The features of studying the stress distribution in an axisymmetric maximum-interference joint, in which the inner part of the joint transforms into a plastic state, are described. The distributions of the circumferential and radial stress tensor components over the insert and bushing of an interference sample are analytically estimated. The calibration coefficients are adjusted with allowance for the stress gradient over the surface in order to calculate the stresses by drilling holes to detect strains by a strain-gage rosette. This made it possible to obtain a more uniform depth distribution of stresses: the coefficient of variation of circumferential stresses decreased from almost 10 to 3

В статье представлены результаты работ по исследованию поверхностного упрочнения плоских образцов из алюминиевого сплава и коррозионностойкой стали в режиме циклического сжатия без накопления односторонней пластической деформации. Описаны процессы изготовления образцов, механические свойства исходных полуфабрикатов, методики их упрочнения, показаны результаты усталостных испытаний. Рассмотрены условия пластического внедрения пуансонов, экспериментально показано, что упрочнение проводилось в области макроскопических упругих деформаций. Проанализированы возможные механизмы упрочнения; показано, что основным действующим фактором процесса упрочнения является усталостный наклеп.

A sample configuration with a specified distribution of residual stresses in depth was developed to elaborate modes for studying residual stresses using mechanical and physical methods. The sample formation technique is based on non-uniform plastic deformation of an aluminum beam of rectangular cross section according to the pure bending scheme. The control of the deformed state during loading was performed on the end surface of the sample by the field of normal deformations obtained using a digital image correlation system. The depth of the plastically deformed layer was 1.3 mm. The theoretical distribution of residual stresses obtained as a result of unloading of a plastically deformed sample was determined from the results of a numerical calculation of a finite element model, with allowance for the physical and mechanical characteristics, elastic-plastic hardening, as well as the deformation curve in true coordinates obtained as a result of uniaxial tension on elementary samples. A study of residual stresses inhomogeneous in depth was carried out by drilling holes in accordance with ASTM E837 on two opposite sides of the sample: the region of tension under loading and the area of compression, respectively. The control of the deformation response resulted from drilling was recorded using three-axis strain gauge rosettes. The results of the actual measurement of the longitudinal component of residual stresses were compared with their theoretical distribution obtained by a numerical method. The root-mean-square error in measuring residual stresses, relative to their theoretical distribution, for an aluminum alloy sample reaches 18.7 MPa. It should be noted that largest measurement errors were recorded at small depths, since they are characterized by small values of deformations comparable to the value of shot noise.

The paper presents the results of a study of the mutual influence of concentrators of the "dent" type, made by a hemispherical indenter with a diameter of 25 mm to a depth equal to the thickness of the sheet, and a "hole" with a diameter of 4 mm in thin 1.5 and 3.0 mm thick aluminum alloy 1163AT on static strength and fatigue. The presence of a dent did not lead to a decrease in static strength, while the combination of "dent + hole" concentrators reduces the strength to ~ 8% regardless of the location of the hole in the dent. The position of the hole relative to the center of the dent practically does not affect the slope of the fatigue curve, while the displacement of the hole from the center to the edge of the dent leads to a decrease in durability.

A method is proposed for superimposing the two-dimensional data fields derived by the finite-element method and by digital image correlation. This entails interpolation of the initial data on an irregular grid by selecting the points on a regular grid closest to the initial points using a k -dimensional tree algorithm (the KD-Tree algorithm in Python). A method is outlined for eliminating the influence of outliers and edge effects on the visualization of the estimates, using a weighting function.

The mutual influence of stress concentrators in the form of a dent (made by a hemispherical indenter 25 mm in diameter to a depth of the sheet thickness) and a hole 4 mm in diameter in a thin 1.5- or 3.0-mm-thick skin on the static strength and fatigue life of structurally similar samples made of a 1163AT aluminum alloy is studied. The presence of a dent does not decrease the static strength, while the strength decreases by 8

A high-strength aluminum alloy 1933 being distinguished by good physicomechanical properties and high manufacturability is widely used in the most critical power aircraft structures, e.g., in a modern AN-148 SSJ aircraft. The alloy is used in production of various parts of articulated joints, thus making study of the durability of the alloy in a complex stress state a relevant goal. We present the results of static and dynamic tests of structurally similar samples (of two types) manufactured according to serial technology and corresponding to the shape of real eyelets of the airframe slats. Preliminary fatigue tests of standard samples (a strip with a hole) were performed to obtain the refined characteristics of the alloy in the T3 state. To analyze the mechanical behavior of the alloy with a different amplitude-frequency character of loading, the asymmetry of the loading cycle ( R = 0.1; 0.2; 0.5; 0.6; 0.76; 0.82) and exposure frequencies (10, 60, and 100 Hz) were varied. In is shown that an increase in the average stress of the loading cycle reduced the number of cycles before the destruction of the eyelets: a 2-fold increase in the average stress resulted in a drop in fatigue life by two orders of magnitude (for an amplitude of 5 kg/mm 2 ).

