Non-geometric Symmetry. Three-point Bending Specimen Fracture And Control

The main purpose of this dissertation is to analyze and discuss the regular fracture of geometrically asymmetric samples. With this subject, the following several aspects are investigated in this dissertation:1. A state-of-art review of several branches of fracture, which are related to the dissertation, are presented, including macroscopic-microcosmic fracture mechanics, fracture dynamic mechanics, 3D crack problem etc. The crack technique, which is the basis and application of controllable regular fracture, are also reviewed concisely but thoroughly. Questions such as the derivation, the current research-situation and the investigation significance of the subject are discussed.2. With the calculation principle and calculation procedure of the finite element method are described, the analytical feasibility of this subject is explained. And on this base, the static and dynamic models of Three Points Bend Sample (3PBS) are established. The FEM analysis step and analysis course are described briefly.3. The static numerical analyses to geometrically symmetric and asymmetric samples are performed with ANSYS software. Through stress, displacement straight-line picture and the strain ratio of two supports before it cracks, the stress and displacement fields of symmetric and asymmetric samples under static conditions are observed. Finally, the identical conclusion can be drawn under the two situations, namely asymmetrical stress-strain field and displacement field do not exist.4. Dynamic numerical analyses to geometrically symmetric and asymmetric samples are carried out with ANSYS software. The change is analyzed according to the dynamic displacement and stress respond curves of the geometry symmetrical points. Result indicates that the displacement field and stress-strain field of the two ends of crack surfaces of geometry asymmetry sample appeared the asymmetric situation under the dynamic situation. So the conclusion is that, asymmetrical dynamic stress-strain field causes the late expanding instability of crack.5. On the problem of supports adjusting in the regular fracture realization of geometrically asymmetric sample, this dissertation proposes a kind of stepping motor control system tentatively. Through the control of 8098 monolithic processors and the power amplification function of the drivers, the signal is passed to the stepping-motor, which is connecting with load (supports) through the ball guide screw, and then supports are moved in axial direction. So automatic control of supports adjusting can be realized.Through static and dynamic numerical analyses of geometrically symmetric and asymmetric 3PBS, the conclusion is drawn: Asymmetrical dynamic stress-strain field causes the crack expanding instability of geometrically asymmetric sample but not static condition. This result is identical with experiments. The stepping-motor control system to geometrically asymmetric sample helps to realize automatic control of stress cutting.