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The effect of cold isostatic pressing of WC-15Co powder blanks on the sintered alloy strength in threepoint bending was evaluated. The specimens treated or nontreated by cold isostatic pressing (CIP) before sintering were analyzed by scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDS), X-ray diffraction (XRD), strength and Vickers and Rockwell hardness test equipment. The strength of CIP-treated specimens in three-point bending was found to be 2410 MPa, which is 13% higher than that of the CIP-nontreated ones and 30% higher than the standardized strength of a VK15 hard alloy of a similar composition. The hardness was shown to be the highest for the specimens CIP-treated before sintering, while average WC grain sizes remained unchanged compared to the CIP-nontreated ones. The predominant crystal modification of cobalt in the CIPtreated specimens was revealed to be HCP-Co, in contrast to the CIP-nontreated ones, where cobalt was mainly in the metastable FCC modification. It is shown that in the specimens CIP-treated before sintering, the cobalt content on the fracture surface is 3-4 times higher than for the nontreated ones, and the fracture crack propagates mainly along the WC matrix phase–Co binder interface. In the CIP-nontreated specimens, the crack propagates in the Co volume, and WC grains on the fracture surface are covered with cobalt interlayers of different thicknesses. Cold isostatic pressing has been proposed as a requisite additional operation of standard hard alloy technology since already at the pressing stage, it provides essential conditions for enhancing the strength of sintered specimens. [ABSTRACT FROM AUTHOR]

Previous studies have shown that with increasing service life of shipbuilding steels, a strong hydrogen charging of their internal near-surface layers occurs, especially if the service life exceeds 3 years or more. This is known to cause changes in the mechanical properties of steels. Therefore, there is a need to conduct additional experimental studies on the effect of hydrogen in a wide temperature range on the degradation of the structural and phase states of steels, in particular, on changes in the crystal lattice and redistribution of cementite, which directly leads to a decrease in the ductile and deformation properties of the metal, especially at subzero air temperatures. The effect of service life and subzero temperatures on the stress state of the a-matrix lattice and its parameters for the 10KhSND and D32 steels was investigated. A tendency to increase in the value of lattice distortion stresses and decrease in the mass fraction of cementite in specimens of these steels after long-term operation was revealed. Metallographic studies showed that with decrease in the temperature of the cooling medium, the volume fraction of hydrides increases significantly, which leads to embrittlement and softening of steels. [ABSTRACT FROM AUTHOR]

A mechanophysical model for crack growth kinetics computation on stress corrosion fracture of modified 06G2BA and 08KhMCHA pipe steels is adequately expressed through the plane stress-strain state dα/dt and dJ/dt ratios that are dependent on the strain crack tip rate. The crack growth accelerated by an aggressive environment occurs under static and cyclic loading due to transient dissolution and repassivation processes at the crack tip. Such accelerations are divided into three categories, determined by the strain rate: mechanical cracking (fatigue crack and stationary plastic crack), corrosion-accelerated mechanical cracking (corrosion fatigue and corrosion-accelerated plastic crack), and sulfide stress corrosion fracture. Metallographic studies revealed the change in the crack nucleation and propagation mechanisms, from transcrystalline to intercrystalline, related to the viscoplastic and brittle structure of steel specimens cyclically loaded and simultaneously affected by a corrosive environment. [ABSTRACT FROM AUTHOR]

Experimental and computational methods to assess the fracture toughness of 15Kh2NMFA-A reactor vessel steel are analyzed based on the local fracture models: Rice–Tracey and developed in Pisarenko Institute of Problems of Strength of the National Academy of Sciences of Ukraine. The master curve was experimentally constructed, and parameters used in examined models were obtained. The Rice–Tracey model modification, which takes into account the averaging of the relative void radius in the vicinity of the crack tip, extending at each loading step, is shown to confirm a previously made assumption as to the fracture toughness assessment regardless of the finite element size near the crack tip. Since in the ductile-brittle transition region, the crack initiation is ductile, i.e., through void nucleation, growth, and coalescence, the fracture toughness assessment based on local ductile fracture models gained theoretical substantiation and demonstrated satisfactory agreement with experimental results. [ABSTRACT FROM AUTHOR]

