Experimental Investigation On Initial Cracking Criterion For Ⅰ-Ⅱ Mixed In Concrete

Study on the destruction mechanism of concrete structures under complex loading and on establishment of a crack initial cracking criterion,which is the foundation to accurate analysis and designing of concrete structures with the concrete materials in a large number of practical engineering application. Owing to the heterogeneities and non-linear behaviors of concrete, different from the ideal brittle materials, the fracture process of concrete is complex and shows three typical stages: crack initiation, crack stable propagation and unstable propagation. In addition, due to the asymmitry of the structure or load and the anisotropic behaviors of the materials, the crack tip is not absolutely classified as mode I,or mode II, or mode III ,but a combined effect of both mode I and mode II or even that of the three. This kind of crack is defined as mixed mode crack. In engineering, the I-II mixed mode crack Occurs more frequently. Therefore, the Whole Proeess of crack initiation, stable ProPagation And unstable ProPagation of the mixed mode fracture, as well as its corresponding fracture Parameters, has been widely concerned. Then the work is done in this dissertation using experimenial and numerieal simulation methods as follows:1. Experimental investigation on intial cracking criterion for concrete beams of different crack-depth ratios900×200×80mm respectively in sizes, the fracture tests of four-point shearing Beams with three different crack-depth ratios were carried out. Measurements of the initial loads , Based on the initial loads measured in the test, the corresponding K_Ⅰand K_Ⅱcan be calculated using finite element method. Besides, a more rational load distribution under four-point shearing condition was obtained and Finally, a crack initial cracking criterion forⅠ-Ⅱmixed mode crack is proposed that is different from the cracking criterion by using maximum loads and its crack-depth ratio effect is also discussed, crack-depth ratio effect on the initial loads, crack initiation angles, K_Ⅰ/K_Ⅱwere also discussed.2. Experimental investigation on intial cracking criterion for concrete beams of different concrete strengths900×200×80mm respectively in sizes, the fracture tests of four-point shearing Beams with three different strengths were carried out. Measurements of the initial loads , Based on the initial loads measured in the test, the corresponding K_Ⅰand K_Ⅱcan be calculated using finite element method. Besides, a more rational load distribution under four-point shearing condition was obtained and Finally, a crack initial cracking criterion forⅠ-Ⅱmixed mode crack is proposed that is different from the cracking criterion by using maximum loads and its concrete strength effect is also discussed. concrete strength effect on the initial loads, crack initiation angles, K_Ⅰ/K_Ⅱwere also discussed.3.The curve obtained from the cracking test compared with the theory curvesThe curve obtained from fitting the criteria corresponding to the cracking test is very different with the curve of the crack criteria under the maximum axial stress, maximum tensile strain and strain energy density factor theory of fracture criterion. Which shows that we try to use a fracture criterion to describe all of the concrete compound fracture is inappropriate, therefore, the theory of fracture criterion of concrete compound remains to be further studied.