Paper Number 2006-01-0464 Current Trends in Bumper Design for Pedestrian Impact
Peter J. Schuster
California Polytechnic State University Copyright © 2006 SAE International
ABSTRACT
Worldwide, the pace of development in pedestrian countermeasures is increasing rapidly. To better understand the state of the art in bumper design for pedestrian impact, a survey of literature and patents has been performed. Two general approaches to reducing the verity of pedestrian lower limb impacts were identified: (a) Provide cushioning and support of the lower limb with a bumper and a new lower stiffener, or (b) U the bumper as a platform for impact nsors and exterior airbags. This study focud on the first approach. Excluding bumper nsors, airbags, and non-design-related articles, a total of 130 relevant technical articles and 147 patents were identified.
The most common method propod for cushioning the lower limb in an impact us an energy absorber (plastic foam or ‘egg-crate’) in front of a mi-rigid (steel or aluminum) beam. There are also proposals for ‘spring-steel’, steel-foam composites, crush-cans, and plastic beams. The most common
method propod for supporting the lower limb in an impact is a condary lower beam, known as a ‘stiffener’ or ‘spoiler’. Most propod lower stiffeners are plastic plates or metal beams supported by the engine undertray, the radiator support, or the front-end module. In addition to the concepts, there are a number of design proposals involving a deploying bumper or lower stiffener. INTRODUCTION
Pedestrian-vehicle accidents are a globally recognized safety concern. Efforts toward modifying vehicle designs to offer more protection for pedestrians began in earnest in the 1970s. In parallel, test procedures to evaluate the performance of the new designs were developed. In industrialized countries pedestrian safety has improved significantly since then. However, as the number of motor vehicles increas rapidly in less developed nations, global pedestrian traffic fatalities remain a major issue.
Beyond the real-world concerns, other incentives for automakers to introduce design features to enhance pedestrian safety are current and planned public domain tests and government regulations. PUBLIC-DOMAIN TESTS
Pedestrian-vehicle impact tests have only recently become part of the mainstream. Since 1996, the
European Union has been subjecting lect vehicles to a battery of tests (frontal, side, pedestrian) as part of EuroNCAP [1]. The pedestrian tests consist of bumper impacts with a ‘leg-form’ impactor, hood edge impacts with an ‘upper leg-form’ impactor, and hood/fender impacts with two different ‘head-form’ impactors (e Figure 1). A vehicle is typically subjected to 3 bumper impacts, 3 hood edge impacts, and up to 18 head impacts. Vehicle results are reported with a 4-star rating system. ANCAP* tests are identical to EuroNCAP. JNCAP† also performs tests simulating pedestrian head impacts onto the hood and fenders, but not lower limb impacts. Vehicle performance in the test ries has been improving, so it appears European and Japane manufacturers are addressing the tests in their designs.
GOVERNMENT REGULATIONS
Pedestrian impact requirements are the subject of two existing regulations in Europe and Japan. Though the requirements differ, there are efforts to introduce a Global Technical Regulation to commonize them [2].
In 2003, the European Parliament and Council approved Directive 2003/102/EC [3], which states that new vehicle introductions must have a specified level of pedestrian impact performance starting in 2005 (e Figure 1).
A recent regulation in Japan specifies vehicle pedestrian head impact protection performance, but not lower limb. New vehicle introductions must meet the requirements in 2005.
In addition to the existing regulations, the European Commission has issued a draft directive regarding the u of frontal protection systems (e.g., bull-bars) [4]. This draft may have an influence on some of the design alternatives identified in this study.
* Australian NCAP, www.aaa.asn.au/ancap.htm
† Japane NCAP, jp/asss/indexe.html
integrityKnee bending< 21°HPC< 1000
(Monitor Only)(2/3 of area) Directive 2003/105/EC Directive 2003/105/EC Directive 2003/105/EC
Figure 1: Pedestrian impact test procedures
PEDESTRIAN LEG IMPACT TEST
津桥留学A brief discussion of the pedestrian leg impact requirements will be helpful before proceeding into th
e design alternatives found in the literature. The purpo of the pedestrian leg impact test procedure is to reduce the occurrence of lower limb injuries in pedestrian accidents. In the pedestrian leg impact test, a ‘leg-form’ impactor is propelled toward a stationary vehicle at a velocity of 40 km/h parallel to the vehicle’s longitudinal axis. The test can be performed at any location across the face of the vehicle, between the 30º bumper corners. The acceptance criteria are illustrated in Figure 2. The maximum tibia acceleration criterion is intended to prevent tibia fractures. The knee bend angle and shear deformation criteria are intended to prevent knee joint injuries such as ligament ruptures and intra-articular bone fractures.