This article describes the features of determining strain curves in true stress–true strain coordinates, using samples of circular cross section from Al–Cu–Mg–Zn aluminum alloy. The calculation and experimental methods of determining true stresses and strains were compared Calculation methods based on the condition of volume constancy may not reflect actual regularities of deformation at the stage of strain localization in the considered material. Nevertheless, the use of systems of digital image correlation (DIC) allows measurements of both the geometrical sizes of deformed sample and strain fields on its surface to be performed, including on the sample neck. It was demonstrated that the measurement error of the sample diameter by the coordinate field was 0.02 mm at the instance of destruction. In order to improve the measurement precision, an increase in the recording frequency in proportion to increase in strain rate was proposed, as well as measuring the surface coordinates from both sides of the sample. It is also possible to supplement the strain curves obtained by DIC optical systems with the measurements of true fracture stress, and the true fracture strain determined by calculations on the destructed sample. The presented methods of analysis of plastic flow by direct measurement of field displacements and strains allow actual regularities between true stresses and strains at the interval of irregular plastic strain to be established. This cannot be achieved by analytical conversion of conventional curve. The obtained hardening coefficients and strain curves can be used for simulation and design of machinery structures and parts.

In this paper, we present the calculation results of the stability of monolithic and layered plates that are obtained by analytical and numerical methods and are compared with experimental data. The results are within the limits of the permissible error. The results of numerical calculations of stability by the finite element method in the ANSYS program were higher than the values determined by the Euler formula and lower than the results obtained by the formula for calculating the stability of plates. To study the bearing capacity of layered samples under compression, their stability was estimated at various numbers and layouts of layers of alloy and composite material. The optimal layouts of layers in the material for designing a composite panel were determined. The results of stability studies of layered plates were used to design a composite wing panel based on sheets and profiles of high-strength aluminum-lithium alloy and layered aluminum-fiberglass. The compressive stability of the composite panel is 7% higher than the experimental values owing to local buckling of its elements, which precedes the general buckling and reduces the critical load.

Abstract—A computational–experimental study of the delamination of carbon fiber reinforced plastic based on a medium-strength SYT49S carbon bundle and a VSE-1212 epoxy matrix is performed. The cohesive zone of a beam with overhanging ends under mode I fracture conditions is simulated. The features of determining the model parameters (interface strength, interface stiffness, cohesive zone length) for estimating the characteristic model element size are discussed.

All-Russian Scientific Research Institute of Aviation Materials, 17, ul. Radio, Moscow, 105005 Russia; e-mail: 89639619741@mail.ruPlastic deformation is a type of material damage which can disrupt the normal operation of the structure. In this regard, the method for assessing the degree of damage to a metal sample has been developed. A corset sample for tensile tests was made from an aluminum alloy of the Al – Zn – Mg – Cu system. The ANSYS finite element complex was used to simulate an inhomogeneous stress state occurred in the sample working zone due to the variable cross-section upon tension. First, the hardness of the corset sample was measured, then it was tested for tension until the onset of necking and the load drop on the deformation diagram. After unloading along the sample working area, the Brinell hardness and surface roughness were measured. The results of the hardness measurements showed that in the areas where the tensile stresses are below the conditional yield stress of the material, the hardness value corresponds to the hardness of the starting material. In the areas where the tensile stresses are higher than the conventional yield stress, the hardness increases and reaches the maximum value in the center of the specimen, i.e., in the zone of the minimum cross-section. Thus, the damageability of the material can be assessed through the change in the hardness and roughness of the surface along the sample length. Proceeding from the results of instrumental spherical indentation, a technique for assessing the mechanical characteristics of the material was developed. The method consists in testing one sample for hardness and tensile strength with subsequent construction of the correlation dependences of tensile and indentation loads using the experimental results to obtain calculated tensile diagrams from the indentation diagrams of the material under study.

The fact that standard specimens made of high-strength bearing steel undergo spall fracture during quasi-static tension is experimentally proved. The uniqueness of this fracture consists in the fact that it proceeds due to the elastic energy of the specimen, in contrast to the classical spallation during high-speed collision or a high-energy pulsed external action. High-speed video filming is used to demonstrate that the mode I fracture of a specimen and its subsequent spall fracture are spaced in time.

