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ABSTRACT
Car accidents are considered as one of the most threatening dangers in daily life. Especially, frontal accidents have a high fatality rate. Due to increased traffic, growing concern of general public, and more stringent legislation have made vehicle safety one of the major research areas in automotive engineering. Euro NCAP regulation is enforced to ensure that the car structures are designed to withstand frontal off-set with minimal structural deformation and occupant injury. This regulation specifies the requirements for the safety of occupants.
In the present work, frontal offset impact test has been simulated according to Euro NCAP safety regulations for an existing car structure. In the simulation, safety restraint systems like driver airbag and seatbelt systems have been included. The existing steering fitted with airbag has been used in this project. As required by the regulation, 50th percentile male dummy has been used and assembled in a FE car. The 40% offset frontal impact with deformable barrier has been carried out in order to assess structural performance and injury levels of dummy with respect to regulations. Based on the predicted injury levels “star rating points” have been calculated for the existing structure. For coming up with new and improved design, axial crush performance of front rails with different cross-sections have been simulated. Comparing the performance of these cross-sections and that of the existing design a new structural design with higher “star rating” can be designed.
INTRODUCTION
Today, more than ever before, safety sells cars. For car buyers it is a key element of their purchasing decision. It's essential that motoring consumers can obtain reliable and accurate comparative information regarding the safety performance of individual car models. Serious or fatal traffic accidents are considered as one of the most threatening dangers in daily life. Especially, frontal accidents on country roads against other cars have a high fatality rate. Due to increased traffic intensity, growing concern of the general public, and more stringent legislation have made vehicle safety one of the major research areas in automotive engineering.
CRASH-BOX
Crash box, with which a car is equipped at the front end of its front side frame, is one of the most important automotive parts for crash energy absorption. In case of frontal crash accident, for example, crash box is expected to be collapsed with absorbing crash energy prior to the other body parts so that the damage of the main cabin frame is minimized and passengers are saved their lives. Conventionally, a crash box is equipped with several ditches as shown in Figure 1, called “crash beads”, so that those crash beads may initiate buckling deformation and make the crash box easily collapse. Recently, it has been strictly required to satisfy both reduction of body weight and improvement of crash worthiness and thus, regarding crash box, it is required to ensure high energy absorption using sheet as thin as possible.
However, there is often the case that the crash beads do not work as designed when a thin sheet is applied as the material for a crash box and thus, it has become difficult to acquire sufficient energy absorption only by the crash beads. In this report, attention is focused upon finding an optimum cross sectional shape of a crash box to ensure high capability for energy absorption without crash bead. By making use of FEM, first a mechanism through which a body part absorbs crash energy in axial collapse was clarified and then, the influence of cross sectional shape of the part on energy absorption was quantitatively revealed. Finally, a new design scheme of cross sectional shape of a crash box was proposed.
1.2 NEW CAR ASSESSMENT PROGRAM (NCAP)
The National Highway Traffic Safety Administration (NHTSA) is an integral part of the United States Department of Transportation (DOT) and its mission is to save lives, prevent injuries, and reduce traffic-related health care and other economic costs associated with motor vehicle use and highway travel. To accomplish this, NHTSA collects and analyzes motor vehicle crash data, and develops, promotes, and implements educational programs, vehicle safety standards, research, and enforcement programs.
In 1979, NHTSA created the New Car Assessment Program (NCAP) to improve occupant safety by developing and implementing meaningful and timely comparative safety information that encourages manufacturers to voluntarily improve the safety of their vehicles. Since that time, the agency has improved the program by adding rating programs, providing information to consumers in a more user friendly format, and substantially increasing accessibility to the information via the website, www.safercar.gov. The program has strongly influenced manufacturers to build vehicles that consistently achieve high ratings, thereby increasing the safety of vehicles. However, the success of the NCAP requires change if manufacturers are to be continually challenged to make voluntary safety improvements to their vehicles. The opportunities for NCAP to be changed and improved are a result of:
• Changes in the vehicle fleet and resulting crash dynamics.
• Advances in injury criteria and test devices.
• The development and deployment of vehicle technologies that have the potential to improve safety.
