It is noted that the reaction of bearing elements of structures and engineering components to service loadings, impacts of physical fields, and corrosive environments is the initiation of not only fields of stresses and strains but also fields of damages. At the same time, depending on states of loading and environment, various mechanisms of accumulation of damages and fracture are realized, whose indicators are essential changes in spectra of a response of technical systems to different impacts. The analysis of parameters of a state of such systems by registration of spectral characteristics of service processes allows one to carry out estimations of levels of their damage in a rather full degree. On the basis of results of research of service states of technical systems on spectral diagnostic parameters, approaches to the analysis of amplitude-frequency characteristics of their actual states both in the regular allowed states and upon transition to dangerous states owing to accumulation in them of dangerous service damages are offered. At the same time, criteria of transition of technical systems from regular to emergency and catastrophic states at accumulation of critical limits of damage are formulated. Examples of the dispersed initiation of microcracks in material of structural parts at a cyclic deformation with their propagation in the main fractures and also examples of the analysis of dynamic states on vibration spectra of a response of simple and compound technical systems with the crack which arose in them and with existence of defects are given. At the same time, the possibility of obtaining initial computation and experimental information on parameters of a stress-strain state and damage of installation with use of methods of the spectral analysis of a response of a compound structure to vibration impact as a result of initiation of damage of its separate parts is shown. Results of such spectral analysis can be used for diagnostics and monitoring of states of regular service of high-loaded installations of a technosphere and transition of their states to limit emergency and catastrophic in relation to objects of power engineering, space-rocket, and aircraft equipment.

A technique for probabilistic risk analysis of technical objects with accumulated irreversible damage is proposed. Damage is considered as a stochastic irreversible cumulative process with an absorbing boundary. The gamma process with a time-dependent probability density distribution is used as a damage accumulation model. The probability distribution function of life before failure and the probability (risk) of failure are determined by integrating the probability density of accumulated damage over the risk area. The damage accumulation rate is considered as a nonstationary function of time. The parameters of the damage distribution function can be determined from the data of nondestructive testing using the maximum likelihood method or the method of moments. The features and potentiality of the method proposed are exemplified in the risk analysis of the corrosion damage of critically important technical objects, e.g., offshore pipelines. The results of calculating the pipeline failure probability at different shape parameters and the scale parameter of the damage distribution function are presented. It is shown that the shape parameter, which directly depends on the time of damage accumulation, affects the damage probability most strongly. The main difference between the proposed technique and other schemes and methods of the damage risk analysis is the explicit time dependence of the gamma model parameters, which makes it possible to predict the probability of failure for a specified interval of service of technical objects. The technique proposed can be adapted to damage of other types, in particular, long-term damage with the growth of corrosion, corrosion-fatigue, and fatigue cracks.

Abstract—It has been noted that, as a rule, during operation of technical systems, the so-called nonstationary loading is implemented, when external loadings increase or decrease arbitrarily. In engineering calculations, of practical interest are the final relations between stresses and strains. It has been shown that simple cyclic loading problems can be solved using state equations in the final ratios, which are based on the small elastic strain theory. In this case, the existence of a uniform generalized strain diagram, which suggests the final relation between corresponding stress and strain components for both the initial and cyclic deformation stages is important for analysis of investigated nonstationary cyclic loading conditions. Basing on this fact, it has been established that, in the analysis of conditions of achieving limit states at all lifetime stages, the criteria and system of computational equations become too intricate because of a variety of factors to be taken into account. Among these factors are, first of all, external and internal natural, technogenic, and anthropogenous impacts, load from them, and reactions of objects to such impacts and loads. A set of criteria defining a limit state is presented in the form of a functional dependence characterizing, on the one hand, a set of force and deformation parameters of the state of a technical system and, on the other hand, a complex of criterion characteristics of constructional materials with allowance for their change during operation. The most important stage in determining conditions for achieving the limit state, which causes fracture, is finding the critical parts that bring risks of emergencies and catastrophes, in which local and main fractures are initiated. In this case, the transition of an operated object from the design area to the zones of initiation of failures, accidents, and disasters occurs when the damage, defectiveness, and risk parameters attain their critical values.

