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S. Agnetti, Frattura ed Integrità Strutturale, 26 (2013) 31-40 ; DOI: 10.3221/IGF-ESIS.26.04 39 ground edge samples, the results in terms of failure strength show that set E present the greater values, that is higher than the failure strength of sets A and C; for this reason set E was excluded from the analysis. Instead the value of σ f (t f ) for the set A is higher than the one of the set C. In the latter case shorter samples present higher stresses that are what we expect by the theory: only sets A and B were compared. The analysis permits to obtain the same value 1.05 form the ratio of the strength σ A / σ C that is equal to the corresponding ( V eff,C / V eff,A ) 1/β . C ONCLUSIONS law characterization on glass beams was studied. The experiments performed, confirm the applicability of the failure prediction theories commonly applied to explain the failure strength of glass. Each specimen was assumed to fail in mode I (in LEFM theory, mode I is a normal-opening, while modes II and III are shear sliding modes); the failures initiated inside the loaded area. The analysis and the measurements executed confirm the relation existing between flaw size and strength: the larger the critical flaw initiating the failure, the lower the strength. An equivalent relation is valid for the mirror radius: the larger the mirror radius, the lower the strength. The edge finishing produces advantage in terms of strength. The strength of ground edge glass is higher than the one of simply cut edge glass. The edge processing produces a reduction of the maximum depth of the flaw and an increase of the strength. Furthermore it was observed that the failure strength had no linear relation to flaw depth. In analogy, the fracture surface analysis shows that the failure strength had no linear relation to the mirror radius. In the LEFM the values of the geometric parameter Y , were estimated by observing the fracture surface, in order to obtain the failure strength for each sample. In the fracture surface analysis, constant values α and σ ar , were confirmed using a linear regression law, fitting the measured data. The observations on the size effect show that the bigger is the length of the beam, the bigger is the probability to find flaws. In general, beams with edge processing, as grinding or polishing is recommended. R EFERENCES [1] Haldimann, M., Fracture strength of structural glass elements – analytical and numerical modeling, testing and design, PhD These n. 3671, EPFL, Lausanne, Switzerland (2006). [2] Haldimann, M., Luibe, A., Overend, M., Structural use of glass, Structural Engineering Documents, IABSE - AIPC – IVBH 10 (2010). [3] Lindqvist, M., Vandebroek, M., Louter, C., Belis, J., Influence of edge flaws on failure strength of glass, In: Proceedings of GPD 2011, Tampere, Finland, (2011) 126-129. [4] Vandebroek, M., Belis, J., Louter, C., Van Tendeloo, G., Experimental validation of edge strength model for glass with polished and cut edge finishing, Engineering Fracture Mechanics, 96 (2012) 480-489. [5] ASTM C1678-10, Standard practice for fractographic analysis of fracture mirror sizes in ceramics and glasses, America Society for Testing materials (2010). [6] Carpinteri, A., Meccanica dei materiali e della frattura, Pitagora Editrice, Bologna, (1992). [7] Lawn, B.R., Fracture of brittle solids. 2nd ed. Cambridge: Cambridge University Press, (1993). [8] Dielhof, M. H., Dortmans, L. J. M. G. , de With, G., Fractography of Borosilicate Tested in Three- and Four-Point Bending Glass, Journal of the European Ceramic Society, 12 (1993): 215 220 [9] Conoway, J. C., Mecholsky, J. J., Use of crack branching data for measuring near-surface residual stresses in tempered glass, Journal of the American Ceramic Society, 72 (9) (1989) 1584–1587. [10] Zaccaria, M., Overend, M., Validation of a simple relationship between the fracture pattern and the fracture stress of glass, In: Engineered Transparency. International Conference at Glasstec, Düsseldorf, Germany, (2012). [11] Fischer, H., Rentzsch, W., Marx, R., A modified size effect model for brittle non-metallic materials. Engineering Fracture Mechanics, 69 (2002) 781-791. [12] Quinn, G. D., Weibull Effective Volumes and Surfaces for Cylindrical Rods Loaded in Flexure, Journal of the American Ceramic Society, 86 (3) (2003) 475–479. [13] EN 12603:2002, Glass in building – Procedures for goodness of fit and confidence intervals for Weibull distributed glass stength data, CEN (2002). F