Inorganic Materials最新文献

A method for assessing the survivability of a pipeline with a longitudinal surface semi-elliptical normal separation crack, located away from a transverse weld, taking into account biaxial strain restraint along its front, is proposed. At present there are no publications that predict the growth of such a crack while accounting for the T_xx and T_zz stresses, which are the non‑singular terms in the Williams expansion for stresses at the crack front in three‑dimensional bodies. For simulation of fatigue crack growth a modified Paris law was used, in which the range of the conventional stress intensity factor (SIF) is replaced by the range of an effective SIF. In the expression for the effective SIF, in addition to the conventional SIF, the T_xx and T_zz stresses are included. The proposed two‑parameter approach allows one to account for deformation constraint in the crack plane: in the direction normal to the front, by introducing T_xx stresses into the expression for the effective SIF, and in the longitudinal direction, by introducing T_zz stresses. The expression for the effective SIF was previously obtained by the authors by refining the maximum tangential stress failure criterion. It is assumed that the tangential stresses in the pre‑failure zone are equal to the local strength of the material. The size of the pre‑failure zone and the local material strength are determined taking T_xx and T_zz stresses into account. Numerical simulation was performed in the finite‑element environment ANSYS Workbench. To build the finite‑element model in the vicinity of the crack front, degenerated 15‑node wedge singular elements derived from SOLID186 were used; these have a built‑in capability to compute the SIF and T_xxstresses. A special APDL macro was developed to compute the T_zz stresses and the effective SIF along the crack front. It is shown that the traditional single‑parameter approach based on the Paris law does not allow one to account for biaxial deformation constraint when assessing pipeline survivability, since, unlike the effective SIF, the conventional SIF for a longitudinal semi‑elliptical crack depends only on the circumferential stresses. It was established that for surface cracks of the same initial depth, increasing the half‑length reduces the pipeline life. Moreover, the life predicted using the standard single‑parameter Paris law is approximately 30% lower than that obtained from the two‑parameter analysis.

This article presents the results of many years of operation, improvement, and modernization of a specialized friction machine designed and manufactured for studying fretting wear of materials and coatings in a gross slip fretting regime. Various modes of behavior of friction contact surfaces during fretting are known: full seizing (sticking) and partial sliding (sticking-sliding), characterized by fatigue failure and crack formation, as well as full sliding with a transition to reciprocating sliding, in which the primary failure of contacting surfaces is wear. The latter is typical of friction units in which the moving contacts of the friction surfaces are specially designed, which is common to a wide range of installations, machines, and mechanisms, particularly friction units of gas turbine engines (GTEs) and power plants based on them. The article presents the main results of fretting wear studies of various friction units at each stage of the machine’s modernization. The advantages of this machine are determined by its ability to control one of the main fretting wear factors—sliding amplitude (relative cyclic displacement)—as well as the use of collet clamps to secure specimens for testing various friction contact patterns, such as sphere–plane, plane–plane, cylinder–cylinder, cylinder–plane, and others. During all stages of modernization of the specialized friction machine, the various coating designs were developed and tested for fretting resistance to protect friction surfaces, particularly fan blade locks and gas turbine engine blade shroud friction pairs operating under gross slip fretting conditions. A comparison of the linear and volumetric fretting wear values of various coating designs and their components revealed that the developed and proposed coatings exhibit good fretting resistance under the specified and implemented conditions on the friction machine. Using an analysis of the dependences of the tangential friction force on the displacement (amplitude) of gross slip fretting, the dissipation energy values (friction energy) were obtained and fretting loops were constructed. These values made it possible to estimate the service life of each of the coatings under consideration, given their known thickness and specified displacement, and to identify the coating with the highest fretting resistance and service life. Based on the results of testing materials and coatings under plane-to-plane friction conditions at a temperature of 175–180°C, the recommendations were developed for the use of these materials and technologies in friction pairs of gas turbine engine blade shrouds. The capabilities of modernized friction machine discussed above allow for laboratory studies of the wear of various materials and coatings in model friction units of various power plants, machines, and mechanisms under specified operating conditions.