The main static design characteristics of sheet semi-finished aircraft materials are obtained from the results of tests for uniaxial tension, compression and bearing. At the same time, the Ramberg - Osgood equation is widely used for the analytical description of strain diagrams under static tension in the region of small plastic deformations. The inverse anisotropy resulted from the crystallographic texture after rolling is characteristic of the sheets of aluminum-lithium alloys. We have studied the mechanical characteristics of 1441RT1 alloy sheets under static loading (tension, compression, bearing) in three rolling directions (longitudinal, long transverse, at 45° angle to the longitudinal direction), with an estimate of the Taylor factor characterizing the texture of the material. Tension tests were carried out at a temperature of-70, +20, + 85, and +125°C, compression and beading tests were carried out at a temperature of 20°C. Test temperatures were chosen proceeding from operating conditions of panel skin materials. The Ramberg - Osgood coefficient was determined from the tensile and compressive strain diagrams. To determine the Taylor factor, the X-ray texture analysis was performed with the construction of inverse pole figures. It is shown that for 1mm- and 3-mm sheets of aluminum-lithium alloy 1441RT1 the test temperature corresponding to the operation temperature has a weak effect on the tensile strength characteristics in the region of small plastic deformations. At the same time, the Ramberg - Osgood coefficient is a characteristic most sensitive to the exposure temperature, rolling direction and sheet thickness. The functional dependence of the Ramberg - Osgood coefficient on the rolling direction and the exposure temperature has been determined for 1-mm and 3-mm sheets of 1441RTlalloy.

The results of the calculations and the obtained experimental data show the need to take into account the distribution of strain fields on samples with stress concentrators for the selection of the correct gage of optical strain gauges. The ability to install certain gage of optical sensors with specific geometric parameters of the sample allows one to obtain the values of the stiffness and deformation characteristics on the sample with holes close to the values of the material characteristics, which can be used, for example, when carrying out research work in conditions of limited materials.

The mechanical properties of square section (up to 350 mm) forgings made of domestic steel VKS-9M (obtained by the duplex process of vacuum induction melting plus vacuum arc remelting) are studied. The mechanical properties of steel VKS-9M and American high-strength structural steel 300M are compared in terms of the calculated characteristics of materials. We extend the concept of «design characteristics of materials» to all the mechanical properties of materials used in determination of the strength and durability of structures. The design characteristics of materials include the strength characteristics under quasi-static tension, compression, crushing and shearing, as well as the strength under variable loads and the crack resistance of the material. The correctness of comparing the calculated values of the static strength of steels VKS-9M and 300M is based on the identity of test methods and the equivalence of processing and presentation of test results. The determination of the strength characteristics was carried out according to domestic standards, which are basically harmonized with the American standards. A difficult situation arises when determining the strength indicators of a newly developed material. Nowel material, as a rule, is developed in laboratory conditions, produced on experimental equipment in small volumes, and, therefore, the statistical assessment of the general population at the development stage is unreliable. In conditions of the full-scale manufacture of industrial semi-finished products, a statistical assessment of their strength characteristics becomes necessary. In the aviation industry, a statistical assessment of the calculated values of the strength characteristics of the material is prescribed by the law. The airworthiness standards set the levels of the calculated values of strength characteristics with a certain probability of non-destruction. For parts, the destruction of which can lead to an accident, the calculated values of the strength characteristics of materials should be selected in such a way that the strength of the material is to be ensured with a probability of 99% — with a 95% confidence interval (basis «A»). Statistical evaluation of the static strength characteristics of two heats of VKS-9M steel on the basis of «A» revealed their insignificant excess in comparison with steel 300M. The data obtained allow us to believe that the calculated values of the static strength characteristics of VKS-9M steel will not be lower than the calculated values for 300M steel. Large-scale fatigue tests of VKS-9M steel were carried out, which proved that the fatigue characteristics of VKS-9M and 300M steels practically coincide, as well as the arrays of their determinations. The equivalence of the strength characteristics of the VKS-9M and 300M steels allows replacement of the American 300M steel by the domestic VKS-9M steel.

A computational-experimental study of the delamination of fiber-reinforced composite materials based on the medium-strength carbon cord SYT49S and the epoxy matrix VSE-1212 has been carried out. Simulating was performed for the cohesion zone of a double-cantilever beam under separation conditions. The features of determining the parameters of the model (interface strength, interface stiffness, and length of the cohesion zone) to estimate the characteristic size of the model element are discussed.

The paper considers the main methods for determining the hardness of materials. The basic formulas for calculating the mechanical characteristics of materials by hardness values are presented. The analysis of methods for calculating tensile diagrams of materials by indentation diagrams is carried out. Approaches to building a finite element model of material indentation are considered. The paper shows the need to improve existing standards and develop computational methods with a detailed description of techniques for recalculating indentation diagrams into mechanical characteristics of materials.

The paper presents an overview of methods for determining residual stresses. Methods such as splitting and segmentation, layer-by-layer removal, slitting (cutting, pliability), profiling, drilling holes (including a «deep» hole) are considered. The description of the methods for mea-suring the deformation used in the determination of residual stresses is given. The most common contact method using strain gauges, as well as non-contact methods: polarization-optical (photo-elasticity), optical speckle interferometry, digital image correlation.

Linear homogenization is applied to cellular structures produced by selective laser melting (SLM). Samples of different materials with cells of different configurations are studied theoretically and experimentally. Their strain curves are compared with the results for bulk and beam models and with experimental data. After homogenization, the rigidity of the homogenized model differs by 5–10% from the beam model and by 8–16% from the bulk model.