Issue 41

A. Mardaliazad et alii, Frattura ed Integrità Strutturale, 41 (2017) 504-523; DOI: 10.3221/IGF-ESIS.41.62 504 Four-point bending test on a middle strength rock: numerical and experimental investigations Aria Mardalizad, Riccardo Scazzosi, Andrea Manes, Marco Giglio Politecnico di Milano, Department of Mechanical Engineering, Via La Masa 1, 20156 Milan, Italy aria.mardalizad@polimi.it, riccardo.scazzosi@polimi.it, andrea.manes@polimi.it, marco.giglio@polimi.it A BSTRACT . Developing a reliable numerical modelling technique is considered as challenge for fracture assessment of the geological materials, which are much subjected to hydrostatic pressure. For this purpose, the mechanical behaviour and the fracture pattern of a middle strength rock material, called Pietra Serena sandstone, is investigated both numerically and experimentally under a Four-Point Bending (also called Flexural) testing program. For the numerical approach, an innovative method, namely FEM- coupled to-SPH, is exploited due to its capabilities in dealing with rock mechanics related issues. Two different material models, which are the Karagozian and Case Concrete (KCC) and the Extended (Linear) Drucker- Prager, are exerted to assess their capabilities. The Flexural strength and the crack initiation area are studied based on the state of the stress in various parts of the specimen in both models, and finally the results obtained from the numerical models are compared with the data obtained from the experimental tests in order to assess the capability of the modelling approach. K EYWORDS . Flexural test; KCC; Dracker-Prager; Pietra Serena; Fracture. Citation: Mardalizad, A., Scazzosi, R., Manes, A., Giglio, M., Four-point bending test on a middle strength rock: Numerical and Experimental Investigations, Frattura ed Integrità Strutturale, 41 (2017) 504-523. Received: 30.04.2017 Accepted: 29.05.2017 Published: 01.07.2017 Copyright: © 2017 This is an open access article under the terms of the CC-BY 4.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. I NTRODUCTION ock materials are highly dependent on the hydrostatic loading pressure, moreover their tensile strength is considerably weaker than the compressive one. Thus, the characterization of the tensile behaviour is of great importance in geophysical applications. The tensile behaviour of rock materials, which is also one of the most important parameters to predict the rock’s boreability, can be determined in several ways, i.e. by a direct tensile test, a Brazilian splitting disk test, a (three- or four- point) bending test, etc [1]. The direct tensile test seems to be the standard way to determine the maximum principal tensile strength of rock materials. However, due to several issues, including the complicated set up, the non-uniformity and the perturbation in the uniaxial stress field introduced by specimen grips and/or slight imperfections in the sample preparation and the material inhomogeneity, etc., its usage is inconvenient [2-4]. Therefore, rock engineers perform the Brazilian and the Flexural (four-point bending) test to investigate the tensile behaviour. These tests are designed to investigate indirectly a normal tensile stress state (at least) in a specific portion of the specimen. The numerical and experimental investigation of the mechanical behaviour of a middle strength rock material which is subjected to a Four-Point Bending (or Flexural) test is the aim of this research study. The Four-Point Bending test is an indirect way to estimate the tensile strength, which consists of a beam in flexure. The strength of the beam, in this test, is expressed mainly in terms of modulus of rupture, which usually tends to overestimate the tensile strength up to one-hundred percent [4]. This overestimation is mainly caused by the hypothesis of the test which R