Issue34

L. Marsavina et alii, Frattura ed Integrità Strutturale, 34 (2015) 387-396; DOI: 10.3221/IGF-ESIS.34.43 391 Determination of density The density determination of the considered foams was performed according with ASTM D 1622-03 [23]. A Sartorius GD 503 Class Balance was used for the mass determination, and the dimensions of the specimens (84 x 35 x 10 mm) were measured using a digital caliper Mitutoyo Digimatic. Fracture toughness determination and mixed mode loading tests The fracture toughness K Ic and K IIc of PUR foams were determined using two types of specimens: Asymmetric Semicircular Bend (ASCB) and Single Edge Crack (SEC), Fig. 1. The choice of using these specimen types was that with the same shape is possible to produce from pure mode I loading to pure mode II loading, and intermediate mixed modes only by changing the position S 2 of one support for ASCB specimens or by changing the loading direction  for SEC specimens. The ASCB specimen with radius R , which contains an edge crack of length a oriented normal to the specimen edge, loaded with a three point bend fixture, was proved to give a wide range of mixed modes from pure mode I ( S 1 = S 2 ) to pure mode II ( S 1 ≠ S 2 ), only by changing the position of one support. The considered geometry of the specimen has: R =40 mm, a =20 mm, t =10 mm, S 1 = S 2 =30 mm (for symmetric loading and mode I fracture toughness determination), 12 mm, 8 mm, 6 mm, 4 mm (for mixed mode loading) and S 2 =2.66 mm (for mode II fracture toughness determination). The dimensions of the SEC specimens were W = 75 mm, t = 8 mm and a = 33.75 mm and the loading angle was  = 0 0 for mode I tests, 30 0 , 45 0 , 60 0 for mixed mode loading, respectively  = 90 0 for mode II tests. Tests were carried out at room temperature (20 0 C) on a 5 kN Zwick Proline testing machine, using displacement control with 2 mm/min loading speed, Fig. 2. Typical load - displacement curves are shown in Fig. 3: a. the effect of applied mixed mode on ASCB specimens for the foam with density 145 kg/m 3 , and b. the influence of density on SEC specimens loaded in mode I. All tested specimens show a linear behaviour and a brittle fracture, which was confirmed by the fact that no plastic deformations remain after testing. The determination of fracture toughness is presented in more detail in [14, 15] for ASCB specimens and in [16] for SEC specimens. The experimental results for density and fracture toughness of investigated foams are presented in Tab. 6. It should be mentioned that the fracture toughness results are determined for specimens obtained in the rise direction (in plane). A discussion regarding the foams anisotropy is presented elsewhere, [14]. a. ASCB specimen b. SEC specimen Figure 1 : Specimens used from fracture toughness and mixed mode loading tests. It could be observed that fracture toughness K Ic and K IIc increases with density. The maximum relative differences between fracture toughness values obtained by ASCB and SEC specimens was 28% for foam with density 100 kg/m 3 for mode II, while minimum relative difference was obtained for the same density for K Ic 1.1%. Also, it could be observed that the mode II fracture toughness values are lower than the mode I ones for all foam densities and specimen type, with the exception of 300 kg/m 3 foam density using ASCB specimens.