Vol 55, No 15 (2019)

Advantages of direct laser deposition of metal providing the directional crystallization of 316L stainless steel specimens are shown. It is found that direct laser deposition of metal offers exciting possibilities for obtaining monocrystalline structures by directional crystallization of the deposited metal. The metal structure of the obtained specimens is represented by dendritic crystals oriented toward the thermal center. The cross-section view of the structure, formed by the crystal axes of the first order, is of the form of honeycombs. The thickness of the applied layer (200 µm) in the central part of the specimen does not prevent the development of the dendrite structure and it contributes to the formation of the monocrystalline structures (having no pores and cracks). It was stated that the growth orientation of dendritic crystals is driven by the thermal center determined by the position of the laser spot. A technique of manufacturing stainless steel products by direct laser deposition using the developed algorithm which allowed connecting the movement of the robotic center and specimen, laser switching, and feeding of the gas-powder mixture was presented. The possibilities of estimating the inclination angle of dendritic crystals in the metal relative to the symmetry axis of the specimen using a metallographic software package for analysis of digital images of the microstructures, as well as the quality control of the items obtained by the X-ray tomographic quality control, determination of the parameters of the laser spot on the substrate surface using high-speed photography, and estimation of the phase composition of the deposited metal using X-ray diffraction analysis, are shown. In such a case, the roughness of the specimens determined by the size of the powder particles may be removed by mechanical treatment.

This paper presents results from studies on the change in the texture and Poisson coefficients of 15YuTA construction steel upon fatigue failure in the range of high-cycle fatigue by the ultrasonic pulse-echo technique. The coefficients of the orientation distribution functions (\(W_{4}^{{00}}\) and \(W_{4}^{{20}}\)) used for construction of the pole figures were obtained using bulk elastic waves and the precise measurement of its propagation time. As a parameter characterizing the texture sharpness, the intensity ratio in the center of the pole figure and at a point remote from the center by 45° along the rolling direction was used. It was shown that change in the texture sharpness is attributed to the development of microplastic strain and accumulation of microdefects. A nonmonotonic dependence of the change in the sharpness of the texture on the number of loading cycles is observed. At the initial (first) stage of loading, there is some increase in the sharpness, which can be attributed to the development of microstrains on the most favorably oriented sliding planes. At the second stage, the decrease in the sharpness is associated with the scattering of the texture upon the increase in the density of microdefects during the destruction of the alloy. The parameter characterizing the sharpness of the texture can be used as a pre-destruction indicator by the ultrasonic monitoring of a construction material.

The state, problems, and prospects of conventional, unified and special laboratory, bench and full-scale tests to substantiate the systematic assessment of strength, durability, survivability, cold resistance, reliability, and safety of the key elements of pipeline transport of crude oil and refined products are considered. These tests correlate with the stages of the pipeline life cycles and basic and reference computational-experimental methods of determining the criterial characteristics of pipe steels, pipes and main pipelines with regard to domestic and foreign practices. The experimental basic mechanical characteristics obtained in standard static tensile tests (yield and strength limits, elastic moduli) are part of the basic static strength calculations of newly designed and operating pipelines. Standard hardness and impact elasticity tests are used to control the pipe steel quality. The results of standard tensile tests provide additional design information for the assessment of static strength with respect to the ductility and degree of steel hardening during elastoplastic deformation. Unified laboratory tests are meant for refining the pipeline strength with allowance for the stress state 3D effects, absolute sizes of the pipeline cross section, strain rate, anisotropy, cold resistance, corrosion, and the presence of welded joints. The assessment of pipe steel crack resistance and pipes according to the criteria of linear and nonlinear fracture mechanics with allowance for technological and operational defects holds a specific place in laboratory and bench testing. The experience in conducting these tests, accumulated in the Russian oil pipeline system is considered.

The reasons for fracture of fasteners made of 30KhGSA steel were analyzed. It was found that, in the case of fracture of bolts in the package connecting the frame elements, the damage of the bolts is the result of the fretting corrosion due to the mutual friction between bolts and elements of skin, belt, and tape. Fretting corrosion develops in the locations of the cadmium coating damage of bolts in the process of friction. The break-in scuffing of the openings and corrosive damage (fretting corrosion) triggering intergranular fracture, which is the most severe in the skin and tape, contribute to the fractures in the belt, skin, and tape. The reason for fracture is structural and technological factors: high stresses in the investigated package, micro-displacement of the mating parts, violation of the cadmium coating, insufficient protection against corrosion and environmental impact. Fracture of the self-locking nuts made of 30KhGSA steel with the anticorrosion cadmium coating used during assembly of an auxiliary gas turbine engine is induced by their severe overheating—the impact of the melted coating on the surface of the loaded nuts and penetration of the liquid metal into the base material along the grain boundaries. Application of the cadmium coating for the operation of the steel parts at a temperature exceeding (even for a short time) the melting temperature of Cd (T_melt = 320.9°C) under the condition of contact with the stressed metal owing to the Rebinder effect leads to a dramatic loss of strength and premature failure of the part. Fracture of the galvanized bolts made of 30KhGSA steel was studied in the process of the repeated static testing. It was found that several factors contributed to the onset of the multiple fractures: rough scratch marks, sandblasting performed before galvanizing with the use of large sand fractions, dents which contribute to the premature origin of cracks, areas with deposits formed during coating with loose adhesion of the layer. The presence of a large number of pits along with the fatigue grooves indicates the operation of the material under high load. The investigation was carried out by the method of optical and scanning electron microscopy applying the X-ray microanalysis.

