Papers by Author: Yan Hua Zhao

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Authors: Yan Hua Zhao, Shi Lang Xu, Zhi Min Wu, Hong Bo Gao
Abstract: The apparent size effect of the specific fracture energy of concrete according to the RILEM procedure has been confirmed by numerous published works. The paper offers an explanation for this size effect by considering the specimen boundary influence on local fracture energy over the ligament length, which is closely associated with the measured fracture energy of concrete. To address this boundary influence, boundary affected length is introduced, over which local fracture energy is different from that in the bulk far away from the surface of the specimen. Based on previous work, a continuous smooth function is hypothesized to simulate the distribution of local fracture energy. At the same time, the model established was compared to the existing models, i.e. Perturbed Ligament Model (PLM) and Bilinear Model (BLM). Some test results from wedge splitting specimen in the literature were used to verify these three models. The results show that the true fracture energy of concrete, irrespective of the specimen size, could be obtained from the measured values directly from RILEM, and is less sensitive to determination approach. The predicted boundary affected length when the crack reaches the specimen surface is more close to the value of the perturbation length in PLM.
933
Authors: Yan Hua Zhao, Hua Zhang, Wei Dong
Abstract: The wedge splitting (WS) test is now a promising method to perform stable fracture mechanics tests on concrete-like quasi brittle materials. Fracture parameters, such as fracture toughness and critical crack opening displacement and et.al, are however not easy to determined since formulae available from stress intensity factor manual are restricted to standard specimen geometry. The paper attempts to compute expressions for commonly used fracture parameters for a general wedge splitting specimen. By means of finite element analysis program, test simulation was performed on non-standard wedge splitting specimen with different depth and initiation crack length, and thereafter expressions were proposed for stress intensity factor at the pre-cast tip and crack mouth opening displacement on the load line. Based on the work above, size effect on the unstable fracture toughness and crack extension were investigated, and the consistency of fracture toughness data for various specimen depth as well as initiation crack length is demonstrated. The crack extension is little sensitive to the initiation crack length, it increases with the depth of specimen, which can be explained by the boundary influence of the specimen.
425
Authors: Wei Dong, Zhi Min Wu, Yan Hua Zhao, Fu Min Yi
Abstract: Investigation on crack propagation is important to understand mechanical behaviors and fracture mechanism in concrete. Till now substantial work has been done on crack propagation under mode-I loading, and mixed-mode fracture mechanics problems, however, are more common for actual crack growth. This study aims to systematically investigate the crack initiation, stable propagation and unstable failure of concrete under I-II mixed mode condition by using photoelasticity technique. For this purpose, reflective photoelastic coatings were applied to the surface of the four-point shearing notched concrete specimens with the maximum size of 2500mm×600mm×200mm. The resulting interference fringe on photoelastic coatings was recorded to reflect a complete crack propagation process. Based on the colored strain sequence, the strain profile around the crack tip could be determined and thereafter the load-crack extension curve. Then finite element analysis was performed to simulate the process of crack propagation and corresponding stress intensity factors were solved for every step. Experimental results show obvious stable crack propagation in concrete before unstable failure under I-II loading condition. The conventional failure criterion with the absence of this stable propagation quantity is therefore conservative. Besides, the fracture mode is ultimately I-mode even the specimen is subjected to I-II mixed mode loading condition, that is, the crack will extend in a direction perpendicular to the peak principal stress.
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