New approaches in the presentation of NCAP ratings information for consumers the agency plans to continue enhancing its NCAP crashworthiness of a vehicle that protect occupants during a crash) and crash avoidance (those aspects of a vehicle that help avoid the crash) activities by challenging manufacturers, and by providing consumers with relevant information to aid them in their new car purchasing decisions. This document describes the opportunities that exist and some approaches to address them.
1.3 EURO NCAP RATINGS ASSESSMENT
By law, all new car models must pass certain safety tests before they are sold. But legislation provides a minimum statutory standard of safety for new cars; it is the aim of Euro NCAP to encourage manufacturers to exceed these minimum requirements. For cars tested before 2009, Euro NCAP has released three ratings: adult protection, child occupant and pedestrian protection. The ratings for adult protection and child protection are achieved as a result of three impact tests that Euro NCAP carries out: frontal, side and pole test. Euro NCAP carries out a separate range of pedestrian tests to reach the score of the Pedestrian Rating.
Euro NCAP chooses these types of tests to cover the range of accidents form some of the dominant causes of serious and fatal injuries. In addition, Euro NCAP rewards cars for having an intelligent seatbelt reminder as part of the adult protection rating. When buying a car tested before 2009, Euro NCAP advises to take all three ratings into consideration for the car buyers. The Euro NCAP will be updated with every new released regulation, where certain criteria have to be met by car manufacturers.
2. BACKGROUND THEORY
The objective of a crash test for Federal Motor Vehicle Safety Standard (FMVSS) No. 208 is to measure how well a passenger vehicle would protect its occupants in the event of a serious real world frontal crash. This is sometimes referred to as the crashworthiness of a vehicle. This report reviews potential test procedures for evaluating frontal crashworthiness. Structural design for crashworthiness seeks to mitigate two adverse effects of a crash:
(1) Rapid deceleration of the occupant compartment.
(2) Crush of the occupant compartment survival space.
In a severe crash, the speed of a vehicle often decreases from its travel speed to zero in a hundred thousandths of a second. One important way to minimize the injury consequences of this abrupt change in velocity is to extend the amount of time necessary to slow the vehicle down includes the less abrupt the change in velocity, the lower the crash forces on the occupant.
EURO NCAP PRINCIPLES
The Euro NCAP programmer is designed to provide a fair, meaningful and objective assessment of the impact performance of cars. It is intended to inform consumers, so providing an incentive to manufacturers as well as giving credit to those who excel at occupant or pedestrian protection. The tests used are based on those developed for legislation by the European Enhanced Vehicle safety Committee (EEVC), for frontal impact protection of car occupants. Economic constraints prevent the tests from being repeated, so to take account of vehicle and test variations a number of actions have been taken:
• Manufacturers have been asked to compare the results from these tests with those from tests they may have conducted and to draw our attention to any anomalies they may find. They have also been requested to supply data from their own tests to us for comparison. Several manufacturers have supplied data for this purpose. Apart from considering the effects of test variation and identifying anomalies, no account of such data is taken in rating the cars and it is kept confidential.
• The overall assessments are based on the combination of multiple results. Variations in any one of these will only have a limited effect on the overall rating.
2.3 WORKING PRINCIPLES OF RESTRAINT SYSTEMS
Sensors detect the impact
As the satellite sensors (collision sensors) attached to the vehicle detects the collision, a signal is sent to the ECU (Electronic Control Unit). When the speed of the vehicle suddenly drops because of impact, a separate vehicle sensitive locking feature located in the retractor detects the negative acceleration (rate of deceleration) and locks the webbing to keep it from further extraction.
Evaluation of the impact
The ECU processes the signal from the satellite sensors and, diagnoses the severity of the impact. If the impact is identified by the ECU to be a collision, the ECU sends a signal to activate the pretensioners (gas emitting devices) attached to the seat belt.
The Pretensioners go into action.
Next, the pretensioners activate and retract some of the seat belt webbing. By pulling the seat belt webbing back, the pretensioners remove some of the slack between the passengers and the belts, thereby, restraining the passengers in their seats more effectively. Removing this slack greatly enhances the passenger protection function of the seat belt system. By using the crushing of the car body efficiently, the system helps absorb the energy of motion that comes to bear on the passengers.