It is shown that it is necessary to take into account biaxial loading and short cracks in crack resistance tests of specimens for a more accurate assessment of the residual life of nuclear reactor vessels. To implement this, a special design of small cross-shaped specimens with linear and semi-elliptical cracks and tooling for their biaxial bending loading on a uniaxial servohydraulic machine was developed. On the basis of a series of finite element calculations, dependences for calculating stress intensity factors and J-integral for a strip for three-point bending with a semi-elliptical crack and for linear and semi-elliptical cracks in a cross-shaped specimen during its biaxial bending tests were obtained. Using the obtained data, experimental data on the influence of biaxial loading on the fracture toughness characteristics of reactor vessel steels were specified and plotted on a graph of fracture toughness versus temperature. It is shown that the crack resistance of specimens with linear and semi-elliptical short cracks is greater than that obtained on standard specimens, and that the biaxial loading of cross-shaped specimens, taking into account the moment of crack initiation, reduces the crack resistance compared to the uniaxial loading of specimens with short cracks. To generalize the experimental data and build a model for predicting the effect of biaxial loading on the characteristics of crack resistance, it is necessary to perform additional experimental studies to obtain a statistically significant result. [ABSTRACT FROM AUTHOR]

It was established that with increased concentrations of chlorides, sulfate-reducing bacteria, heterotrophic bacteria, thionic and iron bacteria the corrosion rate of the studied specimens of reinforcing steels sharply increases. The 40KhN and 09G2S steels had the highest resistance to corrosion, and the St.20 steel had the lowest resistance. The obtained data correlate with the results of numerical experiments reported in the publications of domestic and foreign researchers. For the first time, the data obtained from a series of experimental tests of reinforcing steels for cyclic strength showed that the steels characterized by low corrosion resistance also have a low resistance to cyclic strength, which can be explained by their degradation. The authors believe that, in the process of long-term operation of reinforced concrete structures in an aggressive environment, carbonization and chloritization of concrete, that is, its destruction occur. Further hydrogen charging of reinforced concrete causes embrittlement and reduction of the visco-plastic properties of the reinforcement, which in turn leads to the destruction of metal. The obtained results make it possible to develop metallurgical and technological measures to increase the durability of reinforced concrete structures: the use of chromium, nickel, and manganese alloyed steels for the manufacture of reinforcement with subsequent proper heat treatment; the use of modern high-strength concrete, which will ensure strong adhesion of concrete mortar to the surface of reinforcing bars. [ABSTRACT FROM AUTHOR]

Almost the primary source of fracture for the products from brittle materials is known to be local stress concentrations, which results in repeated carrying capacity losses of the material and even of the whole product due to possible crack propagation. For improving the behavior prediction of brittle materials under such conditions, the local stress concentration sensitivity measure is proposed to be introduced (further Measure). This parameter is defined as the ratio of any crack resistance values calculated at low and high stress concentrations. The crack resistance characteristics are known to be usually determined by the techniques based on the concepts and models of linear elastic fracture mechanics aimed at defining the propagation resistance of an already extending main crack or the latter extending from a stress raiser prenotched in testing on the volume specimen fracture. However, other methods do also exist, which make use of parameters that are measured on local specimen fracture in the experiment. The Measure is defined both for volume and local specimen fracture. As large as possible difference in stress concentrations for volume fracture was provided by notching a raiser of the smallest and largest sharpness, which differed by 20–60 times, and for local fracture, it was effected with diamond indenters of the tip radii R varying by about 73 times. The materials with an increased fracture onset resistance barrier were established to also exhibit a higher Measure. Moreover, the Measure determined from the fracture toughness characteristic for volume fracture is much smaller numerically than its value for local fracture near the specimen edge. It is indicative of the prevailing contribution from the mechanisms of the main crack nucleation and initiation resistance in the surface layer of a brittle material to the fracture onset resistance value. [ABSTRACT FROM AUTHOR]

The paper presents the results of studying the contact endurance and wear resistance of boride coatings, which were deposited on the 40Kh and ShKh15 steels by chemical heat treatment from boron-containing powders and with the addition of copper. The wear kinetics and mechanism of boride coatings in the I-20 lubricant at a pressure of 2140 MPa on the contact area during rolling friction with point contact have been studied. It has been found that the presence of copper in boride coatings increases by a factor of 1.5 their crack resistance compared with those without copper, which favorably affects the life of the coating/substrate composition under cyclic contact load. The life of specimens with boride coatings on steels under rolling friction up to the occurrence of pitting under certain test conditions has been determined. It is shown that the lives of the ShKh15 and 40Kh steels with boride coatings in the presence of copper are 1.9 and 2.4 times longer, respectively, than those of uncoated hardened steels. [ABSTRACT FROM AUTHOR]

