3.4. Determining the mechanical properties of selected samples with dynamic compression against a rigid plate with the initial deformation

The experiment with chosen samples was conducted 60 min after the first measurement, a long enough time necessary to relax the tested sample. The only difference in measurement was that the tested sample was compressed by the upper rigid plate by 20 mm to its initial 50% deformation. After the initial compression, seven measurements with gradually rising frequency f were conducted, from 0.5, 1, 2, 3, 4, 5, and 8 Hz but with the amplitude at Aðz<sup>Þ</sup> ¼ 5 mm for five repeating cycles. The arrangement of the experiment is shown in Figure 10. The resultant courses of the tested PU foam sample for individual frequency values during the fifth cycle are shown in Figure 9.

Results for the PU foam sample with initial 50% deformation in the fifth cycle of the repeated compression (Figure 11) show that the change in frequency also increases the force necessary to compress the material that reaches maximum 246 N and frequency of 8 Hz. This is different, compared to the dynamic measurement of the sample without the initial deformation, because beginning with 5 Hz value, the necessary compression force began to drop. The courses also have a very similar character while comparing samples with different frequency of 0.5, 2, 4, and 8 Hz, which creates so-called "banana curve" shown in Figure 11. While comparing the courses during 8 Hz frequency, a dynamic ratio id expressing the ratio between the maximum force value and minimum force value of the force relief during compression in the PU foam samples is id ¼ 4:86 (min 50 N and max 243 N). The higher the value of the dynamic ratio between maximum and minimum force, the faster the material recovers, because there is a greater energy return to recover the material. From the maximum force value necessary to compress the tested sample during the dynamic measurement, it is apparent that the value is higher than during a static compression; therefore, the rigidity of the sample increases. It is possible to determine the dynamic flexibility module E<sup>D</sup> <sup>P</sup> is greater than static flexibility module E<sup>S</sup> P.

### 3.5. Measuring the relaxation of the chosen material samples

Figure 11. Dependence of force on deformation of dynamically compressed samples with initial deformation.

Figure 10. Determination of mechanical properties of samples of polyurethane foams during dynamic compression with

initial deformation: (a) scheme, (b) realization of the measurement.

84 Aspects of Polyurethanes

Also, it is important to compare the mechanical properties during a long-term compression. As it was stated, the cellular structure of the PU foam becomes more supple under constant pressure, and the increase in deformation grows <sup>ε</sup>ðt2Þj<sup>σ</sup>¼konst <sup>&</sup>gt; <sup>ε</sup>ðt1Þ, i.e. the structure "melts," or during the constant deformation, ε ¼ konst: relaxes and the tension gradually decreases <sup>σ</sup>ðt2Þj<sup>g</sup>¼konst <sup>&</sup>lt; <sup>σ</sup>ðt1Þ. This is true in general for all materials with viscoelastic properties. According to Refs. [1, 6], it is more advantageous to measure the relaxation of material for the evaluation of mechanical properties, because the "melting" of the structure during the constant compression is minimal and almost negligible (the significance grows during long-term measurements—weeks, months—in high temperatures). Comparison of the compressed samples was conducted to evaluate the relaxation of material. The experiment was conducted on the same device just as during the static testing (Figure 4). The relaxation properties were compared for the PU foam samples 100 � 100 � 40 mm. The observed properties obtained from these measurements can be summarized in the following points:

