Boundary Condition Effect on 3-Point Bending Test via FDEM

Abstract

ABSTRACT Three-point bending test is a widely used laboratory test for the estimation of the flexural force-displacement response of the material. It is well known that the results of the three-point bending test are sensitive to the specimen and the loading. In this work, the impact of the boundary effects on a single notch three-point bending test is studied using LANL's combined finite-discrete element method (FDEM) based code, the Hybrid Optimization Software Suite (HOSS). To this goal, a series of virtual experiments with different specimens and boundary conditions are conducted. The influence of the notch location, and fixture shape are studied. Results clearly show that these boundary conditions greatly influence on the fracture initiation and propagation behaviors as well as the flexural force-displacement response. INTRODUCTION Three-point bending test is a long-used technique applied mainly in the oil and gas industry to estimate the fracture toughness of rocks in the laboratory. In this test, the vertical tension is applied to the convex side of a sheet or plate specimen placed on two supporting fixtures with a set distance apart. A variety of numerical approached have been proposed to analyze the crack initiation and growth behavior of three-point bending test in the past several decades (Fakhimi, 2005; Kokot, G., Binkowski, 2011; Ramos, 2015; Klasztorny, 2018: Sauvage, 2020). However, the effect of boundary conditions is still lacking consideration in the previous studies. Motivated by the prior works and the knowledge gaps remaining in effect of boundary conditions, a finite-discrete element method (FDEM) model is applied here to interpret three-point bending experiment results in order to characterize dependence of rock fracture behavior on boundary conditions and notch location. The present study therefore aims to capture the crack initiation and propagation behavior for laboratory experiments using material properties and traction-separation law that has been calibrated by comparing the simulation results with lab experiments provided by Jiang et al. (2021) and Jiang et al. (2022), regardless of notch location. This study aims to revisit the effect of boundary conditions on crack initiation and growth behavior under three-point bending test. For this purpose, a 2D plane-strain model is developed using HOSS, a combined finite-discrete element method (FDEM) based code. Different boundary conditions and notch locations are considered for lab-scale simulations. Indeed, simulation results indicated that the crack initiation location and crack growth behavior can vary with different boundary conditions and notch locations, which can challenge the general belief that the crack initiation and propagation behavior should not be affected by the boundary conditions greatly.