The results of experimental research of low temperatures influence on mechanical characteristics of structural steels of long-term operation in chemically aggressive environments of cooling systems are presented. It is shown that such characteristics of pipe steel as impact toughness (according to Charpy), critical coefficient of stress intensity (fracture toughness), critical crack opening, and microscopic resistance significantly decrease when temperature changes from 20 to -50°C, which indicates a significant decrease in crack resistance of the material. It has been established that the indices of ductile-plastic properties – yield strength, relative longitudinal and transverse deformations decrease sharply in the process of prolonged (up to 30 years) operation of pipe steel. At the same time, intensive flooding of metal takes place. [ABSTRACT FROM AUTHOR]

The procedure of evaluating the crack resistance of 17G1S-U sheet steel after shock and vibration loading was advanced and experimentally tested using the method of complete deformation diagrams. The technical potential was employed to provide the growth of a mixed mode (I+III) macrocrack on the specimens with an identical central circular opening, which guaranteed the self-similar macrocrack propagation. The complete deformation diagrams displayed initial almost straight descending portion. In real constructions of gas mains, rather long macrocracks can arise after this fracture mode. The advanced procedure permits of reliable assessment of energy variations spent for a (I+III) mode crack propagation under any complex combined loading. The straight descending branch slope of the deformation diagram is established to be used for evaluating crack resistance variations of pipe steel subject to thermomechanical loading. Shock and vibration loading of a high frequency (1–2 kHz) is shown to essentially influence the crack resistance of pipe steel and plastic strain in the vicinity of a stress raiser. The impact of a power pulse on the material is dependent on its prestrain level through static tension and damage of its initial structure correspondingly. The controlling factor influencing the change in mechanical properties is the intensity of the power pulse. Basic fracture mechanisms of steel were established on the basis of examination of specimen fractures with scanning electron microscopy. The shock and vibration loading is evidently accompanied by energy contribution not only to the existing damages of the material but also to the initiating ones, which causes the localization of deformation and growth of pores in their vicinity. Since the energy accumulation can contribute to the modification of the material in the vicinity of those damages, the shape and sizes of ductile tear dimples are the informative parameters for evaluating the strength and plasticity of examined steel. [ABSTRACT FROM AUTHOR]

The paper presents methods for determining the elastic modulus, sound speed, logarithmic decrement of vibrations (internal friction), tensile, compressive and bending strength, crack resistance, specific work of fracture of small-sized specimens. The specimens of polycrystals of superhard materials, which are prepared in high-pressure apparatuses, or of other expensive sintered materials (boron carbide, silicon nitride, hard alloys), which have a high hardness and elastic modulus, have typically a small size. Most of such materials experience significant dynamic loads during operation in products (metal-cutting tools, armor protection elements). It has been shown that disk specimens of brittle materials 3-15 mm in size can be used to determine strength characteristics under dynamic loading. The use of such specimens makes it possible to avoid oscillations in the loading diagram and to reduce the measurement error of breaking load. Moreover, it is possible to determine on such specimens the specific work of fracture of brittle materials from energy balance in measuring bars. Methods for determining mechanical characteristics under dynamic loading using Hopkinson-Kolsky bars are described, and examples of using procedures for the investigation of hard alloys, polycrystals of superhard materials based on diamond and cubic boron nitride, boron carbide and silicon nitride, obtained by the Bakul Institute for Superhard Materials of the National Academy of Sciences of Ukraine, are given. [ABSTRACT FROM AUTHOR]

Mechanical tests for evaluating the fracture resistance of ceramics on chipping rectangular specimen edges were first performed with conical diamond indenters of 11-800 μm tip radii. In the experiments elastic silicon-nitride SiN ceramics and inelastic zirconia TS ceramics, displaying a phase transition on fracture, were used, soda lime glass served as the object for comparison. The fracture resistance of those materials was shown to increase similarly when the above indenters are employed. Chip scars formed on the specimen edges of SiN ceramics do not actually differ from those observed in glass tests (pseudoconical, nail-like, arbitrary). With an increase in the tip radius, the first ones grow in number. It does not take place in the tests of TS ceramics. Comparative fracture resistance estimates obtained with the edge chipping method using different fracturing tools were established to be invalid. [ABSTRACT FROM AUTHOR]