The methodology of combined application of acoustic emission diagnostics and vibration diagnostics for monitoring the state of damage of samples made of 17GS1 pipe steel with a combined stress during the development of a dominant crack is considered. In the experiments, the stress concentrator is a transverse weld subjected to local crumbling and a guide scratch to a depth of up to 3 mm across the entire sample width. Such a concentrator simulated the effect of buried reinforcement on the weld when laying a pipeline through a water obstacle.

Rosin strain-sensitive coatings are used to study the deformation fields of parts located in different working environments, including oil vapors at 100

Modeling of the spatial low-cycle crack kinetics for refined service life and survivability of elements of aviation, rocket and space, nuclear, and other critical equipment is studied. The results of experimental studies and numerical calculations of the parameters of shape change are presented, taking into account the characteristics of the microrelief and the complex surface of low-cycle fracture of inclined surface semi-elliptical cracks. A functional relationship is shown between the mesostructural characteristics of low-cycle fragmentation of the surface of the cracks under study with parameters of nonlinear fracture mechanics such as the strain intensity coefficient and the specific elastoplastic work of fracture at a certain point of the defect contour.

The main cyclic thermomechanical loads, as well as changeable in time vibration and aerohydrodynamic loadings accompanying them affect modern energy facilities, space, air, water and ground transport. At the same time the total number of loading cycles taking into account the duration of service turns out to be within in very wide limits. It is shown that a general spectrum of loads changing in time which affect the aforementioned objects is very complicated in terms of load levels, frequency ratios and time of their action. Taking into account a large variation of service impacts in the loading levels, frequency ratio and total number of loading cycles we performed a generalized analysis of the resistance to deformation, damage and fracture of high-loaded objects of modern technics. The results obtained revealed that in conditions of combined mechanical, vibration and aero-, gidro-, acoustic loadings the limit state (by criteria of cyclic strength using the rule of linear summation of damages expressed in deformation parameters) will be attained earlier, than that obtained only with allowance of the main thermomechanical loading. To substantiate the strength and service life of the objects under consideration, traditional standard and unified mechanical isothermal tests for static and cyclic loading are carried out to determine the basic characteristics of the mechanical properties of the material, as well as special mechanical programmed tests with variable modes that simulate complex processes of operational thermomechanical, vibration and aerohydrodynamic impacts. The results of testing are taken into account in computation and experimental estimations of the strength and the fatigue life for the corresponding spectra of operational loads. A refined verification calculation of the cyclic strength and durability is becoming increasingly relevant for modern machines and units operating under conditions of increasing speeds of movement, operating pressures with increased levels of pulsations, as well as in the occurrence of accompanying mechanical oscillations, vibrations and aerohydroacoustic impacts.

Individual deformation diagrams of all materials on metal, non-metal, or composite base under static and cyclic load link stresses and strains. These diagrams are obtained in standard tensile, compression, torsion or bending tests of laboratory samples with the registration of forces and deformations of their working parts upon loading. The diagram for a single static deformation in the stress-strain coordinates in this case covers both the region of elastic deformations and the region of elastic-plastic deformation, when deformations are localized in the neck of the loaded sample up to the moment of its destruction at a critical stress level. It is shown that linear, fractional-linear and power approximation of the obtained deformation curve are widely used in the description of the obtained deformation diagrams. Direct experiments, the theory of dislocations and the statistical theory of strength confirm the priority of power approximation of considered diagrams. At the same time for all construction materials the generalized deformation diagram in relative coordinates is described by a single power equation with the individual hardening parameter which is determined experimentally or theoretically using the dependences linking data on the module of elasticity, yield stresses, strength, and ultimate plasticity of the material. Diagrams of a cyclic elastoplastic deformation in the form of loops of plastic hysteresis are recorded by analogy with static tension diagrams with «stress – strain» axes of in conditional and true relative values. The generalized deformation diagrams for a single static and cyclic loading form a scientific basis for construction of the generalized fatigue curve on the basis of deformation fracture criterion for a wide range of cycles to failure. An effective solution to the problems of strength and service life for the most complex engineering objects such as nuclear reactors, aircraft, rocket and space systems can be achieved through introduction of generalized deformation and fracture diagrams into consideration and corresponding calculations. Their significance will especially increase in the design and implementation of new unique science-intensive facilities.

