Design Of Thin Wall Structures For Energy Absorption Applications

Design of thin wall structures for energy absorption applications:

Past and present developments in polymer bead foams and bead foaming technology.

Characterization of expanded polypropylene bead foams with modified steam-chest molding.

High melt strength polypropylene with wide molecular weight distribution used as basic resin for expanded polypropylene beads.

Lightweight foam impact attenuator design for formula SAE car.

Past and present developments in polymer bead foams and foaming technology

In the modern days, cars are going to be highly spectacular in the field of safety and development. Various safety aspects have already been implement and installed into them and it is seriously great to see the same as the drivers and the co-drivers are considered to be the most valuable assets and there safety is highly needed. The manner in which everyone has worked with the Global institute and FIA, it seems to be one of the highly exciting periods for World Rally Cars (WRC). Therefore, it has set new and latest standards in terms of safety and development and basically each and every area surrounding the protection and safety of the drivers and the co-drivers are ensured.

A brand new safety package has been engulfed into the World Rally Championship cars of 2017 following the joint collaboration in between the Global Institute of Motor Sport Safety, the FIA and the other major manufacturers who are engaged in the series (Poujet and Roucher 2017). The newly traduced protective measures would be specifically affective in protecting the competitors at the time of hard lateral acts, like side-on collisions with the fence posts and tress. The energy absorbing foams on the cars prevents the drivers from getting massive injuries when he underestimates a jump or corner (Tarlochan et al. 2013). Use of such special foam also help with the aerodynamics since the less is the weight of the car the better will be its performance, exclusive of the condition of the road and the driver who is driving the car.

In the year 2008, the World Rally Cross began when the WRC cars were mandated to 200mm space in between the seat edge and the outer door skin filed with some standardized energy absorbing objects, performance necessities were established by the active sled tests and the quasi-static strength tests that had enabled the team of ANDY MELLORS to define the foam specification (Naess 2014). They described that foam with 60litres of volume is necessary and that too with no concessions. Along with that the WRC 2009 technical regulations had prescribed 60gm per liters of closed cell foam that is still present from the three suppliers- ARPRO RG60, BASF Neopolene RG60 and Zotefoam HD60 (Riley 2016). Each of them is very expensive.

• The energy absorbing foam has now the potential of providing a 20% increase in the energy management and also has the higher energy absorption figures than ever.
• Crash protection has been installed
• The cars are also installed with mandatory structural door sill reinforcements. These are designed in order to prevent dangerous obstacles that penetrate into the car’s cockpits such as fence posts.
• On both the sides of the door panel along with around the areas that are close to the seats, energy absorbing foams are installed
• A carbon fiber door panel with some honeycomb aluminum has also been added in order to provide some strength
• Extra energy absorbing foam is also added around the main structure so that the main roll caged structure is coated.
• The door line is also expanded by about 200mm behind the primary structure
• Structural doors have been taken into account in some of the World Rally Cars in order to help in preventing the fence posts that penetrates the cars
• Additional padding system has been installed on the seat head support that will limit the movement of head so that the drivers can safely drive the cars and in a better way
• The sill beam the cars that was present earlier as well is decided to increase by the World Cross Rally every year

Each of the rally cars is based on production. In case of Hyundai, the subcompact i20 has been changed and reshaped in according with the rules of the FIA. The windows are exchanged with the panels made of polycarbonate in the side doors and the standard door fittings are eliminated to provide the side impact protection maximum space and also to provide ideal protection for the driver as well as the co-driver. The Neopolen P has been specially authorized for this purpose. Neopolen P is beads of extended polypropylene (EPP) (Hossieny, Ameli and Park 2013).

Characterization of expanded polypropylene bead foams with modified steam-chest molding

Neopolen is shaped and molded on the machines with steam chambers that are intended for pressure of about 5 bars. There are a total of 5 distinct stages in this process- filling up of the mold cavity, molding, cooling, de-molding and conditioning. In the process of filling up the mold cavity, the increased beads are at first inserted into the mold cavity through booster or injectors that are provided from pressurized vessel. As Neopolen does not contain any expanding agent, the beads present in the cavity are compressed at the beginning (Raps et al. 2015). Through the process of pressure filling and crack press filling, this could be achieved. At the time of processing, the array of the material density could be scaled down and is carried out by submerging the beads into hot compressed air for many hours before they are molded. Thereafter the beads that are packed into the molds are been heated by steam that results in the swelling and fusing of the beads. The moldings are then cooled till there is no risk present for the swelling and splitting while de-molding. Duration of time depends on the wall-thickness and density of moldings. After that, the moldings are then removed with the help of mechanical injectors or compressed air or with the combination of both of these. Thereafter the moldings are conditioned for near about eight hours in warm air at 80 degree centigrade in order to partially restore the volume and remove the interstitial water if present. Nice air circulation is needed around the bead moldings in order to prevent the oven from overfilling.

The Neopolen is characterized by good recovery and cushioning properties along with high energy absorption (Calafut 2013). The heat resistance capacity of Neopolen is very good and is chemically inert as well (Guo et al. 2014). The further advantages of Neopolen include good thermal insulation, environment friendly and easy cleaning.