Fiber-metal laminates (FMLs) composed of metal lamina and reinforcing fibers exhibit an improved crack growth resistance as compared to conventional aircraft materials. The crack resistance of a hybrid FML composite consisting of LY12M aluminum alloy lamina and prepreg layers of HS2 glass fibers and T700 carbon fibers has been experimentally investigated on tensile specimens at stress ratios R = - 1, 0.1, and 0.5. The crack growth rate data at positive stress ratios demonstrate a significant scatter and a large difference between specimen's back and front surfaces formed by 1-mm thick outer aluminum layers. This violation of the test standard requirements for metal materials in the performed FML tests presents a problem of determining a proper failure criterion for the engineering applications of FMLs. Based on the comparative analysis of FML crack resistance results obtained, some qualitative conclusions are drawn on solving this problem. [ABSTRACT FROM AUTHOR]

The fatigue crack growth in single edge-notched specimens made of steel 25Kh1M1F has been investigated at a stress ratio of R = 0 and under controlled loading force. The influence of the loading frequency (0.1 and 1.0 Hz) on the crack growth rate has been estimated. The crack resistance of the bimaterial 25Kh1M1F/15Kh13MF as well as the influence of elastic and plastic properties on the nature of the fatigue crack growth have been investigated. [ABSTRACT FROM AUTHOR]

Dynamical strength and crack resistance experimental results have been analyzed. Space-time fracture model and criterion have been proposed, which describe the space-time fracture pattern. [ABSTRACT FROM AUTHOR]

We study the mechanical properties of Mg–Li alloys obtained by high-pressure die casting in a cold pressing chamber. A procedure of evaluation of the principal strength characteristics obtained under the conditions of high plastic strains is presented. The fracture toughness of materials under quasistatic and dynamic concentrated loads is investigated. [ABSTRACT FROM AUTHOR]

We describe the methods used for the evaluation of strength of butt welded joints according to their crack resistance both by the axial tension of small-sized cylindrical specimens with circular cracks and three-point bending of beam specimens with lateral cracks induced in low-or medium-strength surfacings welded into the high-strength bases of these specimens. We perform the comprehensive investigation of the near-weld zone of butt welded joints, where the lowest crack resistance is exhibited by the metal of the zone of fusion and the highest crack resistance is detected for the source material independently of the type of tested specimens. Metals of the weld and the heat-affected zone are characterized by higher values of crack resistance than the zone of fusion and lower values of crack resistance than the source material. By analyzing, as an example, the case of axial tension of a tubular specimen with internal circular crack in the surfacing, we study the crack resistance of seven types of inhomogeneous welded joints depending on the mode of thermal treatment of 35 and St. 3 steels after normalization and 09G2S, 20Kh, and 30KhGSA steels after hardening, medium tempering, and normalization. [ABSTRACT FROM AUTHOR]

On the basis of theoretical and experimental investigations, we determine the moment of start of a macrocrack by the acoustic-emission method. The amplitudes of acoustic-emission signals accompanying plastic deformation near a notch of the corresponding radius are evaluated. Finally, the acoustic-emission signals are compared with those generated at the moment of the macrocrack start. [ABSTRACT FROM AUTHOR]

We study the temperature dependences of the mechanical characteristics and crack resistance of 17G1S-U, 17GS, 14G2AF-U, and VSt.3kp steels and their welded joints (longitudinal welded pipes). By using the results of tensile testing of smooth microspecimens, we theoretically determine the critical value of the stress intensity factor. The temperature–strain rate dependences of the mechanical characteristics of the metal of large-diameter pipes are established experimentally. [ABSTRACT FROM AUTHOR]

Premature fracture of steam pipelines from 14MoV6 3 steel designed for 100,000 h service life at $\backslash 540\; deg$C has been studied. Test specimens are manufactured from steel in the initial state and after 117 h of operation. Application of the local approach to fracture and the metallographic analysis, in addition to classical methods (tensile, crack resistance, and fatigue strength tests), provided a more precise evaluation of steel properties degradation due to elevated temperatures and stresses. Urgency of further development of the local approach to predicting material fracture after long-term operation at high temperatures has been substantiated. [ABSTRACT FROM AUTHOR]