13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- Table 1. Compression properties of rigid PUR foams Foam density [kg/m3] 100 160 301 Loading direction rise transverse rise transverse rise transverse E [MPa] 40.38 31.92 81.76 50.83 202.25 201.83 σy [MPa] 1.15 1.17 2.09 1.77 5.74 6.69 σp [MPa] 1.06 1.14 1.88 1.83 5.98 6.65 εd [%] 56.2 55.4 54.4 52.4 54.8 52.5 Young’s modulus, yield stress and plateau stress increases with density. For low density foams Young’s modulus values are higher in the rise direction, while for the foam with 301 kg/m3 density Young’s modulus are equal in both direction. Also the densification strain is slightly higher in rise direction for all investigated foams. The yield stress and plateau stresses are quite similar in both directions for low density foams, but are higher in transverse direction for foam with 301 kg/m3 density. Loading rates between 0.6 and 500 mm/min were used to investigate the influence of loading rate. It could be observed in Fig. 4 that increases in loading rate produces an increase of Young modulus (with approximately 50 % for foam densities 100 and 160 kg/m3, respectively with 80 % for 301 kg/m3 foam density) and yield stress (with approximately 67 % for foam densities 100 and 160 kg/m3, respectively with 73 % for 301 kg/m3 foam density). a. Young’s modulus b. Yield stress Figure 4. Effect of loading rate on mechanical properties of PUR foams. 2.2. Digital image correlation investigations The damage mechanisms of the foams were investigated by the 2D (DIC) technique, using a 5 MPixels camera and by applying a random speckle of paint on the specimen surface. Some analyses were done by using the 3D approach and 2 MPixels cameras. In this case the ARAMIS system was calibrated using a 35 x 28 mm caliber and a facet of 27 x 15 pixels; thus a 44 % facet overlap has been obtained. Images were recorded at different loads and then processed with the ARAMIS system. A typical engineering stress-strain curve for the foam with 160 kg/m3 density and major strain plots taken in the relevant parts of the curve is shown in Fig. 5. It can be observed that on the yield point crush bands inclined approximately 450 occur, tendency which extends on the plateau and densification
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