LIMITATIONS
This study is a review the state-of-the-art (as of January 2005) in the design of bumper systems for pedestrian impact. Becau this task relies on work conducted primarily in Europe and Asia, markets with few light trucks, the design trends identified are bad on pasnger cars.
Figure 2: Pedestrian ‘leg-form’ injury criteria
To focus the study, articles and patents were limited to tho specifically describing bumper designs. Articles and patents dealing with the following were excluded: •Other areas of pedestrian impact analysis (e.g.
head, torso, and thigh impacts, accident data
analysis, impact kinematics and biomechanics, test
procedures, and computer simulations)
•Design of other vehicle components (e.g. impact nsors, external airbags, hood, fender, shotgun,
headlamps, wipers, windshield)如何提高托福写作
METHODOLOGY
Standard literature and patent arch techniques were ud for this study. Keyword arches followed by manual asssment of relevance were ud to limit the field to tho documents of interest to this study.
LITERATURE SEARCH
In addition to using the standard library databa arch engines, directed arches were pursued in:
•SAE technical papers ()
•‘Enhanced Safety of Vehicles’ conferences
(www-nrd.v/departments/nrd-
01/esv/esv.html)
• IMechE* technical papers
(uk/ils/catalogues.asp)
• UMTRI† library
(www.umtri.umich.edu/library/simple.html)
The outcome of the arches is believed to be comprehensive in scope. While some technical articles may have been misd, the majority of relevant articles have been identified. Conclusions reached regarding design trends should not be affected by more arches. Following identification, articles were categorized bad on their abstracts. Selected papers were identified for collection and further review. The material prented in this paper is a result of the abstract and lected paper reviews.
PATENT SEARCH
The patent arch relied on governmental patent databas, many of which include international patent listings:
•German Patent Office (depatisnet.dpma.de) •European Patent Office () •Japane Intellectual Property Digital Library (jp/homepg_e.ipdl) •Singapore Patent Office (v.sg) •US Patent Office (v)
•World Intellectual Property Organization
(www.wipo.int)
Following identification, patents were categorized bad on abstracts and drawings. Selected patents were identified for further review. The trends identified here are a result of the abstract and lected patent reviews.
* Institute of Mechanical Engineers (UK)
† University of Michigan Transportation Rearch Institute RESULTS
LITERATURE SEARCH
A total of 130 relevant articles were identified. Of the 61
recent (published since 1990) articles, approximately
25% were authored by OEM’s, 25% by suppliers, and
50% by other groups. Tables 1-3 summarize the number of relevant articles authored by company, and Appendix A provides a list of all articles identified.
PATENT SEARCH
A total of 147 relevant patents (covered by 290 filings)
were identified. Tables 1-3 summarize the assignees
and types of design solutions identified in the patents,
and Appendix B provides a list of all patents.
