13th International Conference on Fracture June 16–21, 2013, Beijing, China -6- Missing cell walls (for each cell only a single wall was eliminated) and filled cells were considered in the FE analyses in order to investigate the effect of the damage of cellular structures on mechanical properties. Figure 10 presents the influence of damage expressed in terms of normalized Young’s modulus (ratio between Young’s modulus for damaged structure and for undamaged structure). It can be observed that 7% of missing cell walls decrease the Young modulus with 57%, while 7% of filled cells produce an increase of Young’s modulus with 20.5%. a. Missing cell walls b. Filled cells Figure 10. Comparison between experimental and numerical values of elastic properties of PUR foams 4. Conclusions Two experimental methods (compression tests and Digital Image Correlation) were used to determine the mechanical properties of rigid polyurethane foams. The effect of density, loading rate and loading direction were investigated. Ranking the investigated parameters it could be observed that the density has the major influence on mechanical properties, followed by strain rate, and the minor influence is the loading direction. DIC measurements prove to be a powerful tool in establishing the local damage characteristics and the bands of deformation for the PUR foams. The density of the foam appears to influence the inclination of the deformation bands. Finite Element Analysis was performed on 3D representative models to predict the elastic constant of polyurethane foams. The predicted values are in good agreement with the experimental ones which validates the numerical methodology. Then the effect of damage, represented by missing cell walls and filled cells, on Young modulus was systematically investigated. Acknowledgements This work was supported by a grant of the Romanian National Authority for Scientific Research, CNCS – UEFISCDI, project number PN-II-ID-PCE-2011-3-0456. Parts of experimental results were obtained at Lubin University of Technology, in the facilities of the Center of Excellence for Modern Composites Applied in Aerospace and Surface Transport Infrastructure (European Union Seventh Framework Programme (FP7/2007 – 2013), FP7 - REGPOT – 2009 – 1, under grant agreement No: 245479.
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