13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- a. density 100 kg/m3 b. density 160 kg/m3 c. density 301 kg/m3 Figure 1. SEM microstructures of closed cell rigid PUR foams (500x). 2.1. Static compression tests Static compression testes were carried out using a LBG 100kN universal testing machine in the Strength of Materials from POLITEHNICA University of Timişoara, respectively on a Walter+bai 50 kN static and fatigue testing machine from Laboratory of Strength of Materials from University POLITEHNICA of Bucharest. Tests were performed according to ASTM D 1621 at room temperature. Cubic specimens with 50 mm length were used for foam densities 100 and 160 kg/m3 and with 25 mm length for the 301 kg/m3 density. The engineering stress-strain diagram shows the non-linear behavior of polyurethane foams in compression, Fig. 2. The following regions could be identified: an initial linear elastic response leading to yield, a small softening in stress after yield, a post-yield plateau and a final sharp rise in compressive stress, corresponding to foam densification. It can be observed that density has a major influence on the behavior of PUR foams. The influence of the loading direction: in plane (rise direction) and out of plane (transverse direction) is shown in Fig.3. Foam behaviour in transverse direction shows a small hardening after the yield point and no softening. Figure 2. Effect of density. Figure 3. Effect of loading direction. The compression properties (Young’s modulus, yielding stress, plateau stress and densification strain) obtained for the three investigated rigid PUR foams are summarized in Table 1 for a 5 mm/min loading rate.
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