expanded polypropylene (epp) – a global solution for...

Document reference: Expanded Polypropylene (EPP) - A Global Solution for Pedestrian Safety Bumper Systems The information contained herein is provided as a convenience to JSP’s customers and reflects the results of internal laboratory tests conducted on samples of material molded from ARPRO® expanded polypropylene manufactured by JSP. While all reasonable care has been taken to ensure that this information is accurate as of the date of issue, JSP does not represent, warrant or otherwise guarantee, expressly or impliedly, the suitability, accuracy, reliability or completeness of the information.

Expanded Polypropylene (EPP) – A Global Solution for Pedestrian Safety Bumper Systems Seishiro Murata, Satoru Shioya Automotive Materials Development Group JSP

Bert Suffis Development Manager JSP

ABSTRACT

The paper outlines some novel approaches to designing pedestrian safe bumper systems using expanded polypropylene, EPP. Traditional advantages of EPP, such as multiple impact performance and resilience are evaluated in the light of compatibility between conflicting impact requirements. The performance criteria of pendulum, insurance and pedestrian impacts are met by integrating the absorber into the complete energy management system and by the optimisation of the EPP design and density. The necessary packaging space remains minimal through the efficient use of the strain characteristics of EPP. Concept ideas related to fore/aft dual density as well as geometry-based dual-density are outlined. The concepts, developed and tested on a full car system, illustrate how soft initial contact can be combined with a high energy absorbing capacity.

INTRODUCTION

Throughout the automotive industry expanded polypropylene energy absorbers have been acknowledged as presenting the most economically interesting mix of physical performance, light weight, recyclability, design freedom, and multiple impact performance. Recent developments of the raw material result in better long-term temperature stability and allow the production of ever more challenging concepts.

The scheduled integration of pedestrian safety systems has significantly transformed the requirements of the front end of the car. Automotive design engineers are challenged to deliver a universal bumper system, which is packaged in a trendy design. An in-depth understanding of the novel design and transformation capabilities of EPP components opens up a wide scope of system solutions. Perceived limitations in design freedom, density range and strain-related bottoming-out can efficiently be overcome by the integration of the EPP component as an engineered part of the energy absorbing system.

Aside the crash criteria, the overall requirements of a new bumper system can be summarised as:

1. Performance throughout the life of the vehicle, requiring multiple impact performance.

2. Light weight, recyclability, system costs equivalent or lower than existing solutions.

3. Limited packaging space, without restrictions on design freedom.

4. Quick development cycle through easy design rules and accurate FEA-assisted virtual development.


MARKET REQUIREMENTS

OEM’s and Tier 1’s are increasingly confronted with the need to render platform-based designs compatible with regional regulations and differentiated customer requirements. The North American and Canadian bumper performance standards are more severe than the European ones. The former stipulate impact speeds of 5mph (8kmh) whilst applying to generally heavier cars, whereas the European pendulum impacts are performed at 2.5mph (4kmh). A bumper system can be designed to pass the European bumper requirements by integrating an EPP absorber in medium density. The cars exported to the North American market can easily be equipped with a bumper core in high density, which can be moulded in the same cavity using the same production equipment.

Criteria like bumper repair cost and ease of replacement become increasingly important marketing features. The non-crushable closed cell structure of EPP results in excellent energy absorbing characteristics and provides the system with a spring-back force limiting catastrophic failure of neighbouring components. Fixation elements, wire harnessing, and fascia support features, including negative draft features, are easily integrated into an EPP part, contributing to the overall ease of assembly.

Numerous papers and extensive in-house development have highlighted the importance of the overall design of the front-end as well as the materials used within the bumper system. The former criterion lies in the hands of the car designers and is largely defined by the targeted market segment and the regional styling requirements. Indeed, North American vehicles traditionally have protruding bumper systems, whereas European OEM’s tend to integrate the bumper system into a fluid bonnet design. The Asian domestic markets have taken the reduction of packaging space a step further by integrating the bonnet into the compact car body.

