Research Of Dynamic Fracture Behavior Of Expanding Ring With A Preset Flaw

Fracture position and fracture forms of materials and structures have been intensively researched by the academic and engineering circulars. Caused by sample personality, cracks usually initiate at different location under dynamic loading, showing its own randomness。In order to observe the local fracture characteristics of materials under high strain rate, a flaw is preset and the crack initiates naturally in the field range。The technique of expanding ring loading is used to achieve quasi one dimension tensile loading. The high-speed photography technique is serviced to gain the preset hole's deformation, the crack initiation and crack propagation。At last the whole fracture process is numerically simulated with LS-DYNA 3D finite element code.(1) A review of fracture study is carried out。Current difficulties, focus and methods of the fracture mechanics are researched。The techniques of expanding ring loading and high-speed photography are used to observe the dynamic fracture characteristics for the subject。(2) To obtain the local fracture characteristics, a flaw is preset。In addition, a magnifying glass is placed before the sample。From the experiments of copper, it is found that different flaw sizes caused different fracture forms。The circular hole's deformation, crack initiation and crack propagation process are obtained, by changing the material.(3) At low explosive power, the crack stopped after a few of time in 20 steel expanding ring with a preset circular hole, while at high explosive power the crack propagating unstably。(4) In addition to the location of crack initiation where preset an extended defect, other cracks initiate in the inner wall or inside for the experiment with a edged notch。As the load changes, defects penetrate through the thickness direction to form a through crack. The fracture induced the destruction of the whole.(5) Details of the crack section are often ignored in actual measurement because of scale effects。It is recommended that the crack velocity can be distinguished as measuring velocity and real velocity。By the difference between these concepts, it is simple to explain why the limit velocity of crack propagation deviated to its theoretical limit speed。(6) According to the image of the 20 steel expanding ring with a preset circular hole, the displacement and the velocity of crack tips are obtained。It is observed that the crack velocity oscillates with time because of the instability。And thus we estimate the dynamic stress intensity factors of the cracks。 (7) The circular hole's deformation, crack initiation, crack propagation and crack stop are numerically simulated with LS-DYNA 3D finite element code. Macro-crack can be found at the location of the hole under dynamic loading caused by stress concentration. Simulated results are in good agreement with experiments. It is found that crack initiation in the inner wall of the ring, which through the thickness direction。And axial cracks eventually are formed。...