Elevated concentrations of corrosive carbon dioxide or hydrogen sulfide in gas and gas condensate both produced and transported through pipelines lead to serious corrosion damage to the internal surfaces of steel infrastructure facilities. The paper presents the results of studying the corrosive effect of the medium flow along the lower component of the gas pipeline, which can exhibit a dynamic, intermittent, or static character. During testing, the effect of both dynamic conditions of the medium flow on the U-shaped cell and static conditions of the permanent impact of the aqueous phase on the pipeline wall during the bubble test was evaluated. Modeling of variable wetting conditions inside the gas pipeline showed that such conditions are typical and occur upon production and transportation of raw gas to the places of gas processing and purification. We have simulated dangerous operational factors that occur inside the gas pipeline: the composition of the aquatic environment, temperature, and the content of corrosive gases. When determining the resistance of steels to local forms of corrosion (pitting, wide and shallow corrosion pits), we revealed that the rate of developing local and general corrosion of steel in aggressive carbon dioxide and hydrogen sulfide conditions can reach 2–3 mm/year. In addition, it has been shown that the use of corrosion inhibitors for protecting the equipment and pipelines of gas facilities can effectively prevent the occurrence of internal corrosion processes. The results obtained can be used in assessing the corrosion activity of operating media and selecting the most proven corrosion inhibitors for pilot testing at gas fields.
The modulus of elasticity is considered a fairly stable physicomechanical characteristic of materials, which is little dependent on their composition and structure. Among the factors influencing the modulus of elasticity, temperature and anisotropy are distinguished. Information about the influence of scale factor on the modulus of elasticity is quite limited and sometimes contradictory. The aim of this study is to investigate the influence of the scale factor on the elastic modulus of steel 45 determined by tension geometrically similar specimens with different initial diameters. The specimens were tested on an Instron 8801 universal testing machine at a deformation rate of 0.1 mm/min at room temperature. Elastic deformations during tension were measured using two methods—with the help of a strain gauge and the digital image correlation method. Both methods showed fairly close results when testing specimens of the same diameter. However, the digital image correlation method allowed for measurements of elastic deformations on specimens with small diameters, where it was not possible to attach a strain gauge. A decrease in the modulus of elasticity with an increase in the initial diameter of the specimen was established. Graphical dependences of the modulus of elasticity on the diameter of the specimen and the area of its cross section were obtained. Possible reasons for the decrease in the modulus of elasticity under the influence of the scale factor are outlined. A decrease in specific surface area and specific surface energy, an increase in the deformable volume, and a decrease in the deformation rate at a constant deformation speed are among the main reasons. The decrease in the modulus of elasticity under the influence of the scale factor should be taken into account in strength calculations and when assessing the residual life of parts and structures with relatively large cross sections and wall thicknesses.
The mechanical characteristics of a metal are determined by a combination of three groups of factors: the chemical composition, the structural features, and the deformation ability of the structure, i.e., the ability of elements to relax internal stresses during deformation through dislocation sliding which does not lead to the crack formation and destruction. The possibility of using microindentation to assess the deformation ability of the structure of structural steels with a relatively high ductility is the goal of the study. The theoretical analysis revealed that an increase in the stiffness and a decrease in the plasticity of a metal leads to a change in the deformation model during indentation and, in particular, to the occurrence of deformation effects of various morphologies on the surface near the imprint, which can be indicative of the metal plasticity. Experimental studies performed on pipe steels of various strength and types of structure confirmed that, as the deformation ability of the metal decreases (primarily as a result of deformation hardening), a system of localized shears is formed near the imprint along the lines of action of maximum tangential stresses. A scale for ranking data of localized shears is proposed, and the optimal load value and shape of the indenter which provide gaining maximum information by microindentation are determined. A methodology for assessing the embrittlement of plastic construction steels on the basis of the results of microindentation has been developed, which can form a basis for creating an effective technology of nondestructive evaluation of the metal state.
