Papers by Keyword: Thin Structure

Abstract: Increasing the operational reliability and durability of parts and mechanisms used to operate under conditions of intense wear, loads, high pressure and temperatures requires the protection of working surfaces with functional coatings. The E.O. Paton Institute of Electric Welding of the National Academy of Sciences of Ukraine has developed a technology and equipment for multichamber detonation spraying (MCDS) of these coatings. This paper summarizes the data of experimental studies of the structure of composite coatings of various systems (Ni–Cr–Fe–B–Si, Cr₃C₂–NiCr, WC–Co–Cr, ZrSiO₄, and Al₂O₃) for different materials. Research has established the influence of technological modes of spraying on structural and phase changes in the coating materials obtained by detonation spraying. Under different processing modes, the materials change volume fraction of phase components, microhardness, (sub)grain structure parameters, size of dispersed phases, and nature and distribution of dislocation density. The peculiarity of the structure of coatings obtained by the MCDS method is the formation of a dispersed structure, the presence of a nanoscale substructure and nanoparticles of hardening phases with a size of 10–100 nm. The formation of a nanostructural state contributes to an increase in the strength, fracture toughness, and crack resistance of coatings obtained by the MCDS method.

Abstract: Electron microscopy and x-ray analysis and mechanical testing have been investigated the influence of severe plastic deformation on structure and mechanical properties of aluminum alloys. It is established that in the initial state in the alloy AMC has a high density of chaotically distributed dislocations with a density of 5-10*109 сm-2. It is shown that microdiffraction paintings in alloy AMC in the bulk of grains are observed uniformly distributed particles of the second phase. It is established that in the initial state in the alloy AMG6 there is a high density of chaotically distributed dislocations with a density of 2-6 *1010 сm-2. Determined that after ECAP the dislocation structure of alloys AMG6, AMC and changes: formed dislocation networks inside the fragments of the dislocation is practically not observed. Determined that after ECAP-12 increase the tensile strength and yield strength of alloys AMG6 and AMC.

Abstract: The influence of specimen thickness on fatigue crack behaviour has been investigated. To this aim the fatigue crack propagation rate has been measured on two different types of test specimens with varying thickness. The change of stress singularity exponent for the crack front due to vicinity of the free surface is considered. To explain the effect of stress singularity changes on obtained experimental results a methodology based on generalized stress intensity factor and strain energy density concept has been used. It is shown that for materials with Poisson’s ratio of about 0.3 the free surface effect does not play a decisive role for specimens with a low level of in-plane constraint but can influence fatigue crack propagation rate in the case of geometries with a high level of the constraint.

Abstract: In many industrial applications is necessary to predict fatigue lifetime of thin structures, where the stress field near the crack front have a real three-dimensional nature. Due to the existence of vertex singularity in the point where the crack front touching free surface, crack propagation in 3D structures cannot be reduced to a series of plane strain or plane stress problems along the crack front edge. The paper describes the influence of vertex singularity on the distribution of the stresses around the crack front for three-dimensional body. The distribution of the stress singularity through the thickness of the specimen gives us indication of the crack behavior in thin structures. The estimation of the thickness of the specimen where the change of singularity plays an important role on fatigue crack growth rate (in dependence on Poisson’s ratio) is carried out. The results contribute to a better understanding of the crack behavior in thin structures, and can help to more reliable estimates of their residual fatigue life.