Energy absorbing foams enables the designers of the passive safety systems by not only in saving space, cost and weight but it also increases the safety (ratings) by having better defined as well as more easily tune-able loading system on the dummy at the times of side impact crashes (Hu and Klinich 2014).  The process of continuous extrusion production safeguards constant quality and high level of stability of the material properties (Maheo and Viot 2013). The foam boards are established in the extrusion process and from this, the parts or pads could be cut out by hot cutting technology. Such an advanced fabrication technique provides additional benefit in eliminating the need for very costly forming tools that are associated with the traditional foam solutions.

Lightweight foam impact attenuator design for formula SAE car

Fig 1. Depiction of a quasi-static stress-strain characteristic of the form

Use of energy absorbing foam for the side impact protection in the motorsports cars has great advantages (Todd 2013). These materials are greatly used in the side impact padding. Due to square-like compression stress-strain response of the energy absorbing foam, nearly a perfect load control could be established. It enables design of the strong and vigorous side impact pads. A correct material model prevails to imitate parts in the CAE analyses. It means that in the growth and development process, the CAE could be hold to optimize an EA countermeasure part design using the density as well as geometry. Up to 50% weight can be saved because of the high efficiency results in the smaller parts and that too along with same performance. The density is usually lower on the top of the smaller parts, like for example, in order to obtain the same pressure or load response with the ePP, the densities is required to be twice as high. However, this is not related to safety but the fuel consumption efficiency as the result of the lower car weights are now-a-days greatly appreciated (Albak et al. 2018). With the same, as the padding could be smaller as well, the packaging space can be saved. If this could be taken into the account earlier in the process of designing, use of extra space could be possible, for example, for the expanded door pockets. Another advantage of it is the short development times that it requires. Firstly, the use of proper CAE models enables much more virtual and accurate testing, along with that; it saves on the testing time as well as the cost (Uddin, Quintel and Zivkovic 2016). Secondly, the prototypes could be produced quickly, easily and more cost effectively as there is no necessity of tooling (Bychkov, Osipov and Parakhoni 2014). The prototypes are equal to the production parts because the way to produce the both is same i.e. by the technology of abrasive or hot wire cutting. With the same, at the time of testing it is very easy to modify the prototypes and an optimum could be found repetitively on the testing spot. Other advantage of it includes- the improved load control.

Conclusion 

From the above analysis, it could be stated that Neopolen P is specially authorized by the FIA for manufacturing the energy absorbing foams. It is the expanded form of Polypropylene and is provided in the form of beads. Neopolen P is further molded on the machines in order to prepare energy absorbing foams and they follow certain steps for the manufacturing of the same, like pre-pressurizing and pressure filling. It too has some special properties that are highly advantageous. The energy absorbing foams are highly benefiting the car drivers for ensuring their safety.

References:

Albak, E.?., Solmaz, E., Kaya, N. and Ozturk, F., 2018. Lightweight foam impact attenuator design for formula SAE car. Turkish Journal of Engineering, 2(1), p.17.

Bychkov, A.V., Osipov, N.L. and Parakhoni, A.A., 2014. Use of three-layer compositions in the support system of the bus body. Journal of Machinery Manufacture and Reliability, 43(2), pp.140-144.

Calafut, T., 2013. Applications of Polypropylene Films. In Plastic Films in Food Packaging (pp. 93-119).

Guo, P., Xu, Y., Lu, M. and Zhang, S., 2014. High melt strength polypropylene with wide molecular weight distribution used as basic resin for expanded polypropylene beads. Industrial & Engineering Chemistry Research, 54(1), pp.217-225.

Hossieny, N., Ameli, A. and Park, C.B., 2013. Characterization of expanded polypropylene bead foams with modified steam-chest molding. Industrial & Engineering Chemistry Research, 52(24), pp.8236-8247.

Hu, J. and Klinich, K.D., 2014. Toward designing pedestrian–friendly vehicles. International journal of vehicle safety, 8(1), pp.22-54.

Maheo, L. and Viot, P., 2013. Impact on multi-layered polypropylene foams. International Journal of Impact Engineering, 53, pp.84-93.

Naess, H.E., 2014. The Spectator Culture of Rallying. In A Sociology of the World Rally Championship (pp. 116-149). Palgrave Macmillan, London.

Poujet, J. and Roucher, D., 2017. Conjoncture in France, October 2017. The Eurozone makes up lost ground.

Raps, D., Hossieny, N., Park, C.B. and Altstädt, V., 2015. Past and present developments in polymer bead foams and bead foaming technology. Polymer, 56, pp.5-19.

Riley, S.R., 2016. Biracial Palimpsests: Racing and Erasure in Black Empire and Haiti. In Performing Race and Erasure (pp. 211-235). Palgrave Macmillan, London.

Tarlochan, F., Samer, F., Hamouda, A.M.S., Ramesh, S. and Khalid, K., 2013. Design of thin wall structures for energy absorption applications: Enhancement of crashworthiness due to axial and oblique impact forces. Thin-Walled Structures, 71, pp.7-17.

Todd, R., 2013. Automotive collision energy dissipation device. U.S. Patent 8,376,451.

Uddin, M.S., Quintel, J. and Zivkovic, G., 2016. On the design of energy absorbing crash buffers for high speed roadways.