| Since the existence of fracture process zone (FPZ) is the primary cause of which the linear fracture mechanics is not applicable to calculate the concrete material, the study on the characteristics of the FPZ has always been one of the hottest research areas in the field of concrete fracture mechanics. In this paper, the characteristics of FPZ in the crack propagation is studied by both of the numerical simulation and tests in the laboratory. Primarily, the size measurement of FPZ and numerical computation of the mode I and mixed mode I-II crack propagation considering FPZ are investigated. The main work is as follows:(1) First, the propagation of FPZ in the three-point bending concrete beams is measured by using digital image correlation method (DICM). This method features non-destructive monitoring, simple collection of the raw data, real-time measurement, high accuracy, simple testing instruments, easy to implement, automatic measuring, etc., and can be carried out in the ordinary laboratory. The size of FPZ and opening displacement of the crack zone can be measured under any load condition by using this method. The results show that in the process of specimen fracture, the length of the FPZ is first growing as the load increases and decreasing as the load decreases after it is fully developed. In addition, the length of the FPZ corresponding to the peak load and full propagation are both increasing with the increment of specimen height, and decreasing with the increment of the initial crack-depth ratio, indicating that the length of the FPZ is correlated with the length of initial ligament.(2) Based on the mode I crack propagation criterion, the discrete crack model is adopted to calculate the FPZ length in the crack propagation of the three-point bending concrete beam by using APDL (ANSYS parameter design language). The comparision between the numerical results and test results by the DICM indicate that the numerical method accurately take into account the resistance effect of the cohesive stress distributed in the FPZ, and can calculate the propagation of the FPZ and load-displacement curve of the concrete structure accurately.(3) The concrete square plate is taken as the computation model, of which the variation of the FPZ length in the crack propagating with different size and initial crack-depth ratio (a0/D=0.05～0.9) is calculated. At the same time, the initial crack-depth ratio effects on the fracture parameters are investigated by the three-point bending beam fracture test. The results show that the initial crack-depth ratio has a prominent effect on the variation of the FPZ in the crack propagation. When the initial crack-depth ratio is very large, i.e. a0/D=0.9, the FPZ cannot be fullly developed even if the specimen totally fracture. When the initial crack-depth ratio is very small, i.e. a0/D=0.05, after FPZ is fully developed, the length of FPZ is continually growing until the crack tip propagates to the ligament area where the crack-depth ratio is grater than0.5, and it starts to decrease.(4) According to the research in this paper, for the small size three-point bending beam specimen, when a0/h≥0.8, it can be found the initial cracking load Pini is equal to the maximum load Pmax.Therefore, Pini can be determined by directly measuring the maximum load of the specimen of which the initial crack-depth ratio is0.8, which is taken as the new menthod for mesuring Pini. This method is more accurate and simple. However, much care should be taken to the fabrication of the specimens with large initial crack-depth ratio. Then K (?) of the concrete material is obtained by substituting (Pmax, ao) into the formula of linear elastic fracture mechanics.(5) The whole mixed mode Ⅰ-Ⅱ crack propagation process of the gravity dam considering the FPZ effect is investigated by combining the tests and numerical simulation. First, the initial cracking and stable propagation of the gravity dam are observed in the test, and the load-crack opening(sliding) mouth displacement and crack propagation trajectory are recorded, which provides both the testing data for estimating the safety usage of the gravity dam with notches and validating data for numerical calculation. Second, the propagating process of mixed mode Ⅰ-Ⅱ is realized based on the mixed mode crack propagation criterion and finite element program. Last, the numerical calculating results are verified by the test results. The calculating results show that the numerical method can calculate the whole propagating process of the mixed mode Ⅰ-Ⅱ crack accurately. Therefore, it can be used to analyze the stability of the concrete gravity dam with notches and predict the ultimate bearing capacity and has a good engineering application value. |
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