Issue 42

M. Peron et alii, Frattura ed Integrità Strutturale, 42 (2017) 214-222; DOI: 10.3221/IGF-ESIS.42.23 215 densities (> 200 kg/m 3 ) they show a porous solid structure, and they are used for fixtures and gauges, master and copy models, draw die moulds, hard parts for electronic instruments [1]. Their mechanical properties have been extensively investigated in the past, showing a relationship, based on the geometry of cellular structure and the relative density, with solid materials used for manufacturing [2,3], and revealing a crushable behaviour in compression, characterized by the capability to absorb considerable amount of energy due to plateau and densification regions. Finally Marsavina [4] reports that they behave as brittle materials under tensile loading, being characterized by a linear elastic behaviour up to fracture. It is worth to notice that usually in the industrial applications components are designed with notches or they are affected by manufacturing induced defects, which are widely reported to reduce both tensile and fatigue strength [5-7]. Several criteria have been proposed in literature for facture assessment of cracked and notched components [8-16], but among all a strain energy based (SED) approach has revealed to be the most robust in the assessment of brittle fracture resistance of several materials [17-20]. The criterion states that brittle fracture failures occur when the strain energy density averaged in a circular control volume of radius R c , which surrounds a crack or notch tip, reaches a critical value W c dependent on the material. The robustness of this criterion has been proved by several works on different notches geometries and on different loading conditions [21-27]. The main purpose of this work is to evaluate the effectiveness of SED criterion on PUR foams, aiming to experimentally evaluate the main parameters of this method, i.e. R c and W c . M ATERIALS olyurethane materials of four different densities (100, 145, 300 and 708 kg/m 3 ) manufactured by Necumer GmbH – Germany, under commercial designation Necuron 100, 160, 301 and 651, were experimentally investigated. At low densities 100 and 145 kg/m 3 the materials have a rigid closed cellular structure, while the PUR materials of higher densities show a porous solid structure (300 and 708 kg/m 3 ). A QUANTA™ FEG 250 SEM was used to investigate the microstructures of the materials at different magnifications. The cell diameter and wall thickness were determined by statistical analysis, together with the density of PUR materials obtained experimentally according with ASTM D1622-08. The elastic properties Young modulus and Poisson ratio were determined by Impulse Excitation Technique (ASTM E-1876-01). Tensile strength was determined on dog bone specimens according with a gage length of 50 mm and a cross section in the calibrated zone with 10 mm width and 4 mm thickness, according to EN ISO 527, and described in the research published by Marsavina et al. [28]. PUR Density 100 145 300 708 Young’s modulus [MPa] 30.18±1.75 66.89±1.07 281.39±2.92 1250±15.00 Poisson’s ratio [-] 0.285 0.285 0.302 0.302 Tensile strength [MPa] 1.16±0.024 1.87±0.036 3.86±0.092 17.40±0.32 Mode I fracture toughness [MPa m 0.5 ] 0.087±0.003 0.131±0.003 0.372±0.014 1.253±0.027 Mode II fracture toughness [MPa m 0.5 ] 0.050±0.002 0.079±0.004 0.374±0.013 1.376±0.047 Table 1 : Elastic, mechanical and fracture properties of PUR materials, by varying the density. The mode I and II fracture toughness were determined on asymmetric semi-circular bend (ASCB) specimens. A detailed description of these tests is presented in [29,30]. The experimentally values of elastic, mechanical and fracture toughness properties are presented in Tab. 1. E XPERIMENTAL INVESTIGATION Tensile test ifferent notched specimens were tested under tensile load. Notched specimens with geometries presented in Fig. 1.a,b,c having lateral V, rounded U and circular holes of different diameters D were tested in tensile. The U notched specimens, with blunt curvature radius ( R = 4.25 mm), were tested for each density, respectively holed P D