A method for assessing the strength of materials in complex stress-strain states (SSS) is considered. The study was conducted on disk-shaped specimens with stress concentrators. The SSS was modeled using the finite element method, taking into account the nonlinear nature of the processes under study. Reliability of the numerical modeling results was assessed using a sequence of finite element approximations. Experimental fracture of specimens was performed on a standard Instron 5989 machine. Data obtained confirmed the feasibility of creating a specified level of principal stress ratio in the test specimens, arising under biaxial tension. Using grade 45 steel specimens, the method’s applicability is demonstrated for both assessing the structural strength of materials and locally approximating their ultimate surface. A modified version of Pisarenko–Lebedev criterion was used as the theoretical basis for the local surface approximation. It has been shown that the proposed disk samples have a relatively simple shape and are characterized by a high degree of variability in the stress state. This circumstance allows for the assessment of the structural strength of materials during laboratory testing.

An important strength characteristic of fiber-reinforced polymer-matrix composites is their resistance to delamination and splitting, which is traditionally determined by bending short beams (in terms of interlaminar shear strength) or by testing specimens with artificial delaminations (in terms of the critical energy release rate, or the specific work of delamination). The implementation of these methods is associated with known difficulties in interpreting and comparing results. In this work a method is proposed for determining the critical value of the stress intensity factor at the onset of a delamination crack near the tip of a transverse side notch, which allows: comparison of the fracture resistance of different composites and metals; assessment of the potential strength of unidirectional fiber composites in failure by the mechanism of multiple splitting; and formulation of “equal-strength” requirements based on the condition of simultaneous initiation of different failure modes. Experimental results are presented for the delamination resistance of composites with various reinforcement schemes in terms of the critical stress intensity factor for crack kinking. The obtained results are compared with the interlaminar strength of the material.

Currently, there is no generally accepted standard setup for assessing the durability of biomedical magnesium alloy specimens in corrosive environments. Each research group uses its own testing system, which does not always account for the many technical factors that influence the final experimental result. This paper proposes equipment that can be assembled using a standard fatigue machine for assessing the corrosion fatigue life of bioresorbable magnesium alloys in various physiological solutions. It includes: a testing machine with a slotted plate for rigidly mounting the chamber; polyamide grips providing galvanic isolation; a peristaltic pump for pumping the medical solution; an ion meter connected to a computer to record changes in pH and temperature; a thermostat for maintaining the chamber temperature within a specified range; an additional solution fed into the chamber through a needle using a small peristaltic pump when a predetermined pH level is exceeded, maintaining the pH within an acceptable range. This setup uses corset-type specimens, although other configurations are also acceptable. A camera can be installed to visualize corrosion processes on the test specimens. The testing system achieves good repeatability and reliability of the obtained results, which has been experimentally verified and confirmed using MA14 magnesium alloy of various melts, selected as a model. Setup proposed can be used to evaluate the fatigue life of magnesium alloys for medical purposes. It is easily reproducible under standard laboratory conditions.

Long-term and safe operation of complex mechanisms and assemblies requires timely diagnostics of frictional units. This work presents results of determining the friction torque in self-lubricating sliding bearings from temperature data. A system of sliding bearings made of a polymer composite material, which support a rotating shaft, was investigated. A mathematical model of the thermal process in the considered system is given, accounting for the spatial distribution of temperature and its temporal variation. Temperature was recorded with thermocouples at several points on each bearing. Friction torque was determined from frictional heat generation using a solution of the inverse heat transfer problem (criterion: agreement between measured and calculated temperatures). Temperature data with measurement errors were processed statistically. Refinement of the inverse problem solution was stopped upon convergence of successive approximations. To ensure continuous data processing and determination of friction torque during long tests, the inverse heat transfer problem was solved on successive short time intervals, and the obtained solutions were then stitched together. It is shown that the discrepancy between values of the total friction torque obtained from temperature-based calculations and from measurements with an inductive sensor does not exceed 15%. The proposed thermal diagnostic method for friction can be used to determine friction torques in self-lubricating sliding bearings in both bench and in-service tests of machine friction units, and to improve the reliability of bearing condition diagnostics.

A methodology for studying the distribution of residual stresses across the thickness of thin-sheet rolled materials is presented. The tested specimens were narrow strips cut from the blank along and across the rolling direction. Stresses in the specimen were determined using a gradually deepening transverse notch-groove method (slotting method). The recorded response was the angle of the resulting mutual rotation of the specimen ends. For reliable recording, the non-contact optical methods such as electron speckle interferometry or digital image correlation are considered optimal. The choice of one of these methods is determined based on the test conditions. The procedure for interpreting primary experimental data in terms of residual stresses is formulated as an inverse problem of solid mechanics. Two possible mathematical approaches to data processing are considered, taking into account the problem’ ill-posedness. Stabilization of solution is achieved, in part, through the use of a regularization algorithm. The developed method effectiveness is demonstrated using a study of stress distributions in thin-sheet steel samples produced by the Novolipetsk Steel Plant (Industrial complex). It is shown that different approaches to mathematical processing yield virtually identical results.

