The thesis for a candidate of technical science degree in the specialty 05.02.09 – dynamics and strength of machines.– National Technical University of Ukraine “Igor Sikorsky Kyiv Polytechnic Institute”, Ministry of Education and Science of Ukraine, Kyiv,2020.
The dissertation work is devoted to the estimation of durability at low - cycle fatigue of superficially strengthened constructive elements.
After analysing existing works of various authors devoted to the topic on the assessment of durability in low-cycle fatigue for structural elements with stress concentrators in the form of structural holes. It was found that in existing approaches to assess durability, the damage is not taken into account. Which leads to less accurate results. It is known that during the operation of structural elements there are elastic-plastic deformations. In which there is a phenomenon of degradation of mechanical characteristics due to the occurrence and accumulation of damage.
The concept and essence of damage were laid down, even L.M. Kachanov and Y.M. Rabotnov as microstructure changes in structural materials caused by inverted processes from the action of loads of different nature. The use of this parameter, the damage, in the calculations can be used as a vector, scalar or tensor. It allows the introduction of this parameter in a large number of existing models.
Moreover, in the study of the analyzed works, it was shown that not all calculations for low-cycle fatigue for structural elements with stress concentrator take into account hardening due to surface plastic deformation. That occurs due to technological processes such as stamping holes of segmental configuration, rolling and mandrel. After analyzing the presented technological processes, it is determined that the dormant process is widespread due to its simplicity, economy and reliability.
The dependence of the change in microhardness along the radius on the center of the hole on the surface of the material of aluminum alloy D16chT for perforated samples at 1%, 2%, 3% level of inverted plastic deformation was obtained. From the obtained results, three different areas of the hardness level of the material are shown. The zone of a sharp change in the value of microhardness, the zone of moderate change of microhardness and the zone of stabilization of microhardness caused by the mandrel process.
Numerical simulation of the diving process was performed using the numerical package Abaqus Student Edition. It is determined that the level of maximum damage is in the area of the mandrel out of the hole. Damage in the middle of the hole is 2-5 times less (depending on the level of pre-deformation) relative to the maximum value.
To assess the impact of operating loads on the strength of power structural elements with a hole after dunking. To the preliminary results of the calculation of the dredging process, we apply an additional stage of loading, corresponding to the operating load. As a result of the calculations, the residual stress fields were obtained, they are presented in equivalent stresses according to 4 strength theory (Mises stress). It is estimated that as a result of mandreling in the area of the functional hole there are significant compressive stresses at all levels of plastic hardening.
The regularities of the influence of residual stresses on the strength of power structural elements with a hole after mandrel are established. The regularities of the influence of the level of residual stresses after mandreling on the number of cycles before failure were experimentally established. It is shown that for samples with a hole after mandrel at 3% residual plastic deformation for stresses up to and at 2% residual plastic deformation for stresses up (for a given material), the stress concentrator does not affect durability.
According to the developed model, a generalizing surface of low-cycle fatigue of structural elements (in the form of samples with a hole) made of aluminium alloy D16chT with pulsating tension (T = 293K) is constructed.