The paper deals with the effect of Ti2NbAl intermetallic additives on the friction processes of hot-extruded B83 babbitt specimens. Optical and electron microscopy and energy-dispersive analysis were used. The structure, friction surface, and wear products were studied. Tribological tests were performed under dry sliding friction conditions using a universal testing machine according to an axial loading scheme of a steel sleeve to a disk made of the test material. The values of temperature near the friction zone were recorded during tests. The applications of the material depend on the wear regimes and mechanisms occurring in the tribocontact. Changes in wear regimes and mechanisms were assessed in terms of differences in the behavior of the friction coefficient and temperature, differences in the condition of friction surfaces and wear rates, and products of wear. The results suggest that hot pressing of powder containing the alloy B83 and discrete particles of the high-strength intermetallic phase Ti2NbAl is a promising method for producing composite materials with better tribological properties than the babbitt alloy. The introduction of reinforcing high-modulus particles of intermetallic compounds changed the structure of the material and affected the friction processes in the babbitt alloy, delaying the moment when wear regimes shifted into the zone of more severe friction conditions. A substantial reduction in the wear rate of the produced composite materials compared to the initial alloy makes it possible to predict the increase in the service life of tribounits. These data can help determine and recommend the regimes for increasing the service life of tribounits based on B83 alloy as volumetric liners and plain bearings (or sliding bearings), as well as produce new functionally structured layer compositions having enhanced tribological properties, which are based on structural steels and surface coatings using not only B83 babbitt alloy but also its composite materials.
Estimates of the reference temperature T0 obtained for the base metal and the weld-seam metal of the Cr–Ni–Mo–V type (shell 200 mm thick) on the basis of statistical modeling by the Monte Carlo method are presented. T0 was determined according to the ASTM E1921 standard taking into account the inhomogeneity of the material. The sample size of the fracture toughness values KJC for T0 modeling was 12, 24, and 70. The Monte Carlo method was used for analysis of the correctness of metal identification (homogeneous/inhomogeneous). It is shown that sampling of 12 samples does not provide a reliable determination of whether the metal is homogeneous or inhomogeneous (incorrect results were obtained in 50% of cases for the base metal and in 37% of cases for the weld-seam metal). When the sample size increased to 24 samples, incorrect results were obtained in 5% of cases. The T0 values with allowance for the material inhomogeneity were determined in two ways: using a screening procedure and proceeding from the actual bimodal representation of the fracture toughness distribution (parameters of the bimodal distribution were determined by the maximum likelihood method). It is shown that both methods give close results for the base and weld-seam metal, the magnitude of the shift toward positive values in the average T0 values determined with allowance for the inhomogeneity being about 22°C. Using the obtained T0 estimates, the lower envelopes of the temperature curves of the fracture toughness are constructed (master curves for 5% failure probability).
Many properties of polycrystalline materials depend on their crystallographic texture, which can be fully described by the orientation distribution function (ODF). The main task of quantitative texture analysis is to reconstruct the ODF from its two-dimensional projections, which are obtained through X-ray or neutron diffraction methods. In this work, the results of ODF reconstruction for materials with low lattice and sample symmetry using the harmonic method are presented. The method is based on expanding the ODF in a Fourier series using three-dimensional symmetric spherical functions. Real functions which are linear combinations of corresponding complex spherical functions were used. A model single-component texture and the texture of a magnesium alloy sample subjected to equal-channel angular pressing were investigated. Both textures exhibit hexagonal lattice symmetry and triclinic sample symmetry. In both cases, the RP-factor values and the error of ODF calculation, used to check the adequacy of the solution, showed good agreement between the calculated and original data. It was also found that the ODF of the magnesium alloy sample contains two texture components ((bar {1})2(bar {1})6)[(bar {1})211] and ((bar {1})2(bar {1})6)[(bar {1})2(bar {1})1] with maximum intensities of 13.81 and 2.23, respectively. The obtained results can be used in texture studies of ceramics, rocks, and other nonmetallic materials with low symmetry.
