Abstract
Symmetrically notched beam specimens of concrete and mortar, loaded near the notches by concentrated forces that produce a concentrated shear force zone, are tested to failure. The cracks do not propagate from the notches in the direction normal to the maximum principal stress but in a direction in which shear stresses dominate. Thus, the failure is due essentially to shear fracture (Mode II). The crack propagation direction seems to be governed by maximum energy release rate. Tests of geometrically similar specimens yield maximum loads which agree with the recently established size effect law for blunt fracture, previously verified for tensile fracture (Mode I). This further implies that the energy required for crack growth increases with the crack extension from the notch. The R-curve that describes this increase is determined from the size effect. The size effect also yields the shear fracture energy, which is found to be about 25-times larger than that for Mode I and to agree with the value predicted by the crack band model. The fracture specimen is simple to use but not perfect for shear fracture because the deformation has a symmetric component with a non zero normal stress across the crack plane. Nevertheless, these disturbing effects appear to be unimportant. The results are of interest for certain types of structures subjected to blast, impact, earthquake, and concentrated loads.
Résumé
On a procédé aux essais en rupture d’échantillons de béton et de mortier à entailles symétriques dans le voisinage desquelles s’exerçaient des efforts concentrés qui ont déterminé une zone de concentration de contraintes de cisaillement. Les fissures à partir des entailles ne se sont pas propagées dans la direction normale à la contrainte maximale principale mais dans une direction où dominaient les contraintes de cisaillement. L’endommagement est donc dû d’abord à une rupture de cisaillemnt (Mode II). La direction de propagation des fissures paraît déterminiée par le taux maximal de restitution d’énergie. Les essais de corps d’épreuve géométriquement similaires indiquent des charges maximales qui concordent avec la loi récemment établie de l’effet de dimension pour les ruptures qui étaient précédemment vérifiées pour les ruptures en tension (Mode I). Ceci par la suite implique que l’énergie requise pour la croissance de fissure augmente avec l’extension de fissure à partir de l’entaille. Plus la fissure est loin de l’entaille, plus il faut d’énergie. La courbe de résistance que décrit ce phénomène se détermine à partir de l’effet de dimension. Celui-ci détermine également l’énergie de rupture en cisaillement qui se trouve être environ 25 fois supérieure à celle de la rupture en tension et concorder avec la valeur annoncée par le modèle de bande de fissure. Le corps d’épreuve de rupture est simple d’emploi mais pas parfaitement adapté à la rupture par cisaillement du fait que la déformation présente une composante symétrique avec la contrainte normale à travers le plan de fissuration. Néanmoins, les effets perturbateurs semblent être négligeables. Les résultats présentent de l’intérêt pour certains types de structures exposées à des charges concentrées sismiques d’impact et explosives.
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Bažant, Z.P., Pfeiffer, P.A. Shear fracture tests of concrete. Materials and Structures 19, 111–121 (1986). https://doi.org/10.1007/BF02481755
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DOI: https://doi.org/10.1007/BF02481755