Abstract
Tests were carried out using three independent jacks orthogonally disposed, making it possible to apply a purely tensile force, so that the secondary flexural stresses, if kept under control, constitute a degree of error comparable with the values allowed for normal testing apparatus. The method enables a stress versus strain curve to be plotted with the descending (softening) branch up to the point where the cross-section of the tensile specimen breaks away. The principal purpose is to avoid any spurious effect that might provide a fallacious explanation of the recurring size effects on apparent tensile strength and fictitious fracture energy. Once the secondary effects have been excluded, only the disorder and fractality of the concrete microstructure remain to explain such fundamental trends. In the case of tensile strength, the dimensional decrement represents self-similar weakening of the material ligament, due to pores, voids, defects, cracks, aggregates, inclusions, etc. Analogously, in the case of fracture energy, the dimensional increment represents self-similar tortuosity of the fracture surface, as well as self-similar overlapping and distribution of microcracks in the direction orthogonal to that of the forming macrocrack.
Resume
L'expérience a été conduite en utilisant trois vérins indépendants placés orthogonalement. Celà a permis d'appliquer une charge de traction pure de façon que les contraintes secondaires de fléxion, si elles sont contrôlées, donnent des erreurs dont l'ordre de grandeur est comparable au valeurs tolérées pour les appareils utilisés normalement. Ce gendre de méthode expérimentale permet de suivre la partie descendante (écrouissage négatif) de la courbe effort-déformation jusqu'au point où la section transversale de l'échantillon sous traction se casse. Le but principal est d'éviter tous les faux effets qui peuvent nous conduire à des explications erronées à propos des effets d'échelle récurrents sur l'allure de la résistance à la traction apparente et de l'énergie de rupture fictive. Une fois que les effets secondaires ont été exclus, seuls le désordre et le caractère fractal de la microstructure du béton permettent de justifier cette tendance fondamentale. Dans le cas de la résistance à la traction, la diminution dimensionnelle représente un affaiblissement des liaisons du matériau, dû aux pores, aux vides, aux défauts, aux fissures, aux granulats, aux inclusions, etc. D'une manière analogue, dans le cas de l'énergie de rupture, l'accroissement dimensionnel représente la sinuosité de la surface de rupture, ainsi que la superposition et la distribution des microfissures dans la direction perpendiculaire à celle des microfissures en formation.
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Carpinteri, A., Ferro, G. Size effects on tensile fracture properties: a unified explanation based on disorder and fractality of concrete microstructure. Materials and Structures 27, 563–571 (1994). https://doi.org/10.1007/BF02473124
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DOI: https://doi.org/10.1007/BF02473124