The processes of elastic and elastoplastic deformation in concentration zones are analyzed for the case of different levels of rated stresses and resistance with respect to plastic strains. Based on the calculated and experimental data, it is shown that, with a decrease in the strengthening index in the inelastic region, the inhomogeneity of the strain distribution exhibits an increase. At the same time, the strain concentration coefficients increase and the stress concentration coefficients decrease. Special cases are considered when the latter are less than unity. This is important for justifying the strength, service life, survivability, and safety of new objects of the technosphere made of special structural materials and operated under extreme thermomechanical loading conditions, including the conditions inherent in nuclear power plants in normal modes and emergency situations.

Standard strength calculations for bearing units of extremely loaded structures, including nuclear power plants, allow an inelastic deformation of the materials of these units. At the same time, the calculations of the low-cycle fatigue require taking into account the factors that are not observed upon single loading, i.e., kinetics of cyclic strains, cyclic creep, and the effect of change in the modes of the inelastic cyclic deformation under normal operating conditions. It is known that, in this case, a material can be cyclically hardened, softened, or stable. For the first type of materials under soft loading with constant amplitude of stresses in cycles, the range of strains decreases with an increase in the number of cycles, but increases for the second one. Under hard loading with constant strain amplitude, the maximum stresses in a cycle increase for a hardened material and, on the contrary, decrease for a softened one. In addition, the soft loading of a softened material with an increased number of loading cycles results in one-sided accumulation of plastic deformations. These phenomena must be taken into account both in the analytical description of the kinetics of deformation diagrams and in the corresponding calculation equations used in the strength standards. At early stages of developing the calculation techniques for these conditions, the stresses were calculated under the assumption of perfect elasticity of a material. This approach was used owing to the lack of available calculation techniques for the problem of an inelastic cyclic deformation, which is complicated in the formulation. The subsequent development of the theory of the cyclic elastoplastic deformation and the analytical and numerical solutions of cyclic boundary-value problems and the development of numerical computational methods and powerful computer software codes fundamentally changed the situation, providing the possibility of analysis and modeling of the physically and geometrically nonlinear deformation processes. It is shown that the transition from the elastic adaptability (with an elastic deformation of the structure in a stable cycle) to a sign-alternating flow is smooth and continuous and is similar to the transition from the elastic to plastic deformation under a single loading. This mechanism is similar to the conditional boundary of the transition from low-cycle to high-cycle fatigue under a cyclic strain. In this case, we propose to use in calculations the existing rather simple models and experimentally determined parameters of the cyclic deformation diagrams of materials. In the modern formulation of the considered problems, it is of fundamental importance to take into account both the kinetics of cyclic and one-sided accumulated deformations and make allowance for the occurrence of creep effects in cycles. This approach also makes it possible to take into account the acceleration of unsteady cyclic creep as a result of the previous plastic deformation of the other sign, which can be rather significant.