Table 1: Number of recent non-corporate pedestrian
bumper publications and patents
Recent Articles Patents
Individuals 9
18
Government Labs 4 2
Universities 9
- Consultants 5
- Consortia 4
-
TOTAL: 31 20
Table 2: Number of recent OEM pedestrian
bumper publications and patents
PATENTS Company R e
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1½½
DCX 162½
1½ 1½
½ Fiat 11 1
Ford/Jaguar317 2 4 2 1½2½14
GM/Opel - 31½ ½ ½ ½
Honda 34½ 111½
Hyundai 21 1
Kia 1-
Mazda 28½
4½ 111 Mitsubishi - 44
Nissan - 4 1 2½
½
Peugeot - 1 1
Rover - 1 1
Subaru/Fuji110 2 3 1 ½3½
Toyota - 6 1 2 21
Volkswagen- 3 2 1
TOTAL:147013½ 20 6 34½6413
Table 3: Number of recent supplier pedestrian
bumper publications and patents
PATENTS
Company
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T o t a l
F o a m
S t i f f e n e r
B e a m
E g g -c r a t e
A c t i v e
B u m p e r
A c t i v e S t i f f e n e r C r u s h -C a n s
O t h e r
l. - 1 ½ ½ Aisin Seiki - 1 ½ ½ Alcan 1 -
Atlas Auto - 1 1
Bayer - 1 ½ ½
Benteler - 1 ½ ½ Calsonic Kani - 2 1 1 Cellbond - 2 2
Decoma - 3 1 2
Denso - 1 1
Dow 3 2 2
Dynamit Nobel - 3 1 2 Faurecia 1 - FMB Fahrzeug - 1 1 FPK - 1 1
G P Daikyo - 2 1 1
GE Plastics 5 6 1 5
Inoac - 1 1 JSP Corp 3 5 4 1 Kobe Steel - 1 1
Linpac - 2 1 1
Man Nutzfahrz. - 1 1
Mitsuboshi Belt. - 1 1
Netshape 1 3 1 2 Peguform - 2 1 1 Plastic Omnium - 6 2 2 ½ 1½
Raufoss Auto. - 1 1
Siemens 1 -
Solvay 1 -
SSAB Hardtech - 1 1 Tatsuno - 1 1 Valeo - 1 1
ZF Boge - 3 2 1
TOTAL: 16 57 13 8 9 10½ 3 2½ 2 9
ANALYSIS OVERVIEW
An asssment of the pedestrian bumper design publications identified two propod approaches: • Design the vehicle front-end components to provide
the appropriate stiffness to cushion the impact while at the same time providing support of all parts of the
limb to limit knee joint lateral bending. This alternative is the focus of this paper.
• Design an active pedestrian safety system, utilizing
nsors and external airbags to cushion and support the lower limb. While there are a growing number of publications in this area, this alternative does not drive bumper design specifically, so is not discusd further here.
CUSHION (ENERGY ABSORPTION)
The cushion function of the bumper in a pedestrian
auntimpact is directly related to the acceleration impact criterion shown in Figure 2. It is intended to reduce the verity of bone fractures in a pedestrian impact. This
function is not entirely dissimilar from the traditional
function of a bumper system (absorbing energy of a vehicle impact). But, there are two key differences: the
impact energy and the acceptance criteria. Impact Energy A vehicle-to-vehicle impact requires a local energy
absorption ‘density’ approximately double that of the pedestrian impact, as can be en through this brief analysis: The pedestrian leg-form test device has an effective width of 70-mm. Assuming that a typical bumper energy
absorber is 150-mm tall, the contact area is (70)x(150) =
10500-mm 2. The total impact energy at 40 km/h is ½mv 2 = ½(13.4 kg)x(11.1 m/c)2 = 825 Joules. As a result,
the required energy-absorption ‘density’ of the bumper energy absorber for a vehicle-to-pedestrian impact is approximately (825/10500) = 0.08 J/mm 2. A pendulum impact engaging only the top 50-mm
(typical worst ca) of the energy absorber compress an area (50)x(500) = 25000-mm 2. The total impact
energy for a 1500-kg vehicle at 5-mph is ½mv 2 = ½(1500 kg)x(2.22 m/c)2 = 3696 Joules. So, the required energy-absorption ‘density’ for a 5-mph vehicle-to-vehicle impact is approximately (3696/25000) = 0.15
J/mm 2.
Acceptance Criteria For the leg-form impact, the acceleration of the test device must be 150-g or less. For the vehicle impact, the cascaded requirements are maximum force at the
frame rail (to prevent damage to the structure) and
maximum intrusion (to prevent damage to other
components).
The maximum force allowed for vehicle low-speed impact is significantly higher than that tolerated by the human lower limb (as measured by the acceleration criterion). In addition, the intrusion limit,
combined with the desire to limit the front-end vehicle length, tends to drive the bumper stiffness as high as possible while still meeting the allowable force limit. This difference
between the acceptance criteria is the main cau for conflict between the two impact requirements.
