takeshi Kuboike, Motomu Oki, “Heavy Duty Vehicle Crash Test Method in Japan”, NHTSA, Paper number 98-S4-O-13, 1998

[3] Jeffrey C. Elias, Lisa K. Sullivan, Linda B. McCray, “Large School Bus Safety Restraint Evaluation” NHTSA, Paper No. 345, 2001

[4] Mátyás Matolcsy, “Technical Questions Of Bus Safety Bumpers”, NHTSA, Paper number 05-0161, 2005 Hoag, Kevin L., (2006),

[5] Willibrordus J. Witteman, “Improved vehicle crashworthiness design by control energy absorption for different collision situations”, CIP-data library technical

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