This article discusses the concept of assessing the risk of defects in offshore subsea pipelines according to risk criteria, taking into account the characteristics of methods and means of nondestructive testing used in practice. The expansion of the practice of assessing the danger of defects in pipelines according to risk criteria will clarify the assigned safety factors and increase the reliability of calculations of the permissible sizes of defects. The work describes the most promising methods of pipeline repair, and in their range, special attention is paid to the repair method using crimped composite sleeves, which performs repairs at an existing facility in real operating conditions. Based on the results of testing the proposed repair technology on a real object, the method repairs reliably most defects in the base metal and welded joints of pipelines for the entire service life and restores its full functionality.

Scientific and methodological aspects of ensuring the safety of offshore subsea pipelines according to accident risk criteria are considered. A conceptual approach to assessing the hazard of defects according to risk criteria is formulated by analyzing modern norms and requirements on pipeline design. A probabilistic model for assessing the risk of accidents at offshore subsea pipelines considering the randomness of defects and damage from accidents is developed. The criterial conditions for the permissibility of defects in pipelines are formulated. Two conceptual directions for developing a procedural framework for calculating the permissible defects versus risk criteria are proposed on this basis. The first direction consists in elaborating semi-probabilistic calculation procedures using differentiated margin factors, considering the accident risk level. The second direction consists in solving the problem of the probability of pipeline failure, taking into account the restrictions in the form of a preset risk value. The probability and scale of accidents are linked through a risk matrix. The calculation methodology is developed using a semi-probabilistic concept for the most typical defects in offshore subsea pipelines. The suitability of the pipeline for operation after in-line diagnostics is determined using a three-level assessment of permissible defect sizes. The first level is basic and determines permissible defect sizes according to the strength criteria for pipelines exposed to the main loads, i.e., internal overpressure and external hydrostatic external head. The second level is extended and determines permissible defect sizes according to the strength criteria, taking into account the impact of additional longitudinal and bending loads on pipelines. The third level is special and determines permissible dimensions of cracks and cracklike defects according to the crack resistance characteristics of the pipeline metal. The novelty of the method consists in the justification of the margin factors through the failure probability levels corresponding to a preset class of damage and losses. A scheme for making decisions on the permissibility of defects according to risk criteria has been developed. An example of the assessment of the hazard of defects in offshore subsea pipelines is presented.

The operability of structures in complex combined loading modes depends on a significant number of combinations of operational parameters of thermomechanical impacts in terms of loads, temperatures, times, numbers of cycles, and strain rates. The main strain patterns of structural materials under complex conditions are established using combined standard, unified, and special tests under laboratory conditions. With the use of representative substantiations of physicochemical models for strain diagrams in a wide range of loading conditions and with allowance for the scale diversity of models, the material structure, and the responsibility of structures, we propose a step-by-step consideration of the corresponding strain types: elastic, sign-alternating flow, progressive strain accumulation, and their combinations. In this case, structural calculations can be built as a hierarchical system, in which each next level refines the boundaries of permissible impacts toward expansion of the range of acting loads, temperatures, rates, and strain modes, which is associated with an increase in the amount of required initial data and complicates calculations. The proposed methods for schematization of physicomechanical properties and types of state equations for describing strain curves account for the compactness requirements of the initial data and the need to use both standard and unified methods for determining the characteristics of cyclic inelastic deformation and special methods. Both from a theoretical standpoint and from the viewpoint of practical applications, power equations are the most reasonable to describe the kinetics of strain diagrams under the considered conditions. Exponential dependences are suitable to reflect the role of the temperature factor while power dependences are suitable to take into account the time and strain-rate factors and two-frequency loading conditions. Ensuring the maximum possible use of the strain and strength reserves of materials and structures, refined calculations at higher, more complex levels of the considered hierarchy must be based on kinetic dependences describing low-cycle deformation in complex loading modes.