In order to accommodate the changing styling requirements the design engineers are increasingly trying to limit the overall space of the energy absorbing system. The performance criteria, on the other hand, now include the traditional low speed impacts (4kmh or 5mph pendulum), the insurance tests (Danner or IIHS evaluation), as well as the upcoming pedestrian safety lower leg impacts.

SYSTEM REQUIREMENTS FOR PEDESTRIAN SAFETY

Industry experts agree on the need to accompany the leg over its complete length so as to limit the geometry induced bending and shear of the knee joint. An additional ankle support system is deemed necessary to create an upward rotation avoiding the pedestrian being overrun.

The control of the deceleration dictates the use of a ‘soft nose’ upon first contact of the lower leg with the bumper fascia. Pedestrian safe bumper fascias thus tend to be less rigid and leave more energy to be absorbed by the rest of the system. The energy absorbing system has to be sufficiently soft to respect the g-limit of the pedestrian requirements, but it must also absorb the impact energy of the pendulum hits. To achieve both required properties, the energy absorber has to include relatively soft foam for the pedestrian lower leg impact and the traditional relatively hard energy absorbing foam for the 2.5mph (or 5mph) pendulum impact, within the available packaging space.

Another major consideration upon developing a bumper system is its compatibility with the other pedestrian impact requirements. High speed camera monitoring has revealed no buckling during compression experiments of expanded polypropylene foam. This absence of Poisson effects eliminates all lateral loads onto sensors and harnesses during the impact. The sensing components and triggering mechanisms that control the active hood systems can thus be seamlessly integrated into an EPP absorber.


GENERIC EPP BUMPER TECHNOLOGY

The inherent performance characteristics of EPP dictate the basic design rules of an EPP energy-absorbing component. The closed cell nature of EPP foam results in its typical compression characteristic, shown in Figure 1. The initial elastic region is followed by a compression plateau, which progresses into the bottoming-out region. For strains higher than about 65-70%, the overall energy absorbing efficiency of EPP reduces drastically.

Figure 1. Typical Stress-Strain curve of EPP.

In order to increase the energy absorbing efficiency of an EPP component a wide range of design and production techniques have been developed. Since the early 90’s JSP has produced energy absorbing parts with multiple densities. This moulding technology allows locating the necessary density at the exact location to maximise the part performance whilst optimising the part economics. Other possibilities include the use of geometrical features to control the stress progression of the overall part with increasing strain.

The use of high density EPP with a rib structure is illustrated in Figure 2. The geometrical stiffness of the high density ribs results in a rapid force increase, which approaches the ideal rectangular energy absorption characteristic. The fine-tuning of the rib design features, such as spacing, thickness, and height, combined with the EPP foam density allow a system specific optimisation. Incorporating compact PP inserts, with a specific buckling limit, into the rib structure can increase the overall energy absorption capacity.


Sample: 300mm x 100mm x 55mm height EPP Density: 180g/l Impactor Shape: 70mm diameter cylinder 21.4kg Impact Speed: 4.8m/sec Impact Energy: 250J Fascia: PP material (3mm thick)

Figure 2. Compression curve for EPP with rib structure.

The ideal rectangular compression characteristic, however, is in most cases linked with an irreversible deformation or destruction during the compression process. The rib concept mentioned above will be effective for side impact pads, but for bumper energy absorbers required to perform during more than one impact, it is not the best solution.


Sample: 100mm cube EPP Density: 60g/l Impactor Shape: Flat Impactor Impact Speed: 11.7m/s Max. Strain: 85% Recovery just after impact: 94%

Figure 3. High-speed compression test of EPP.

The primary benefit EPP offers over other traditional energy absorbing materials is its guaranteed performance throughout the vehicle life. The resilient nature of the expanded polypropylene foam results in only negligible performance modifications upon a series of consecutive impacts.

