The Analytical Study On The Energy In The Fracture Process Of Concrete

Because of its wide use as a major construction material in practice, concrete has attracted much attention regarding its fracture and failure study. A general consensus exists related to the influence of slow crack propagation ahead of the crack tip upon the nonlinearity observed in the typical load-deflection response of the concrete specimen, namely, concrete exhibits somewhat quasibrittle behavior, which has been verified by the inapplicability of the conventional linear elastic fracture mechanics(LEFM) and fracture models suitable for metal-like materials. With this in minds, many researchers have proposed a variety of models for describing the nonlinear fracture of concrete for the past decades. Amid them most take the critical stress intensity factor KIC as the fracture toughness, a sign of ability to resist the crack extension. While it is known that Glc the critical energy release rate based on the energy balance method should have the same effect as KIc in depicting the fracture of concrete. Therefore, this paper aims to study the fracture process of concrete from the standpoint of the energy method.1. The fracture energy GF of concrete is one basic material characteristic, representing the energy necessary to create a unit area of fracture surface. The method of experimental determination of GF is one of the hot subjects hi the field of concrete fracture mechanics. With comparisons among those methods generally used in determining GF, the wedge splitting method is more appropriate and thus chosen to quantify GF and its variation with the ligament length of specimen. During the test procedure, the compliance test method is adopted to increase the load cyclically until the crack extends over the height of the specimen. Then the record data are rationally smoothed and calibrated for next use, at the same time, the response on the tail of measured curve is expressed in mathematical function to eliminate its errorous influence on GF.2. Many experimental investigations reveal that crack in concrete structures undergoes two evident stages: crack stable propagation and crack unstable fracture. Whereas GF, although an energy index, only denotes the average energy absorbed during the whole concrete fracture process, ignoring those two procedures mentioned above. Wherefore, other two energy-based fracture parameters are put forward: the stable fracture energy GFS, indicating the average energy dissipation during the crack stable propagation and the unstable fracture energy GFU, corresponding to the crack unstable fracture stage. Wedge.?splitting specimens are tested with a view to understanding those two parameters' size effect in terms of the initial crack length a0.3. Different test methods in determining those three fracture energy parameters arise one problem, that is whether GF. GFS and GFU are consistent as obtained from differentmeasuring methods? hi this paper, concrete specimen of the same composite in shape of three-point-bend beam and wedge splitting specimen are tried out to ascertain the influence of specimen shape upon the fracture parameters. A slight difference is observed from the test results, which needs further investigations.4. Paralleling to double-AT criterion with SIF(stress intensity factor)-oriented K?(theinitial fracture toughness) and AT?(the unstable fracture toughness) as fracture toughness, double-G fracture model based on energy balance method is presented, hi which two basic governing parameters hi the form of ERR(energy release rate), i.e., G?(the initialfracture toughness)and G?(the unstable fracture toughness) are taken as fracturetoughness of concrete. Cohesive force distributed along the fracture process zone ahead of the crack tip is taking into account hi this model, together with the application of theconventional LEFM in aspect of analytical expression for G?andG? Also, these two parameters are not independent, the discrepancy is Gjcc, the energy assumption devoted by cohesive force. With G^ obtained by introducing...