STUDY OF THE DOUBIE-K FRACTURE PARAMETERS OF CONCRETE BASED ON EQUIVALENT PURE BENDING BEAMS

In a four-point bending (FPB) test, the beam segment near the mid-span cracking section is under pure bending. Therefore, the mode Ι fracture model can be better applied in theory. Based on this, FPB tests on notched beams were carried out. The calculation formulae of double-K fracture parameters were established by assuming that the pure bending segment of a FPB specimen under external load is equivalent to a pure bending specimen and by using the linear superposition principle. Moreover, the effect of different initial crack-depth ratios (0.2, 0.3, 0.4, 0.5 and 0.6) on the fracture parameters was evaluated. The results indicate that with the increase of the initial crack-depth ratio, the initial fracture load, P_ini, the maximum fracture load, P_max and the critical relative effective crack propagation length, (a_c-a₀)/H, decrease gradually while the initial fracture toughness, K_ICⁱⁿⁱ and the unstable fracture toughness, K_IC^un keep almost constant. Consequently, FPB tests can be applied to determine the double-K fracture parameters of concrete.