Figure 3 illustrates the excellent recovery and absence of Poisson effects of EPP after impacts during which the material was compressed to 85% strain. Materials such as PUR, EPS, and injection moulded PC/PBT… suffer from gradual or catastrophic collapse, even when subjected to minor pendulum impacts.

The key to developing a bumper system capable to fulfil the upcoming performance requirements as well as the design criteria lies in a differential stiffness profile over a restricted total deformation. At the beginning of the compression, the stress should be kept low to comply with the pedestrian leg criteria, after that the stiffness increases with increasing stroke to cover the energy absorption necessary for the 2.5mph/5mph impact.


EPP BASED PEDESTRIAN SAFE BUMPER SYSTEM

OPTION 1: INTEGRATED BEAM / ABSORBER

In accordance with the generic EPP bumper technology the easiest way to increase the efficiency of the energy absorber without increasing the offset is its partial integration into the support structure. Fitting the foam absorber in a depression of the beam, as shown in Figure 4, allows the effective use of the full range of the efficient EPP strain.

Figure 4. Integrated beam / absorber concept.

Indeed, the effective-strain portion of the energy absorber is located in front to the bumper beam. Tuning its density and geometry profile allows a controlled compression during the pendulum impacts to this available strain. The absence of Poisson ratio avoids all lateral loading, ensuring the integrity of the beam depression throughout the impact. In order to check the compatibility of the pendulum and lower leg impacts drop tests according to the schedule in Figure 5 have been performed. The EPP sample was compressed to a strain of about 65% using a flat impactor, equivalent to a pendulum hit. A waiting period of 30 minutes at room temperature symbolises the continued years of use of the car, during which the EPP part recovers more than 90% (Figure 6). The EPP component is then subjected to an impact energy of 180J using a cylinder of 70mm in diameter, representing the pedestrian lower leg.

Sample: 200mm x 40mm x 80mm height EPP Density: 82g/l Impactor Shape: 70mm diameter Cylinder Impactor Weight: 4.07kg Impact Speed: 9.4m/sec Targeted Energy: 180J Target Strain: About 65% Fascia: PP material (3mm thick)


Figure 5. Test conditions for characterisation of multiple impact behaviour of EPP.

Figure 6. Recovery characteristic of EPP upon repeated impacts.

Figure 7. Force-Deflection curves for repeated impacts.

Figure 7 illustrates the dynamic compression performance of the EPP part. After the first impact there is a small residual strain at which the curve of the second impact starts. The overall Force-Stroke characteristic, however, remains equivalent between the successive impacts. This confirms the multiple impact performance of EPP without significant loss of energy absorbing capacity. It can be seen that a safety factor of about 5mm offset will ensure the compliance of the bumper system under multiple impacts. The partial integration of the energy absorbing foam into the beam geometry offers a very easy system for small or compact cars, especially when targeted to the Asian or European 2.5mph markets. The beam, frequently extruded roll-formed aluminium, can be designed to meet the pendulum criteria, whereas the geometry and density of the foam component is easily dimensioned to keep the deceleration and force levels under control during the pedestrian impact. In the case of the design of a bumper system for a full size car, it is difficult to cover the 5mph impact requirements with the


bumper beam only. A combination of beam – absorber integration with other design solutions is required. Figure 8 shows some of the variations when current dual density technology is integrated into the design of the bumper system.

Figure 8. Variations using dual density technology.

OPTION 2: FORE / AFT DUAL DENSITY

In order to combine a soft nose with high-energy absorbing capacity, JSP has developed the fore-aft dual density technology. The production technology is based on a staged injection of specific raw material grades into a tool, which allows 3D geometry partitioning. The resulting moulded part has a strategic combination of 2 densities arranged in the fore-aft direction, which is subsequently perpendicular to the impact direction.

Figure 9. Energy absorber with high-density core and low-density outer layer.