• The load limiter goes into action as the occupant moves forward.
The shock resulting from the accident reaches the passengers and the inertial force moves them forward. At this very moment when a specified level of weight is transferred to the webbing, the seat belt load limiter activates and is allowed to move webbing from the housing to help absorb the early weight burden on the occupant.
• Energy of movement is absorbed.
At this moment the seat belt has positioned the occupant to take advantage of the supplemental restraint system (airbags). The energy of movement of the passenger’s body is absorbed by the crushing of the vehicle, the load limiter and the airbags.
• Absorption of the energy of passengers’ movement is completed.
Seat belts are the primary device for passenger safety. The “airbag” is “supplemental restraint system” (SRS). As the meaning of the name suggests, the airbag plays a supplementary role in restraining and protecting passengers and are most effective when the occupants have their seat belts fastened.
EURO NCAP PHASE IN FROM 2009
As of 2009, Euro NCAP only releases one overall star rating for each car tested with a maximum of five stars. This overall safety rating is composed of scores in four areas: adult protection, child protection, pedestrian protection and safety assist.
The overall score is calculated by weighing the four scores with respect to each other, while making sure that not one area is underachieving. The underlying dynamic tests are identical to those before 2009, except for the addition of a test for Whiplash neck injury protection in rear impact. Also, Euro NCAP now rewards not only Seatbelt reminders, but also Speed Limiters and the standard fitment of Electronic Stability Control.
Euro NCAP claims that it provides motoring consumers with a realistic and independent assessment of the safety performance of cars. The following Figure 3.1., representing the complete point’s distribution to meet safety requirements.
Eugenio Toccalino has been carried out the study on crash performance and interior energy management. Based on the study, three of the most widely adopted rating criteria, i.e., Euro-NCAP, US-NCAP and IIHS, it is clear that testing conditions are more severe, i.e. increased barrier speed, and the biomechanical readings required to obtain good rating are more stringent versus the legal prescription. The adoption of different dummies (Euro-SID vs. US-SID), different testing conditions (full rigid frontal barrier vs. ODB frontal barrier, different size mass ground clearance for the side impact barriers, different impact speed), and different rating criteria highlight different critical areas and make the harmonization between the three programs are quite complex.
From the three different rating systems it has been clear that the main approach to improve rating are through the use of vehicle body structure reinforcements, which add weight and an increased number of air bags, which add both weight and cost. If properly optimized, interior based solutions using crash friendly trim design, collapsible structures, integrated ribs, stand alone cartridges, and foam padding could be further utilized to improved star rating and lower vehicle weight and cost. It is particularly true of solutions, which offer high efficiency and adequate load control, to meet the two key attributes that characterize energy management in the critical areas.
3.2 EURO NCAP ADULT OCCUPANT SAFETY
The following Impact tests have been carried out to assess points for occupant protection:
i. ECE-R94 Deformable Barrier frontal impact
ii. ECE-R 95 Side Impact
iii. Side Pole Impact
iv. Low Speed Rear Impact
3.3 EURO NCAP FRONTAL OCCUPANT SAFETY
From the Table 3.1 it is clear that the majority of protection is given for adult occupant inside a car and a weight factor of 50% is given. Since the areas of assessment for adult occupants is described in Figure 3.1. In vehicle to vehicle crashes, it is unlikely the vehicle will deform in axial direction, hence the deformable barrier with 40% offset for the vehicle represents the real life scenario.
The frontal impact configurations are represented in Figure 3.2. The frontal impact test performed by Euro-NCAP is based on the European ECE-R96 frontal impact protocol with impact speed, i.e., 64km/h. 50th percentile male HYBRID III dummies are fitted into the driver and front passenger seat. Both dummies are belted during the test with the airbags for driver and frontal passenger. Barrier is made of aluminum honeycomb, absorbs some of the energy and 40% offset of the vehicle leads to transfer of entire impact on just one corner of the vehicle. As a result the vehicle crushes more and such tests impact higher injuries to the occupant due to higher intrusions in the compartment. This test help to evaluate the vehicle deformation and the injury sustained to vehicle occupants due to very high vehicle intrusions from the impact.