Acoustic emission (AE) diagnostics was carried out during tensile testing of 20KhN2MA steel samples to study the loss of ductility after the impact. We used V-notched samples (3.3 mm in depth) with overall dimensions of 300 × 20 × 6 mm. The impact in the concentrator zone caused the depletion of the plasticity of the material, which was accompanied by a decrease in the partial fraction of ductile fracture and an increase in brittle fracture. The test samples were divided into six batches. The samples of the first batch were not subjected to the impact. In the second batch, the impact energy was 50 J, in the third — 75 J, in the fourth — 100 J, in the fifth — 125 J and in the sixth — 150 J. The rupture tests were carried out at room temperature and at a speed of the movable traverse of 1 mm/min. The kinetics of damage in the notch zone during loading was monitored using the acoustic emission (AE) method and video recording. Processes of brittle and ductile (caused by cleavage and shear, respectively) destruction of the crystal lattice of a metal differ primarily in the speed and duration of stress waves. To separate AE pulses generated by these processes, spectrograms of time-frequency transformations and waveforms were analyzed. Pulse selection was carried out using a complex parameter reflecting the steepness of the amplitude drop at the phase of signal attenuation. Boundary values were determined that allow separation of the recorded pulses into flows caused by ductile and brittle structural damage to structural steels. It is shown that manifestation of the effect of impact on the exhaustion of the plastic properties of steel 20KhN2MA becomes noticeable when the level of specific work exceeds 50 J/cm 2 . Moreover, with an increase in the specific work up to 150 J/cm 2 , the weight content of location pulses characterizing the kinetics of brittle destruction of structural bonds increased by 3 – 4 times, relative to that recorded for the samples without impact. This result correlates with the duration of the rupture test of the samples, which was reduced by three times when the level of the specific work increased to 150 J/cm 2

Early prediction of an emergency situation is of great importance, especially for objects with high accident costs. This is due to the fact that, according to the signals of early diagnosis, it is possible to stop the development of an emergency before it enters an irreversible phase, when it is already necessary to consider the consequences of the accident. There are two important aspects to this work. Firstly, the timely detection of the signal at the beginning of the development of abnormal operation of the equipment; secondly, it is necessary to clearly filter out false signals, since they can lead to unmotivated shutdown of the equipment and economic losses. The way out of this situation is seen in the combination of different methods, so that different interferences, which are fundamental for one method, have little effect on the results given by another method. This approach, together with appropriate processing of the results, will provide reliable data at the beginning of the emergency development of the process. This article discusses highly sensitive methods of early diagnosis to determine the time of safe operation of equipment when a signal of the onset of an emergency process is detected. This al-lows an informed decision to be made in the presence of an emergency development model: to stop work immediately or to continue operating the equipment without serious risk until the next scheduled repair. The article considers combined methods of diagnosis: fragile strain-sensitive coatings, acoustic emission and aerosol particle generation, technical vision. A methodological evaluation of the physical and mechanical properties of the coatings, the relationship between the stress and strain values and the parameters of the output signals of the devices under laboratory conditions was carried out on beams of various profiles, which made it possible to improve the accuracy of the measurements and the quality of the new strain-sensitive coatings.

Дан краткий анализ развития концепций риска аварий технических систем. На основе концепции минимизации стоимости жизненного цикла предложена новая трактовка риска технических систем, включающая потери как от аварий, так и от нарушений работоспособного состояния. Рассмотрены задачи оценки риска в предлагаемой формулировке. Ввиду ограниченности возможностей аналитических методов оценки риска основное внимание акцентируется на использовании методов Монте-Карло. Рассмотрены возможности оценок риска на основе статистических данных об авариях и нарушениях работоспособности. В качестве примера выполнены оценки риска аварий наземных и морских подводных трубопроводных систем с учетом нарушений работоспособности. Показано, что предлагаемый подход дает более высокие оценки риска по сравнению с традиционным подходом The article gives a brief analysis for development of the concepts for risk analysis of accidents in technical systems. The new interpretation of the risk of technical systems is proposed, including both losses from accidents and losses from violations of the working condition and based on the concept of minimizing the cost of the life cycle. The problems of risk assessment in the proposed formulation are considered. The main attention is paid to the prospects of risk assessments using Monte Carlo methods due to the limited capabilities of analytical methods for risk assessment. The capabilities of risk assessments based on statistical data on accidents and malfunctions are considered. The assessments of the risk of accidents and malfunctions of onshore and offshore underwater pipeline systems are carried out. It is shown that the proposed approach gives higher risk estimates compared to the traditional approach