The compression strength curve, Figure 10, consists of both the lower density material curve and the higher density material curve. The resulting part behaviour presents a unique stress-strain shift, linked to the respective densities and thicknesses of the respective layers. This allows JSP to tailor the specific part design and densities within the part to fine-tune the interaction of the respective system components and optimise the performance characteristic of the complete system.


Figure 10. Tailored Stress-Strain response of fore-aft dual density part.

Figure 11. Production system analysed.

To demonstrate the effectiveness of a fore/aft dual density energy absorber, EA, on the lower leg protection, an FEA was conducted comparing a traditional EPP EA, a PC/PBT EA, and a fore/aft dual density EA. For this comparison, an American production vehicle weighing 2045kg was selected and had a centerline absorber depth of 100mm (see Figure 11).

The original EPP energy absorber was a tri-density EA that was designed to pass the FMVSS/CMVSS and IIHS impact requirements. The PC/PBT EA was initially designed to pass the FMVSS/CMVSS and IIHS requirements and then the centerline section was softened to provide the best possible tibia deceleration, lateral knee shear, and knee rotation results. To create the fore/aft dual density EA, the original EPP absorber was reduced by 25 mm and then a 25 mm, 30g/l (1.87pcf) EPP layer was added to the fascia side of the absorber (see Figure 9).

The results of the lower leg impact analyses can be seen in Table 1 and Figures 12 to 14. As can been seen from the analysis results, the fore/aft dual density EPP EA significantly outperformed the original EPP EA and the PC/PBT EA in terms of tibia deceleration and lateral knee shear. However, the amount of knee rotation was not significantly affected by the softening of the EPP EA. In this system, the fascia profile was not designed for pedestrian protection and therefore limits the influence of the absorber. In order to reach the maximum knee rotation requirement of 15º on this system, modifications to the fascia design will be necessary.


Tibia Deceleration (G’s)

Lateral Knee Shear (mm)

Knee Rotation (Degrees)

Requirements 150 6 15

Original EPP EA 317 4.2 32.0

PC/PBT EA 288 3.9 31.7

Fore/Aft EPP EA 148 1.6 31.4

Table 1. Lower leg protection analysis results.

Figure 12. Tibia deceleration vs. Time.

Figure 13. Lateral knee Shear vs. Time.

Figure 14. Knee Rotation vs. Time

In addition to the pedestrian lower leg impact analyses, 8kmh flat barrier impact (IIHS) analyses were performed on each EA design. The results of these analyses can be seen in Table 2 and Figure 15.


EA Mass (kg)

Max. Rail Load (kN)

Barrier Intrusion (mm)

Requirements 120 90

Original EPP EA 1.35 106 82.9

PC/PBT EA 2.0 98 87.9

Fore/Aft EPP EA 1.26 111 90.3

Table 2. 8kmh flat barrier impact results.

Figure 15. 8kmh flat barrier Force vs. Displacement.

As can be seen from these results, the original EPP EA as well as the PC/PBT EA perform slightly better than the fore/aft dual density EPP EA in the 8kmh flat barrier impact. However, the fore/aft dual density EPP EA meets the maximal rail load and barrier intrusion requirements, the pedestrian safety requirements and provides a significant mass savings over the other two designs.

OPTION 3: GEOMETRY-BASED DUAL DENSITY

Extensive work has been done on the development of a mono-density system to mimic the unique stress-strain response linked to fore/aft dual density components. The project definition, based on a specific system geometry, beam set-up and material characteristic, outlined that a low density EPP foam of 30g/l passed the necessary pedestrian safety criteria. The system, however, did not pass the pendulum requirements at this low density. Figure 16 shows a cross section of one of the geometry options, which yielded the best results.


Sample: 290mm x 110mm x 75mm height EPP Density: 30 and 45g/l Impactor shape: 70mm diameter Cylinder Impactor weight: 13kg Impact speed: 24kmh Fascia: PP material (2.5mm thick) Figure 16. Test conditions of the performance validation.

