Fracture Experimental Study On Three-point Bending Notched Beam And Flattened Brazilian Disk Of Concrete

Concrete is a typical kind of heterogeneous quasi-brittle material. Traditional strength theory cannot describe its failure behavior and fracture process accurately. The results of the theoretical deduction are very different from those of actual tests, and there are serious problems in practical engineering application. In order to describe and predict the fracture behavior of concrete accurately and apply the theory to practical engineering, the concrete fracture mechanics become the most important theory to study the fracture behaviors of concrete. Fracture mechanics of concrete has very high requirements in both theoretical and experimental aspects. It not only needs theoretical study, but also needs experimental study to prove the known theory and discover the unknown theory. Fracture experimental study is a vital part in the research of fracture mechanics of concrete.In this paper, we studied the R-resistance curves of three-point bending notched beams with different relative crack lengths (a0/h) through size effect model and two-parameter fracture model. The relative crack lengths are 0.2、0.3、0.4、0.5、0.6. We studied the influence of relative crack lengths (a0/h) on the R-resistance curve in the same model and the influence of the type of model on the R-resistance curve with the same relative crack lengths (a0/h). We also got the P-CMOD curves through the two models using linear elastic fracture mechanics and linear elastic superposition hypothesis. Then we compared them with the real P-CMOD curves. The results show that:the R-resistance curves increase with the increase of the relative crack length. The R values of size effect model are smaller than those of two-parameter fracture model. The max load and critical crack opening displacement of size effect model are both smaller than those of two-parameter fracture model and near the experimental values. And the critical effective crack length and critical crack growth resistance of size effect model are both bigger than the values of two-parameter fracture model. The test results indicate that R-resistance of two parameter fracture model applies to the range of small relative crack lengths (a0/h) and R-resistance of size effect model applies to a wider range.In this paper, we also tested some three-point bending notched beams and notched flattened Brazilian disks with same mix proportion and section size. The relative crack lengths are 0.2、0.3、0.4、0.5、0.6. Through the comparison among the double-K fracture toughness of these concrete specimens, we can know the influence of relative crack lengths on fracture toughness and the relationship between fracture toughness values of three-point bending notched beams and notched flattened Brazilian disks. The results show that:With the increase of the relative crack length, the double-K fracture toughness values of three-point bending notched beams and notched flattened Brazilian disks decrease. The values of three-point bending notched beams are bigger than those of notched flattened Brazilian disks. The test results indicate that using notched flattened Brazilian disks to measure the double-K fracture toughness values is available.