Influence Of T-stress On Fracture Toughness Of Concrete

It was once believed that stress intensity factor (SIF), which describes the singular stress field ahead of a crack tip, was the single controlling parameter for the fracture process in cracked specimens. Nevertheless, further explorations have shown higher order terms of the crack tip asymptotic field are also of great importance, especially the second nonsingular term, the so-called T-stress. The T-stress which acts parallel to a crack corresponds to the constant term in the stress series at the crack tip, and its influence on the stress field can not be ignored. In order to explore the influence of T-stress on fracture toughness of concrete, this paper adopted three-point bending beam and four-point shearing beam which were generally used for testing fracture parameters in ordinary laboratories.T-stress at the crack tip must be determined to see whether its value would influence the fracture parameters. In the present paper, finite element program ABAQUS was used to get the T-stress for a three-point bending beam with a span to depth ratio of4and a single-edge notched beam subjected to pure bending. For the sake of accuracy, we compared it with the previous literature. Under this circumstance, a general expression for T-stress of three-point bending beams of arbitrary span-to-depth ratio was predicted based on St. Venant Principle. T-stress expressions were provided for three-point bending (TPB) beams and compact tension (CT) specimens and then its influence on mode I fracture toughness of concrete was investigated. The study showed that T-stress is dependent on the specimen geometry and material property as well, and for TPB and CT specimens of regular size, T-stress was so small that its consequence can be neglected. The study also indicated that concrete specimen size should be carefully chosen to make sure the existence of K-dominance ahead of the crack tip, thus fracture toughness extracted from these specimen configurations can be reliable.Secondly, T-stress was determined the at the crack tip in different conditions of four-point shearing beam. A series of different relative crack length, loading position, crack location were used to study the influences of T-stress on the crack properties and Ⅰ-Ⅱ mixed mode fracture toughness of concrete. The research showed that when the relative crack length is relatively small, the J-stress is not large. When the relative crack length close to one, it had a notably significant effect on the stress intensity factor of Ⅰ-Ⅱ mixed mode fracture. In contrast, the crack will deviate from the original direction. When the load point was applied at the center position of the beam, the value of T-stress was so large that the influence on the stress intensity factor was more remarkable. This paper summarized the T-stress calculation formula of four-point shearing beam through large amounts of the fitting formula. Finally, we obtained the influence on Ⅰ-Ⅱ mixed mode fracture is especially significant comparing with the literature on experiment. This paper had proved the importance of T-stress on fracture properties of concrete research.