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Figure 17 clearly illustrates how a 45g/l foam element can be designed to perform as its lower density equivalent. The key element to passing the pedestrian criteria lies in a controlled and delayed deceleration peak upon first contact. The small EPP section, which is activated at high strain, enables this. As the inertia of the pedestrian leg decreases, more and more EPP is being activated to take on the weight factor of the impact energy.

This concept has been implemented and validated in a full size pedestrian lower leg impact. Figure 18 shows a cross section of the car set-up, in which a 20g/l EPP foam absorber is mounted on to a rigid beam. A 60g/l EPP absorber is fitted to the lower ankle support.

The performance curves validate the fact that a system solution based on an active EPP foam absorber can pass the ACEA Phase 2 (2010) pedestrian safety requirements (Figure 19). It is to be noted that the same system successfully passed the pendulum impacts.


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Figure 18. Cross section of pedestrian safe EPP bumper system.

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Figure 19. Lower leg performance passing Phase 2 (2010) criteria.

CONCLUSION

Compatibility of the different crash requirements across the globe as well as design constraints form the major challenge in the development of pedestrian safe bumper systems. Novel approaches to designing a bumper system where the absorber is integrated into the system functionality are essential. Expanded polypropylene foam offers a high energy absorbing efficiency, multiple impact performance, light-weight, design freedom, and recyclability at a competitive cost. Integrating part of the foam absorber into the beam structure increases the efficiency of the foam. Innovative transformation technology allows the moulding of parts with fore/aft dual density. Together with geometry optimisations, tailor-made stress-strain performance is easily achieved. The combination of traditional bumper design techniques together with the concepts outlined above ensure that EPP is one of the energy absorbing material best positioned to bring pedestrian safe solutions to the global market.

REFERENCES

1. “Improved Test Methods to Evaluate Pedestrian Protection Afforded by Passenger Cars (December 1998 with September 2002 Updates)”, EEVC Working Group 17 Report

2. T. Frank, A. Kurz, M. Pitzer, M. Sollner, “Development and Validation of Numerical Pedestrian Impactor Models”, 4th EUROPEAN LS-DYNA Users Conference

ACKNOWLEDGMENTS

The authors would like to thank their colleagues at JSP for their help in writing this paper. Special thanks also for joint the development effort with Peguform GmbH on the full size pedestrian safety tests.

Document reference: Expanded Polypropylene (EPP) - A Global Solution for Pedestrian Safety Bumper Systems The information contained herein is based upon the results of internal laboratory test samples of material moulded from expanded polypropylene resin manufactured by JSP. There can be no assurance that similar results will be achieved in simulated tests or actual use of commercial products moulded by customers of JSP. Product performance may vary substantially depending upon the particular application or processing involved. The listed properties are illustrative only and not product specifications. All suggestions and recommendations are made without warranty since the conditions of use are beyond JSP control. Processing and applications of JSP foam products can influence moulded part performance in many ways. Consequently, processors and/or users are advised that there may be a need to conduct independent tests and experiments in order for them to determine the extent to which they may choose to rely upon such information in their business operations. JSP disclaims any liability in connection with the use of the information and does not warrant against infringement by reasons of the use of its products in combination with other material or in any process.

DEFINITIONS, ACRONYMS, ABBREVIATIONS EPP: Expanded Polypropylene mph: mile per hour kmh: kilometres per hour IIHS: Insurance Institute for Highway Safety S-S curve: Stress – Strain curve PU: Polyurethane PS: Polystyrene FEA: Finite Element Analysis EA: Energy Absorber CONTACT

Seishiro Murata Automotive Materials Development Group JSP Kanuma 2nd plant 5 banchi, Satsuki-cho, Kanuma-shi, Tochigi 322 Japan Phone: +81-289-76-3271 e-mail: [email protected]

Bert Suffis - Development Manager JSP Z.I. Le Bois Chevalier 60190 Estrees-Saint-Denis - France Phone: +33.344917000 e-mail: [email protected]

expanded polypropylene (epp) – a global solution for...

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