The Experimental Study On Fracture Energy And Double-K Fracture Parameter Of Concrete

As one heterogeneous composite, the fracture mechanism of concrete is very complicated and difficult to study because of the physical characteristics, combination aspects and state of stress of the component of concrete.But concrete is in the massive use with architectural project, which needs the researchers must to strengthen the research of the concrete. With the development of national economy,it is decided to study the characteristics of the concrete deeply. So, the research of the fracture mechanism of concrete has important realistic meaning and project value. What the author does in this text is to study the influence of specimens size and initial ratio of the notch depth to the beam depth on fracture energy G_f and double-K fracture parameter via physics test, which the fracture parameter including initial cracking toughness Klcini and unstable fracture toughness KIcun are studied as well as the size effect.Through reading a lot of references and papers and using some achievements and experiences that other researchers have already made, the author carries out physical tests of notched three-point bend beams with different sizes and ratio of the notch depth to the beam depth. The author utilizes high-accuracy AD board to sample intensive data in the process of the test, and analyze the test data carefully.There are a large number of noises in the process of the test because of the voltage fluctuation and electromagnetic interference. The interference of the noise brings inconvenience to analysis of the test data, and influences the accuracy of the calculation greatly. According to the concrete conditions of the test data, the author tries to use Savitzky-Golay fitting, low-pass filtering and wavelet method to reduce the noise in the test data. Because the noise reduction by wavelet method is optimum, the author adopts the wavelet method in the thesis fo improve the SNR of the test data.Through the analysis of many group's test data, the author gets some conclusions of the following, some of them are different from the past to some extent:1. The influence of the specimen size on the fracture energy of concrete is investigated by using three-point bend beams with different size. Results are found that the fracture energy Gf is a constant without the relationship of specimen height, and does not change with the change of specimen size, that is to say that the size effect does not exist.2. The concrete fracture energy Gf calculated by load-displacement (P) curve of three-point bend beam specimens is almost a constant, and it is not influenced by the ratio of the notch depth to the beam depth ao/h.3. The influence of concrete specimen size on double-k fracture parameter is also investigated by using notched three-point bend beams with the different sizes. The result shows that specimen initial cracking toughness KIcini and unstable fracture toughness KIcun increase with the increase of concrete size, which the size effect exists. It also shows that the precritical crack propagated length and the critical crack tip opening displacement CTODc value increase with the increase of concrete size.4. The initial cracking toughness Kicini and unstable fracture toughness Kicun of notched three-point bend beams are almost the constant with the different ratio of the notch depth to the beam depth ao/h, which do not change with ao/h on the whole. The precritical crack propagated length and the critical crack tip opening displacement CTODC value decrease with the increase of the notch depth to the beam depth.5. For the three-point bend beam, it was found that the initial cracking toughness Kicinicalculated based on the different cohesive force equation is not similar.