Bondarets O. Model of scattered damage accumulation in anisotropic structural materials under plastoelastic deformation

The dissertation is devoted to the development of a generalized model of accumulation of scattered microdamages taking into account the parameters of anisotropy for solving practical engineering problems to refine the stress-strain state of the load-bearing elements of structures. It allows to significantly reduce the number of basic experiments to determine the components of the damage tensor for anisotropic structural materials. The development of the basic principles of the thermodynamics of irreversible processes with respect to the equations of the kinetics of the accumulation of anisotropic damages during elastic-plastic deformation is considered and detailed. It is shown that for today a parameter has not yet been determined that would unambiguously describe the process of accumulation of microdamages in an anisotropic structural material. The boundaries of these approaches are shown and substantiated for the experimental determination (assessment) of the damage parameter by the method of changing the elastic modulus, based on the concept of equivalence of deformations, increment of additional elastic energy and equivalence of elastic energy. Based on these concepts, a generalized phenomenological model of damage anisotropy in structural materials under elastoplastic deformation was developed and substantiated. An engineering method has been developed for determining the parameters of the damage model and the anisotropy coefficients of structural materials based on measuring the change in the elastic modulus. A method is proposed for studying the kinetics of scattered damage accumulation, taking into account the anisotropy of mechanical properties. The dependence of the limiting values of scattered damage on the anisotropy coefficients is shown. A complex of experimental studies on the laws of the kinetics of the accumulation of anisotropic damages in structural materials by various methods and their relationship with the indicators of anisotropy of plastic deformation has been carried out. The results of calculating the corresponding parameters of the approaches and models are presented. The tests were performed on flat samples made of aluminum alloy AMg2, 5025 and on alloy type 2024-T351 by laser cutting from sheet. The samples were cut at angles of 0, 45 and 90 degrees to the direction of rolling. To measure the values and variable damage D were conducted a series of experiments on uniaxial tensile with unloading of samples cut at angles of 0, 45 and 90 degrees to the direction of rolling. A set of diagrams of changes in the modulus of elasticity and damageability versus the level of deformation is obtained, depending on the rolling direction of the material. The maximum degradation of the elastic modulus has reached 41% compared to the initial value. In this case, the dependence of the change in the initial elastic modulus on the angle is not pronounced. Based on the experimental data, we can conclude that in specimens cut at an angle of 45 degrees, the transverse deformation is 19% greater than in specimens cut at an angle of 0 and 90 degrees. It is shown that the Lemaitre model gives overestimated values of the damage parameter in comparison with the Chow and Luo models, which are based on the energy approach and give very close numerical values. The energy approach is more acceptable for describing the process of damage accumulation in aluminum alloys in comparison with the one proposed by Lemaitre. The obtained conditional and effective deformation diagrams were compared with real distributions of effective and actual stresses obtained numerically at various levels of deformations. A significant effect of the anisotropy of plastic deformation on the kinetics of damage accumulation is shown. A complex of kinetic diagrams of damageability of anisotropic metal structural materials is presented. It is shown that the initial modules of elasticity of a material in different directions hardly differ from each other, which gives grounds to carry out calculations in the elastic region for materials as isotropic. However, already the conditional yield strength and ultimate fracture resistance differ significantly, which must be taken into account when calculating the strength of structures made of this material. The proposed energy approach is more suitable for describing the process of damage accumulation in aluminum alloys in comparison with the one proposed by Lemaitre. The damage curve obtained from the change in electrical resistivity was taken as true. As a result of experimental studies, it was found that the limiting degradation of the elastic modulus reaches 41% compared with the initial value for aluminum alloys. Samples cut at 45 degrees have 19% more transverse deformation than samples cut at 0 and 90 degrees. The relationship between the anisotropy coefficients of plastic deformation and the anisotropy of damage is established, as evidenced by the obtained generalized anisotropy diagrams.