This article discusses the possibilities of estimation of safe sizes of integrity defects on the basis of risk criteria. Such defects occur at all stages of the lifetime of structures. In most cases the estimation of their hazard and determination of allowable sizes attract attention when the defects can lead to brittle or quasi-brittle fractures. In this case, the models of linear and nonlinear destruction mechanics are applied, when the defects are considered as internal elliptical or surface semielliptical cracks. The stochastic variety of shapes, sizes, locations, and orientations of defects has a significant influence on the failure mechanisms. Therefore, the probabilistic problem of estimating allowable sizes of defects according to the criteria of risk of failure is relevant. This paper examines a general approach to estimation of the hazards of defects according to risk criteria. Two formulations of the probabilistic problem of risk estimation are presented: on the basis of single-parameter and two-parameter failure criteria. The risk function is used as the calculated characteristic, represented as the probability of failure according to a given criterion. An equation of the risk function based on single-parameter failure criteria is presented. The main focus is on the probabilistic model based on the two-parameter Morozov failure criterion. This criterion provides a wide range of opportunities for analyzing various failure mechanisms with variations in the size of defects. An expression for the risk function based on the family of two-dimensional Lu–Bhattacharya probability distributions of Weibull type is derived. It is shown that correlations between failure mechanisms can significantly influence the probabilities of failure and, consequently, the allowable size of defects.

A structural-phenomenological concept of monitoring the residual strength of composite materials is proposed. The structural-phenomenological concept was developed taking into account the kinetics of damage and destruction of polymer composite material (PCM) at the micro-, meso-, and macroscale levels, which generate acoustic emission pulses (AE) recorded by the receiving transducers of the antenna array. A correspondence between the ongoing destruction of the composite material structure at the micro-, meso-, and macroscale level and the AE pulses recorded at the same time and their weight content provides the possibility of monitoring of the damage kinetics in the loading mode at all structural levels and, consequently, the possibility of control of the residual strength of the product. An algorithm and software have been developed that made it possible to divide the recorded AE signals into clusters of lower, middle, and upper energy levels corresponding to micro-, meso-, and macroscale disruptions of the structure of a composite material and calculate the AE activity and the weight content of location pulses in energy clusters, thus displaying the dynamics of their changes every second. Comparison of the current values of the most informative parameters of the weight content of location pulses in energy clusters with the threshold values recorded during the destruction of the material provides monitoring of the residual strength of the product in the loading mode. The validity of the developed concept, algorithm and software was proved during tests of elementary and structurally similar samples of PCM under different loading conditions. An example of using the developed technique for revealing the areas of the most intense damage accumulation in a MS-21 fuselage panel at a stepwise increase in the compressive load is presented. In addition to the possibility of identification of the area of intense accumulation of damage and failure of the structure of the composite material, the structural-phenomenological concept of the AE diagnostics provides also the possibility of tracing the damage kinetics at different scale-structural levels, controlling the level of the residual strength of the panel upon the stepwise compression.

To the 85 anniversary of the Mechanical Engineering Research Institute of the Russian Academy of Sciences

The effect of arrangement of two pipes on the parameters of their force interaction in a fluid flow has been considered. The result of the study in this paper will be selection of the parameters for the mutual arrangement of pipes, which is justified by the calculation of hydrodynamic forces and confirmed by the physical or numerical experiments. The software used is designed for pipe bundles and reproduces hydroelastic excitations with periodic vortex separation. This paper provides a description of the known physical experiments and their comparison with the authors’ numerical experiments.