3.4 OFFSET DEFORMABLE BARRIER
Figure 3.4 represents Offset Deformable Barrier (ODB) which is been used by Euro NCAP and most of leading car manufacturers worldwide. This deformable barrier is used for frontal offset impact while the specifications developed by EEVC WG11.
Frontal offset crash test program
About half of all passenger vehicle occupants killed on U.S. roads die in frontal crashes. Major strides have been made in frontal crash protection, thanks in large part to the crash test program the government began in the late 1970s and the offset tests the Institute began in 1995.
The Frontal Offset Test.
In the Institute's test, a vehicle travels at 40 mph toward a barrier with a deformable face made of aluminum honeycomb. A Hybrid III dummy representing an average-size (50th percentile) man is positioned in the driver seat. Forty percent of the total width of the vehicle strikes the barrier on the driver side. The forces in the test are similar to those that would result from a frontal offset crash between two vehicles of the same weight, each going just under 40 mph.
3.6 Rating criteria
Engineers consider three factors to determine a vehicle's frontal rating: structural performance, injury measures, and dummy movement. 3.9 Rating criteria
3.7 Structure/safety cage:
To assess a vehicle's structural performance, engineers measure the amount of intrusion into the occupant compartment after the crash. Measurements are taken at nine places around the driver's seat (right).
3.8 Injury measures:
Injury measures from the dummy in the driver's seat are used to determine the likelihood that a driver would sustain various types of injuries in a similar real-world crash. The numbers recorded from the head, neck, chest, legs, and feet of the dummy indicate the level of stress or strain on that part of the body — in other words, the risk of injury.
3.9 Restraints/dummy movement:
Even if injury measures are low, it's important to consider the dummy's movement during the crash, since not all drivers are the same size as the dummy or seated exactly the same way. A close call for the dummy could be an actual injury for a person. Before each crash test, technicians put grease paint on the dummy's head, knees and shins. After the test, the paint shows what parts of the vehicle came into contact with those parts of the dummy. The paint, combined with high-speed film footage of the crash, allows engineers to evaluate the dummy's movement.
4.1 PROBLEM STATEMENT
In automotive world more emphasis has been given to the safety of passenger car occupants. For both fatally and seriously injured occupants, frontal impacts are the most important crash type. As studied in the various literatures and surveys, it has been found that in real life scenarios most of the life threatening crashes occur in offset between the vehicles. In order to reduce injury levels structures should be well developed to meet safety regulations.
4.2 OBJECTIVES
• To review the literature on the crash performance of a car structure and NCAP ratings.
• To conduct full vehicle 40% offset frontal impact of FE model of passenger car with the existing type of frontal structure.
• To investigate the crash performance of an existing car frontal “S‟ rail structure.
• To determine injury levels of the car frontal occupants and correlating for Euro NCAP points (Seat belts and Airbag mandatory).
• The new designs will be developed for the “Crash box‟ and “S‟ rail frontal structures of the passenger car and to perform impact analysis to study energy absorption capabilities.
• To implement developed ‟S‟ rail frontal structures into full vehicle and conducting frontal 40% offset impact and to study its effects on occupant injury (Seat belts and Airbag mandatory).
• To quantify crash performance for NCAP rating. To quantify crash performance for NCAP rating.
4.3 METHODOLOGIES
• Literature review of impacts of cars, accidental statistics, newly released NCAP regulations for safety of occupants was carried out by referring reviewed journals, books, manuals and related documents.
• Refinement and pre-processing of the finite element model of a passenger car is done using Hyper mesh 9.0
• Frontal 40% offset impact simulation of a FE model of passenger car with the existing type of structure is carried out as per Euro NCAP regulations using RADIOS as a solver.
• Injury levels to the occupants will be found out reviewing the results by using Hyper view and correlating to injury level points.
• Component design for basic shape and size of the energy absorbing and determining the dimensions is done using CATIA V5R16.
• Various geometric designs for “S‟ rails will be developed.
• Good absorbed design structure showing minimum deceleration levels is implemented to full vehicle.
• Frontal crash analysis will be carried out for redesigned structures to analyze the effect on the occupants and to check for NCAP points.
• A newly obtained rating point under frontal impact is compared with the existing car points.