A New Test Method For Mixed Mode Ⅰ-Ⅱ Fracture In Concrete

The cracks in concrete structures are usually mixed mode I-II and the process of fracture propagation is more complex than that of mode I.So far,experimental investigations on mixed mode I-II fracture in concrete had been systematically carried out at home and abroad and a series of results had been obtained.However,when the ratio of mode II stress intensity factor to mode I stress intensity factor（KⅡ/K_I）at the tip of the initial crack in concrete specimen is large,it is difficult to obtain the complete load vs.displacement curves.Therefore,it is necessary to propose a new method for testing the complete load vs.displacement curves in the mixed mode I-II crack in concrete.In this paper,the complete crack propagation process of the three-point bending beam with polyline crack in the mid-span was studied by means of experiment and numerical simulation.The main work and conclusions are as follows.（1）The mixed mode I-II fracture in concrete test was carried out by using the three-point bending beams with polyline crack in the midspan whose initial crack length-depth ratio（e）are 0.2,0.3,0.4,0.5 and initial bending angle（α）are 22.5o,45o,67.5o,90o.The crack trajectories,the complete load vs.crack opening displacement（P-CMOD）curves,the complete load vs.crack opening shear displacement（P-CMSD）curves,the initial cracking load(P_ini)and peak load(P_max)of different ratio at crack tip,which are K_II/K_I respectively,were measured.The results show that the specimens are mixed mode fracture and the complete fracture process of the test was stable.The complete load vs.displacement curves were measured in the test when the ratio of KⅡ/K_I was up to 1.23.（2）The complete crack propagation process of the specimen under the various conditions in（1）was simulated by using the mixed mode I-II crack propagation criterion of concrete.The crack trajectories,the complete P-CMOD curves,the complete P-CMSD curves,the initial cracking load(P_ini)and peak load(P_max)of were calculated.Compared with the experimental results,the calculated crack trajectories are basically in the envelope region of the experimental crack trajectories.The calculation results can well predict the extension path of the mixed mode I-II crack in concrete.The calculated P-CMOD and P-CMSD curves are in good agreement with the experimental results,so the complete load vs.displacement curves calculated by the mixed mode I-II crack propagation criterion of concrete are relatively accurate.Under the most conditions,the peak loads calculated by the criterion are close to the test results.The results are in good agreement.The research in this paper shows that the test of three-point bending beam with polyline crack in the mid-span can measure the complete load vs.displacement curves when the ratio of KⅡ/K_I at the initial crack tip is large,and the test method verifies the complete process curve of load vs.displacement calculated by the mixed mode I-II crack propagation criterion of concrete.