Author details

8. Conclusion

106 Aspects of Polyurethanes

distribution of contact pressures.

strong dependence of stiffness on the strain rate.

For static and dynamic measurements of samples, experimental devices were designed and implemented. Results showed that mechanical properties of PU foams are dependent on the strain rate, because the air in the initial stage cannot escape from the structure under dynamic loading, thus the air participates on the resulting foam properties. These results in increase of the main stress in the peripheral parts of the PU foam sample, as shown in the simulation of the sample loaded with harmonic signal. From the results of measurement of the relaxation, it was observed. Using the FEM simulation, the distribution of contact pressures was also compared. It was showed that the strain of the PU foam in directions perpendicular to the direction of compression leads to uneven stress distribution, which is reflected by the uneven

Sample Deformation [%] Principal stress [kPa] Total stress [kPa] Contact pressure [kPa]

Figure 29. FEM model: comparison of contact pressures distribution at 37.5% deformation.

Table 5. Results of stress in dynamically compressed sample.

PU foam 37.5 2.398 2.422 9.608

The described behavior influences properties of products containing PU foams. This behavior is significantly reflected in the construction of car seats. For example, headrests are used to capture the head in the rear impact (so-called whiplash). The energy absorption may be insufficient due to the setting of the headrest. If the backrest is fully inserted, a significant deflection of the spine in the area of the cervical vertebrae occurs, while if the headrest is correctly set, the spine is not deflected. Also, a dependence of material stiffness on strain rate has certain influence. The solution could be not only in the modification of the geometry and the anatomical design of the seat, but also in the change of the headrest stuff material which is not so dependent on the strain rate. The solution could be in a usage of foams with a highly porous structure and foams in the combination with fibrous structures that do not exhibit

Michal Petrů<sup>1</sup> \* and Ondřej Novák<sup>2</sup>

\*Address all correspondence to: michal.petru@tul.cz

1 Institute for Nanomaterials, Advanced Technology and Innovation, Technical University of Liberec, Liberec, Czech Republic

2 Department of Nonwovens and Nanofibrous Material, Textile Faculty, Technical University of Liberec, Liberec, Czech Republic