The goal in the design of bumper components to cushion a pedestrian impact is to limit the ‘leg-form’ acceleration without either (a) sacrificing vehicle damageability, or (b) significantly increasing the depth of the bumper system.
Cushioning Methods
The literature and patent review identified different approaches to perform the cushioning function. The are summarized below in order of decreasing popularity, as measured by the number of patents describing each propod solution. An example patent is listed for each. Foam Energy Absorbers – 35 collected patents describe alternative methods for absorbing pedestrian impact energy using plastic foams. The goal of all of the designs is to improve the energy absorption efficiency of existing foam absorbers, and therefore minimize the increa in vehicle length to meet both pedestrian impact and vehicle impact requirements:
•Foam dimensions (13 patents, e EP 1422110) – by changing the contacting shape of the foam, the
respon of the leg-form device can be tuned. For
example, the foam does not have to absorb all the
impact energy, it can convert some into leg rotation. •Multi-density foam (7 patents, e EP 1046546) – by placing low- and a high-density foams in quence
in front of a bumper beam, bumper stiffness can be
tailored to different impacts.
•Fluid-filled foam (7 patents, e WO 9725551) – The patents describe alternative fluid-foam
composite materials to improve energy efficiency. •Depression in beam (5 patents, e US 6764117) – by providing an area within the beam for the
compresd foam to sit, more of the foam depth canpractical
be ud for energy absorption. This is important
since typical foams only compress 70%.
•Foam coring (3 patents, e JP 2004224106) – by removing material on the backside of the foam, the
effective energy-absorption efficiency can be
improved.
Molded Plastic Energy Absorbers – 21 patents describe plastic structures to absorb the impacts. In general, the structures replace existing plastic foams, and are intended to improve the energy absorption efficiency for both vehicle and pedestrian impacts:
•‘Egg-crate’ molded shapes (13 patents, e US 6726262) – Relatively complex molded plastic
structures can be ud to deflect and crush in low-
and high-energy impacts. Early versions of the
designs remble the inside of an ‘egg-crate.’ •Variable stiffness concepts (4 patents, e US 6554332) –Plastic structures that provide different
stiffness for different contacting objects have been
propod. For example, a thin object encounters
stiffness X, while an object four times as thick might
encounter a stiffness of 16X.
•Open shell & other shapes (4 patents, e EP 1365945) – Replacing the energy-absorber with
empty space and a simple bumper cover can
provide enough stiffness to stop a pedestrian leg-
form. However, the designs do not necessarily
address vehicle impacts.
Air-filled Energy Absorbers – 11 patents describe air bladders ud as energy absorbers, as a means to improving the efficiency. In five of the (e DE 2645823), the stiffness is the same for all impacts. In the rest (e JP 09020192), valves are ud to vary the stiffness varies bad on the object struck.
Flexible or Plastic Beam – 8 patents describe changes to the bumper’s structural member to make it more compliant for a pedestrian impact (e US 6494510), with or without an additional absorber.
Deploying Bumper – 7 patents describe bumpers that provide for additional energy absorber depth without increasing vehicle length by retracting the bumper under normal conditions, and only pushing it out when an impact is predicted (e GB 2368565).
Crush-Cans – 7 patents describe deformable bumper beam attachment structures such as crush cans or pistons. This allows for the impact energy to be absorbed not just in front of a beam, but also behind. In four of the (e DE 3434844), the stiffness is fixed. In the remainder (e JP 2000025540), the stiffness is varied bad on the type of impact.
Add-ons – 6 patents describe parate deformable structures added outside the vehicle to protect the pedestrians (e EP 0797517). The structures appear similar to ‘bull-bars’ but are designed s
pecifically to provide energy absorption and protection of pedestrians.
Foam-encapsulated metal – 3 patents describe methods of encapsulating a metal bumper beam inside the energy-absorbing foam (e US 6793256). The goal is to optimize the interaction between the two pieces and reduce the required foam depth.
Steel energy absorbers – 2 patents describe steel spring structures to store impact energy from different impacts (e US 6398275). The may be ud in conjunction with or independent of plastic foams.
