**1. Introduction**

336 Polyurethane

Czech)

Scucka, J., Soucek, K. (2007). *Architecture and Properties of Geocomposite Materials with Polyurethane Binders*, Institute of Geonics of ASCR, ISBN 978-80-86407-15-9, Ostrava (in

*Trade mark GEOPUR, no. 148 179, 1999 Czech Republic*, Arrangement et Protocole de Madrid

no. 864 266 EU, GEOPUR, no. M0002030 , 2000 Hungary

The use of cellular materials in general in the automotive industry, and polymeric foams in particular, has been increasing significantly for the last few decades. These materials are used within a particular vehicle for many different purposes, among which are, for example, sound and thermal insulation, vibration damping, fire protection and, of course, crashworthiness. Thus, crashworthiness, safety and protection parameters are strongly influenced by the materials used and, as a consequence, polymeric foams play a major role in the vehicle's crashworthiness levels. In absolute terms, the energy absorption capability of this class of materials can lead to significant improvements on the vehicle's passive safety, better protecting the passengers from aggressive impacts, by absorbing impact energy in a gradual and controlled manner. In addition, design limitations due to environmental constraints are growing steeply as are safety concerns. Whilst the former often leads to a reduction in the weight of the vehicle, the latter will most probably lead to the opposite. Therefore, the combination of properties such as low density, low cost and design flexibility with a great energy absorption capability, is what makes cellular materials so attractive for the automotive industry.

Presently, vehicle structures with high levels of crashworthiness protection are almost always light-weight and must deform in such a way as to dissipate the largest amount of impact energy possible. Several distinct mechanisms may contribute to this, such as, for example, plastic deformation, wrinkling, heat generation, etc. [20, 35]. One way to achieve these effects is to fill tubular or hollow metallic or composite structures with cellular materials, such as foams. During the last few decades, many researchers have been working on these issues [22, 28, 30–32].

©2012 Paulino and Teixeira-Dias, licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. ©2012 Paulino and Teixeira-Dias, licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### 2 Will-be-set-by-IN-TECH 338 Polyurethane On the Use of Polyurethane Foam Paddings to Improve Passive Safety in Crashworthiness Applications <sup>3</sup>

Polyurethane foam is nowadays being widely used in many energy absorption engineering applications such as cushioning and packaging [4, 15, 50]. Its use in automotive industry as an energy absorbing material in passive safety mechanisms goes beyond the protection functionality since it also provides more comfort, insulation and sound absorption. Thus, the role of this class of materials in vehicles is of special interest from both the consumer and the manufacturer points of view.

consists of a foam like impact padding confined inside the lateral doors of the vehicle. In order to be efficient, this foam padding must be mounted aligned with the occupant's pelvis, protecting one of the most critical areas in this type of collision. From the experiments and analyses made it is expected that this padding may absorb a significant part of the impact energy, and thus minimise both the forces transmitted to the body of the occupants and, most importantly, the magnitudes of the decelerations experienced, consequently reducing possible

On the Use of Polyurethane Foam Paddings to Improve Passive Safety in Crashworthiness Applications 339

The design of structures and the choice of materials for crashworthiness and protection systems within a vehicle are also of major importance and relevance for the overall safety of the driver and passengers. These are two of the research fields where there is still quite a large margin for improvement [16, 17, 24, 41, 46]. In the present work, the authors will present, discuss and compare the applicability of several distinct types of cellular materials for impact

The behaviour of three polymer based structural foams under compressive impact loading polypropylene, polyamide and rigid polyurethane foam — has been investigated by Avalle *et al.* [4]. As a conclusion, these materials are indicated as players of a very important role in passive safety systems. These authors obtained stress-strain curves in both static and impact loading (dynamic) conditions for the materials examined for different densities at room temperature. They analysed in detail, using energy-absorption and efficiency diagrams, the energy absorption characteristics of each material. Among the materials tested, these authors observed that polyurethane foam is the less sensitive to strain-rate and the one that presents the longest intermediate plateau stage. These facts distinguish this material from the remaining foams studied. The authors also concluded that the rigid polyurethane foam exhibited one of the highest efficiency levels, however, it lost its integrity during compression. The energy absorption behaviour properties of polyurethane foam were also investigated by Anindya and Shivakumar [2]. The authors evaluated the energy absorption attributes of polyurethane foam in various forms — flexible high resilience, flexible viscoelastic and semi-rigid — as a function of the overall foam density, based on the load-displacement