The results of analysis results of residual life of crane girders with operational defects based on numerical studies of the stress-strain state, the equations of limiting states, and the kinetic dependences of fatigue crack propagation formulated under criteria of strain and fracture mechanics are presented. A high level of wear of the fixed capital assets of engineering structures and metal constructions, as well as operation of the facilities in beyond-design-basis terms, contributes to origination of emergencies caused by damage accumulation. Crane girders operated under emergency conditions caused by the presence of fatigue cracks of different length in the most dangerous zones of the girder wall with an eccentric application of crane load are studied. According to the results of a numerical experiment, the lines of stress intensity influence at the crack tips appearing at the upper zone of the wall when different values of loading eccentricity caused by the rail displacement from the vertical axis of girder cross section were revealed. The dependence of the fatigue crack length and of the stress intensity at the crack tip on the number of load cycles of the girder is determined. To evaluating the bearing capacity of crane girders operated in emergency conditions, it is proposed to use the effective values of the stress intensity factors calculated for specific loading conditions, taking into account the structural features of the girders, the sizes of emerging fatigue cracks, and their locations in the crane girders. According to the results of analysis, the diagrams making it possible to predict the remaining residual life of crane girders with a crack are constructed. On the basis of traditional design procedures of structures for cyclic crack resistance, the calculation algorithm for individual life of crane girders which includes the obligatory analysis of the stress-strain state of the crane girder with a crack at the upper zone of the wall is formulated and implemented.

The change in acoustic emission parameters is observed upon a single static loading of a flat metal specimen coated with a brittle strain-sensitive coating. The acoustic emission signal activity (energy, number, and frequency) was recorded as the tensile loading time increased. The dislocation mechanism of deformation in the area of small elastic and elastoplastic deformations, being commensurate with yield stress deformations at further loading to the ultimate strength, is shown to be induced by the processes that occur at the nano- and micro-levels with the appropriate acoustic emission signal frequencies. On the other hand, the evolution of local deformations with the emergence of defects at the micro- and mesoscales should be accompanied by different signal structures with higher amplitudes and lower frequencies. This is likely due to the formation of mesocracks at the grain boundaries and dislocation cluster rich zones. In this case, the number of high energy signals is expected to decrease at simultaneously decreasing acoustic frequencies. When tensile tests are performed on relatively coarse grained steels of high strength and low ductility, the sound effects can be heard even by the human ear. In addition to the traditional analysis of these parameters, the acoustic emission spectra have been plotted for specified loading intervals. Using new integral parameters of those spectra, the general regularities of spectral changes from the region of higher to lower frequencies are shown with increasing number of signals. This fact indicates the oncoming of hazardous damaged states caused by stress propagation through a coated aluminum sample. The generalized parameters of spectral changes are given as well. This allows the indicated elastic and elastoplastic deformation features to be characterized by means of spectral tools and new integral diagnostic parameters.

The operating modes of loading elements of machines and structures exhibit, as a rule, more complicated character of their loading cycles compared to sinusoidal used in the practice of calculations and experiments. It is noted that, in a number of cases, the actual conditions of load changing can be idealized by dual-frequency loading modes with superposition of the high-frequency component of the main workload attributed to the effects of vibrations, aero- and hydrodynamic impacts, regulation of the working process, etc. Testing of three steel samples with different cyclic properties has shown that such two-frequency regimes lead to a decrease in the durability in comparison with single-frequency loading equal in the amplitude of maximum stresses. This reduction depends on the parameters of the basic low-frequency and imposed high-frequency loads. Evaluation of this reduction can be performed both using the laws of summation of the damage expressed in the strain terms and using an analytical expression considered below, which includes the calculated or experimentally determined durability for single-frequency loading with the maximum (total) amplitude of the effective stress and the durability coefficient characteristic of each type of material and determined by the ratio of amplitudes and hours of low and high stresses. A computational-experimental analysis of the effect of the amplitude of low-frequency and superimposed high-frequency loading under two-frequency modes of stress change on the cyclic durability has shown that the imposition of the high-frequency component of cyclic deformation on the main low-cycle loading process leads to a significant decrease in the cyclic durability; the level of the decrease correlates with the level of ratios of amplitudes and frequencies of the summarized harmonic processes of load application.