Shyshkovskyy R. Evaluation of strength and life time of structural elements under complex loading using the energy approach

The thesis is devoted to the analysis of the present state of the problem of strength and durability of structural elements under complex loading, the results of theoretical and experimental researches are analyzed. According to the results of the analysis and experimental data, a methodology for determining the strength and durability of structural elements based on the energy criterion is formulated. On the basis of the balance of elastic-plastic deformation and energy losses spent on fracture in the local volume of material, the energy criterion is developed to determine the boundary-equilibrium state of a body under tensile and shear loading. According to the criterion, damages in the local volume are accumulated under elastic-plastic deformation in a solid body. When energy of elastic-plastic deformation reaches the energy of material fracture, the body is split into fragments, in the case of complex loading the energy of elastic-plastic deformation is given as the sum of elastic-plastic deformation under tension and shear. The method is developed to determine the specific energy of material fracture by tension of a cylindrical specimen and construction of complete equilibrium diagrams using a special device under rigid mode of loading. The true deformation is determined using the digital image correlation method (DIC) in the local area where a homogeneous stress-strain state is achieved. The value of the true stresses is determined and the diagrams on the basis of which the specific energy of fracture, taking into account internal damages (defects) in the sample deformation are constructed. The DIC method is based on operations on speckle images of the optically rough surface and software processing of a number of digital images recorded during loading to track the movement of the points of the sample deformable surface and to calculate deformations. The method for determining the specific energy of shear fracture, based on a force chart of cylindrical specimen torsion, is developed. The method is developed and researches have been carried out on specimens of different degree of deformation by passing acoustic wave and the dependence of the wave propagation speed on the damage magnitude is constructed. It is established that the density of the material in the fracture zone decreases with the change in the wave propagation speed. Investigations are carried out, complete deformation diagrams are constructed, the true stresses and the specific fracture energy for the AMG-6 alloy are determined. On the basis of strength characteristics the stress-strain state is established in the wall of the carrier- rocket fuel tank. It is established that the specific fracture energy is invariant value and does not depend on changes in the external load rigidity. According to the developed approach and the program of certification tests at SE "Pivdenne" DB, the developed methodology has been implemented and the maximum working pressure in the fuel tank of the carrier-rocket is calculated. Using the finite element method, the simulation is performed and the stress-strain state of the turbine shaft is calculated. The turbine shaft is loaded with torque and bending moment, as well as the tensile force. Experimental studies are conducted for the shaft material and the specific energy for tensile and torsion is determined. Considering that the shaft operates under cyclic loads, the change in energy losses of the material in one load cycle is estimated. The dependence of the change in the magnitude of the deformation energy in one cycle on the number of load cycles is constructed. According to the results of the studies, the life time of the turbine shaft is calculated.