Taher *et al.* [47] investigated the use of polyurethane foam with a density of 47 kg/m<sup>3</sup> as a core filler of a composite keel beam as a way of preventing global buckling and improving crashworthiness performance in aeroplanes and helicopters. The results obtained by these authors revealed that the energy absorbing mechanism can meet the requirements for the

Likewise, the behaviour of polyurethane foam filled thin-wall structures was investigated by Ghamarian *et al.* [13] in terms of crashworthiness improvement for the aerospace industry. The quasi-static crushing behaviour and efficiency of empty and foam-filled structures was investigated experimental and numerically and the efficiency and the authors were able to demonstrate that the filled tubes presented higher energy absorption capabilities than that of

injury levels of the occupants of the vehicle.

**2. Polyurethane foam as an energy absorbing material**

behaviour of the material under compressive loads.

purpose desired together with substantial savings.

the combined effect of the empty structured and the foam.

and energy absorbing paddings.

On a microscopical level, most cellular materials, including polyurethane foams, have the ability to absorb energy while deforming due to the mechanics of cell crushing. In the process of absorbing impact energy, cell walls deform plastically and get damaged (e.g. fractured) [7, 18].

Vehicle-to-vehicle side-impacts and vehicle rollover are presently among the most common types of car accidents and collisions. Additionally, these are also frequently the most serious accidents in terms of occupant injuries [6, 8, 39, 48]. Among these, frontal and side impact are the most severe. As a consequence, quite a large effort has been widely focused in improving passive and active safety mechanisms for frontal impact situations for the last decades. However, more recently, the number of serious injuries resulting from side-impacts has brought the attention of many researchers to the importance of developing similar or adapted mechanisms for such collisions [12, 33, 53]. In this type of collisions, the risk and/or the severity of the resulting injuries is frequently a direct consequence of the contact between the occupants and the lateral structure of the vehicle, given the reduced space between the occupant and the door [21]. Pelvic and chest areas have been reported by many authors as the the two areas most affected in this type of car-to-car collision [29, 43, 44].

In the late 1990s Morris *et al.* [37] observed, through a series of numerical simulations of side-impact collisions, that the space available between the structure of the vehicle and the passengers is one of the most important parameters with direct influence on the levels of occupant's injuries. This statement was also supported by many other researches, as can be seen, for example, from the works of Tencer *et al.* [49] and Schiff *et al.* [44]. Morris *et al.* state that the space available not only has influence on the impact velocity but also on the point of the velocity profile at which the door initiates contact with the occupant. These authors also evaluated the benefits of the use of paddings of different sizes in the door interior and of lateral airbags. Lim *et al.* [26] also studied, numerically, the inclusion of padding material for protection of the occupant pelvic area and concluded that it significantly reduced the severity of the resulting injuries. Additionally, Majumder *et al.* [29] studied the dynamic response of the pelvis and established fracture limits in side-impact collisions. These authors supported their conclusions with the results from numerical simulations using finite element modelling software. One of the most important conclusion these researchers derived from their work was that with a more appropriate design of the lateral door and the inclusion of padding material on the level of the pelvic area, the risk and/or severity of occupants' injury could be significantly reduced.

Based on the previous considerations, the authors propose the use of cellular materials, among which polyurethane foams, within an energy absorbing system specifically designed in such a way as to significantly improve passive safety on the event of side-impacts. This system consists of a foam like impact padding confined inside the lateral doors of the vehicle. In order to be efficient, this foam padding must be mounted aligned with the occupant's pelvis, protecting one of the most critical areas in this type of collision. From the experiments and analyses made it is expected that this padding may absorb a significant part of the impact energy, and thus minimise both the forces transmitted to the body of the occupants and, most importantly, the magnitudes of the decelerations experienced, consequently reducing possible injury levels of the occupants of the vehicle.

The design of structures and the choice of materials for crashworthiness and protection systems within a vehicle are also of major importance and relevance for the overall safety of the driver and passengers. These are two of the research fields where there is still quite a large margin for improvement [16, 17, 24, 41, 46]. In the present work, the authors will present, discuss and compare the applicability of several distinct types of cellular materials for impact and energy absorbing paddings.