Ultrasonic flaw detection equipment is typically configured using signals scattered by artificial defects of a given shape. However, the characteristics of grooves used for this purpose depend largely on their manufacturing method. This paper presents the results of a study of longitudinal and shear wave scattering by cracks using grooves manufactured by traditional methods (milling and electrical discharge machining) and laser cutting. Model cracks manufactured using friction stir welding were also examined. To detect defects, a time-of-flight diffraction (TOFD) method of ultrasonic testing was used. The paper presents data from a comparative analysis of signals diffracted by the tip of planar defect models and an assessment of the tip coordinate and defect type based on the recorded signal’ amplitude. The effect of rotation angle of the source—receiver pair relative to the crack model on the amplitude of ultrasonic waves scattered by the tip of an artificial reflector was also analyzed. Measurements were conducted at frequencies of 5 and 10 MHz with longitudinal (along the reflector) and transverse (parallel and perpendicular to the weld axis) movement of the source—receiver pair. It was shown that the best results for crack tip modeling can be achieved using laser cutting. This method allows for the creation of planar defect models that simulate real defects with a tip opening of approximately 0.01 mm. These results can be used to create artificial defects in specimens for ultrasonic testing.

Engine oil should be changed on a regular basis hence it is required to monitor the mechanochemical wear process (breakdown) of detergent additives. We report the results of studying the process of deteriorating detergent during natural aging of M-6₃/12G₁ engine oil following end-face friction resulting in generating wear particles at the early stages of engine run. Testing involves metallographic studies and nuclear magnetic resonance. It is established that the proton relaxation time decreases in the engine oil as the number of ferromagnetic particles—which are wear products—grow in the colloidal solution. At the same time, with growing number of wear particles in oil, the size of the molecular complexes formed by detergent additives also gets higher due to coagulation (since the additives acquire the minimum energy state). These complexes surround the wear particles keeping them suspended in a colloidal solution. It is shown that the end-face friction for 0.5 and 3.0 hours results in an increase in the number of wear particles. If the samples of oil undergo 10-day retention (when the engine is shut down) followed by shaking, the spin-spin relaxation time of oil does not increase. On the contrary, it reduces significantly (by 1.6 and 2.8 times, respectively for above batches) due to oxidation triggered by interaction with oxygen (natural aging). After natural aging, oil without wear-generating compounds has parameters reducing slower by a factor of 2–3 if compared with the spent and aged oil. The results obtained can be used for tribodiagnostics of the early degrading the engine oil detergent additives.

The results of slow strain rate tensile tests (SSRT) for samples of corrosion-resistant steels of ferritic, austenitic, austenitic-ferritic, and martensitic classes, as well as an aluminum alloy, in order to determine their susceptibility to sulfide stress cracking (SSC) are obtained. The selected materials are used in oil and gas production industry and are operated in acidic aqueous environments saturated with hydrogen sulfide. A strain rate at the level of 10^–6 s^–1 is recommended by NACE TM 0198 standard for evaluating the stress corrosion cracking susceptibility of corrosion-resistant steels and alloys. The SSRT method in a corrosive-active environment proves its effectiveness for comparative assessment of the resistance of steels of different classes to sulfide stress cracking. The strain-stress tensile curves of the studied materials in a hydrogen sulfide environment make it possible to establish that the destruction of samples of the 08Kh13, 08Kh18N10T, and 08Kh22N6T steels occurs only in the region of plastic deformation, and the destruction of samples of the 17-4RN steel occurs in the region of elastic deformation, on the linear section of the tensile curve. At the same time, the 17-4RN steel of martensitic class is the most sensitive to the effects of hydrogen sulfide and its threshold stress is below the permissible level. The 1953T1 aluminum alloy doesn’t not show a tendency to sulfide cracking (the tensile curves in air and in a hydrogen sulfide environment are almost completely identical) and can be considered a standard for resistance to this type of corrosion destruction. Metallographic analysis of the structure of studied steels and the character of propagation of the resulting corrosion cracks make it possible to confirm the influence of the structure, phase composition, and strength level of the material on the development of the sulfide stress cracking process.