The Experimental Study On Ⅰ-Ⅱ Mixed Mode Crack In Concrete By Means Of Photoelastic Coatings

The mechanic behavior of concrete is closely associated with crack propagation and the research on crack propagation theory and analysis has become the foundation for concrete. However, owing to the heterogeneity and non-linear behavior exhibiter in concrete, different from the ideal brittle materials, the fracture process of concrete material is complex and shows the typical three stages: crack initiation, crack stable propagation and unstable propagation. In addition, due to the asymmetry of the structure or load and the heterogeneity of the materials, the crack in structure is usually under combined stress, which results the stress field at the top of crack should not absolutely be classified as I mode (opening), or II mode (sliding), or III mode (tearing), but a combined effect of both I mode and II mode, or even that of the three. This kind of fracture should be defined as mixed mode crack. In engineering, the I-II mixed mode crack occurs largely. Consequently, the whole process of crack initiation, stable propagation and unstable propagation of the mixed mode fracture, together with its corresponding fracture parameters has been widely concerned.Considering these problems, study is done in this dissertation on crack propagation process of I-II mixed mode crack in concrete by means of photoelastic coatings. 19 four-point shearing specimens at five different sizes are utilized to systematically investigate the crack initiation, stable propagation and unstable failure of concrete by means of photoelastic coatings. Using camera, the complete crack propagation process of concrete that appeared as interference fringes on the photoelastic coatings is recorded. On this basis, the propagation lengths under various loads are obtained. The whole process of stable propagation and unstable development of the main crack is recorded by photographic pictures, and based on the colored strain sequence, the strain distribution around the crack tip are also obtained. According to the results of the test, it was discovered that for those specimens in the same size, with the increase of shearing strength, the initial load together with the critical crack propagation length will be strengthened. While for those specimens in the same size but with different crack position, the initial load will be increased with the lengthening of the scale. Moreover, experimental curves are formed on the basis of obtained K_I/K_IC andK_II/K_IC datas from specimens. And they are compared with avaiable mixed mode of concretfracture theory. According to the results of the test, the experimental curves show obvious size effects, and the experimental curves are more close to the maximum circumferential strain fracture criterion.