A phenomenological approach to the actual problem of determining the inhomogeneous residual stress-strain state in the components of high-tech engineering systems at the stages of their design and operation is presented. The approach is based on physical and mechanical methods of measuring displacements. Current physical models describe the physical regularities of the residual states attributed to changes in the structure by the interaction of defects and dislocations in the field of micro- and meso-stresses. At the same time, there are the problems of the transition to the macrolevel, the construction of multilevel models, and the conversion of these models in engineering practice. In the framework of phenomenological approaches, in the general case, the solution of this problem requires the solution of three-dimensional inverse problems of thermoelasticity. A well-known mechanical method for determining a uniform field of residual elastic stresses recommended by ASTM E837 is described. The method proposed earlier by one of the authors for determining an inhomogeneous (in the plane) field of residual elastic stresses is discussed. A method of the three-dimensional inhomogeneous residual elastic stress-strain state determination based on the experimental determination of the displacement vector components by the method of step-by-step point hole-drilling and data of digital speckle interferometry and digital image correlation is developed. The constitutive relations for the components of the displacement vector are written in the form of Volterra integral operators. The basic operator functions are the functions of four variables, i.e., the coordinates of the cylindrical system (r, θ, z) associated with the hole and the hole depth h. A method for verification of the basic functions is presented. The problem is reduced to the determination of three displacement functions of three variables: hole radius r, h, and z. Numerical simulation of basic functions is carried out. The obtained results are consistent with the known experimental data and calculated values of the deformation on the surface depending on the depth of the hole according to the ASTM E837 standard.
The shear and interlayer characteristics of polymer fiber composites, in contrast to metals, play a decisive role in the deformation and fracture processes. In view of this, special methods have been developed to determine the interlayer flexional strength of a short beam and the interlayer shear modulus by the deflection correction. At the same time, the accepted hypotheses about the distribution of shear stresses, for example, those based on the Zhuravsky formula, are too simple and do not provide the determination of the correction and calculation of the shear modulus with a high accuracy. The use of the Saint-Venant–Lekhnitzky solution for an orthotropic beam instead of the simplest parabolic distribution potentially makes it possible to take into account all shear stresses occurring in a beam and their distribution over the beam height and width, which should increase the accuracy of determining the deflection correction and interlayer shear modulus, respectively. Since the strict solution is presented in a series of hyperbolic functions, its practical use is rather difficult. In this study, an exact approximation of the strict solution by simpler quadratic dependences is proposed, which makes it possible to determine the deflection correction and shear modulus with a high accuracy. It is shown using the proposed approximation that, for real beam-type composite specimens, the use of the refined shear stress distribution with allowance for the nonuniformity of stresses over the beam width yields a deflection correction negligibly small as compared with the case of the simplified parabolic distribution according to the Zhuravsky formula. The numerical verification using the finite element method has been carried out. Special three-point bending tests of fiberglass specimens of different widths have also showed no deflection growth with increasing beam width, which points out an insignificant impact of the heterogeneity of shear stresses on the deflection.
The results of fatigue tests of two geometrically identical and similar in design models of the lower wing panel of a commercial aircraft were analyzed. The panels differed in the way of installing mounting bolts, which connect the skin and stringers. Cold expansion of holes drilled both in the skin and stringer was performed for the first panel before joining. The second panel included no additional treatment after drilling pilot holes and final reaming. The bolts were mounted with an interference fit varying from 1.3 to 2.1% and from 2.9 to 3.2% for the first and the second panel, respectively. Changes in the interference fit were the consequence of a scatter attributed to the presence of a tolerance zone for the diameters of both bolts and mounting holes. A two-step comparison of both technologies was based on the experimental study of residual stress fields. The first stage, being a subject of the present study, included the analysis of residual stress fields arising after removal of the bolts and separation of the skin from stringers. Hole drilling and gradual crack growth were used to determine the components of residual stresses. The deformation response was measured by electronic speckle pattern interferometry. High quality interferograms, which provided a reliable resolution of the interference fringes of ultimate density over the hole edge or directly along the notch borders, were obtained for both ways of local removal of the material. The first (pointwise) method, based on drilling a probe hole, provided a quantitative determination of the residual stress components, starting from 1.4 mm distance from the assemblage hole edge. The second technique implements the crack compliance method of subsequent lengthening of the notch, starting directly from the mounting hole edge. This approach provided for a quantitative analysis of residual stress fields, related to different bolt mounting technologies, proceeding from the comparison of SIF values. A high level of compressive residual stresses near open holes was characteristic for both types of panels. Both experimental approaches showed the benefits of joints, where bolts are mounted into cold-expanded (reinforced) holes. For this case, the estimation of the relaxation parameters of the principal component of residual stresses in the direction of the external load is presented.
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