L. Marsavina et alii, Frattura ed Integrità Strutturale, 34 (2015) 387-396; DOI: 10.3221/IGF-ESIS.34.43 387 Focussed on Crack Paths On the crack path under mixed mode loading on PUR foams L. Marsavina, E. Linul, T. Voiconi University Politehnica Timisoara, Romania , , D. M. Constantinescu, D. A. Apostol University Politehnica Bucharest, Romania, A BSTRACT . In this paper are presented the crack initiation angles obtained in polyurethane (PUR) foams under mixed mode loading. Closed cell rigid PUR foams having three different densities 100, 145, and 300 kg/m 3 were investigated. Experiments were performed using Asymmetric Semi-Circular Bend and Single Edge Crack specimens. The obtained crack initiation values were compared with four fracture criteria to introduce: Maximum Tensile Stress, Strain Energy Density, Maximum Energy Release Rate and Equivalent Stress Intensity Factor, and a good agreement was observed. This allow to conclude that the theoretical fracture criteria developed for solid material could be used with success to predict the crack propagation angles in cellular materials like PUR foams. K EYWORDS . PUR foams; Crack path; Mixed mode. I NTRODUCTION he main characteristics of polyurethane (PUR) foams are lightweight, high porosity and good energy absorption capacity, [1,2]. One of the main application of foam materials is their utilization as cores in sandwich structures. Of particular interest is the fracture toughness in mixed modes because foam cracking weaknesses the capacity of carrying load of the sandwich structure. There are several studies to determine the mode I fracture toughness and to investigate the influence of density, loading speed and loading direction [3-11]. A linear correlation between Mode I fracture toughness K Ic and relative density of the foam was observed by Danielsson [7] on PVC Divinycell foams, Viana and Carlsson on Diab H foams [8]. Brittle fracture without yielding in mode I loading was observed by Kabir et al. [9] for PVC and PUR foams applying the procedure described by ASTM D5045. They investigated the effect of density, effect of specimen size, effect of loading rate and effect of cell orientation. Density has a significant effect on fracture toughness, which increases more than 7 times when the foam density increases 3.5 times. Burman [10] presented fracture toughness results for two commercial foams Rohacell WF51 (density 52 kg/m 3 ) and Divinycell H100 (density 100 kg/m 3 ). The mode I fracture toughness K Ic was obtained on Single Edge Notch Bending specimens and has values 0.08 MPa  m 0.5 for WF51, respectively 0.21 MPa  m 0.5 for H100. He also determined the Mode II fracture toughness using End-Notch Flexure (ENF) specimen with values of 0.13 MPa  m 0.5 for WF51, respectively 0.21 MPa  m 0.5 for H100. Poapongsakorn and Carlsson [11] also presented fracture toughness results for PVC foams determined on Three (TPB) and Four Point Bending (FPB), investigating the cell size T