Static Crack Resistance Research Articles

Determination of metal crack resistance on half-size Charpy specimens at cryogenic temperatures

The work consists in the manufacture of experimental equipment for fracture toughness tests at cryogenic temperatures in the range from -196 °Cto -80 °Cand the analysis of the efficiency of determining the reference temperature of a metal by the maker curve for static crack resistance in accordance with ASTM E 1921 and ASTM E399. Liquid nitrogen and its vapours were used to cool the samples to the required temperature. The temperature stability during sample loading was ensured within ±0.5°C around the required temperature. Pre-cracking of the samples was carried out at room temperature in accordance with ASTM E647. Experimental studies on static crack resistance are carried out for three-point bending using half-size Charpy specimens (55×10×5 mm) for the material - 10GN2MFA steel, which is used as the main metal for the main circulation pipelines of VVER-1000. The reference temperature is determined by processing a set of relevant experimental data.

On Unusual Fractures Occurring under Testing Static Crack Resistance

On Unusual Fractures Occurring under Testing Static Crack Resistance

Brittle fracture resistance of the Fe–17%Ni alloy of extraterrestrial origin (the chinga meteorite) under static and dynamic loading

The values of conditional yield strength, impact strength, and the parameters of static and dynamic crack resistance of the Chinga meteorite substance (the Fe–17%Ni alloy) are determined by mechanical testing for uniaxial tension and static bending and instrumented impact testing of V-notched prismatic samples at temperatures ranging between 20 and −196 °С. It is shown that the dynamic nature of material loading according to the three-point bending scheme contributes to a noticeable decrease in impact strength (KCV) and crack propagation energy (KCT) during low-temperature tests (−196 °С) and the appearance of brittle quasi-cleavage sites on the fracture surface. On the contrary, as the test temperatures decrease from room temperature to the temperature of liquid nitrogen during the static tests of the Chinga meteorite samples with induced cracks, the parameter of static crack resistance KC (K1C) increases and the fracture surface of the samples after low-temperature testing is characterized by predominantly viscous dimpled topography.

Structure and Crack Resistance of Maraging Steels under Single Loading

Abstract—The effect of structural parameters on the strength and crack resistance of maraging steels under single loading has been studied. It has been established that the increase in strength due to dispersion hardening with intermetallic compounds in the studied steels is determined by the titanium content and the presence of cobalt. It was shown that the change in crack resistance under single loading with increasing tempering temperature has a complex character. It has been established that, in contrast to dynamic crack resistance, the minimum level of static crack resistance characteristics of the studied steels corresponds to the maximum strength. Possible reasons for such a change in the characteristics of static and dynamic crack resistance are discussed.

Effect of Hydrogen on the Static Crack Resistance of 05Kh13N8M3 Cast Martensitic Steel

Effect of Hydrogen on the Static Crack Resistance of 05Kh13N8M3 Cast Martensitic Steel

Assessment of the residual life of turbine runners with operational defectiveness

The residual life of the runners of hydraulic turbines in the presence of operational defects is estimated. The main problems of the operation of hydraulic turbines associated with technological defects and exhaustion of the standard resource are described. The main requirements for initial data to be used in estimation of the residual resource and the requirements for predicting the residual resource of runners based on the results of surveys and analysis of their technical condition are specified. We have classified and briefly described the applied approaches and techniques used in estimation of the residual resource. The main damaging factors affecting the residual life of the runners are revealed: deformation aging of the metal, cavitation, corrosion and fatigue damage to the elements of runners. The most characteristic defects are divided into three groups: zones of cavitation erosion; corrosion-fatigue cracks; and weld defects. Particular attention is paid to corrosion-fatigue cracks identified using flaw detection. The mechanism of crack formation and the most probable location of the cracks in the runner are shown. Statistical data on the number of cracks at the onset of the runner operation and at the time of shutdown maintenance are presented. The main statistical parameters of the sample and the parameters of crack size distributions including the distribution law are determined. The distribution law is exponential for the crack length parameter; whereas for the crack opening width it is log-normal. The revealed multidirectional cracks are located at the surface, subsurface or inner layer of the metal. They arise from operational defects (ulcers, craters, undercuts or delamination) and grow during operation of the turbine units. We also present the design schemes of elements with cracks used for quantification of resources according to the criteria of fracture mechanics. The results of calculations for static and dynamic crack resistance are presented as the dependence of stress intensity factors on the crack size. The levels of the total accumulated damage to the runners, the values of the residual life at the stage of crack nucleation and development were determined for 11 hydraulic units in the «start-stop» and «working» cycles. The main conclusion is that the total operating time of the hydraulic turbine runners significantly exceeds the standard operating life, while the residual resource is insufficient for a further period of long-term operation.

On Experimental Procedure Development for Evaluating the Effect of Biaxial Loading on the Static Crack Resistance Characteristics of Reactor Vessel Steels

On Experimental Procedure Development for Evaluating the Effect of Biaxial Loading on the Static Crack Resistance Characteristics of Reactor Vessel Steels

Strength and Fracture Mechanism of an Ultrafine-Grained Austenitic Steel for Medical Applications.

In this paper, we study the corrosion-resistant austenitic steel Fe-0.02C-18Cr-8Ni for medical applications. The microstructure and mechanical properties (tensile mechanical properties, torsional strength, impact toughness, and static and cyclic crack resistance) under different types of loading of the steel are investigated. The results are compared for the two states of the steel: the initial (coarse-grained) state and the ultrafine-grained state produced by severe plastic deformation processing via equal-channel angular pressing. It is demonstrated that the ultrafine-grained steel 0.08C-18Cr-9Ni has essentially better properties and is very promising for the manufacture of medical products for various applications that experience various static and cyclic loads during operation.

Crack Resistance Evaluation for 38KhN3MFA-Sh Structural Steel According to Fracture Characteristics and Elastic Wave Velocities

The results of studies on the effects of tempering temperature on the static strength and crack resistance of 38KhN3MFA-Sh steel are presented. The fractured surface texture has been studied after different heat treatment modes using electron fractography analysis. Relationships between the fracture characteristics and the critical stress intensity factor have been established. The effect of tempering temperature on the velocities of ultrasonic bulk waves has been investigated. A linear relationship between the velocity of elastic waves and the critical stress intensity factor of 38KhN3MFA-Sh steel has been found. The revealed relationship makes it possible to estimate the changes in crack resistance of steel using a nondestructive testing method with variations in the tempering temperature. The method of electron fractography has been used to analyze the fractures in the samples of structural high-quality 38KhN3MFA-Sh steel with a crack. The investigations of the fractured surface texture after different heat-treatment modes have shown that the microrelief is represented by flattened cone-shaped pits. The increase in the tempering temperature is accompanied by an increase in the diameter of flattened pit-cones on the fracture surface. A quadratic relationship between the crack resistance parameter and the diameter of the pits has been revealed. It has been shown that the contribution of the structural condition of the ferrite matrix to crack resistance is much more significant than the contribution of isolated carbides. The elastic wave velocities in steel have been measured; their values exhibit an increase with an increase in the tempering temperature. The strength and crack resistance parameters of the structural steel exposed to high-temperature tempering have been predicted on the basis of the values of transverse wave velocities. The deviations of the predicted values of crack resistance K1c and ultimate strength σu from the experimental values do not exceed 5.4 and 12.6%, respectively.

Static Crack Resistance of Heat-Resistant KhN43MBTYu Nickel-Chromium Alloy in Gaseous Hydrogen

The effect of hydrogen at a pressure of up to 35 MPa and a content of up to 29 ppm on the strength, ductility, short-term and long-term static crack resistance of four KhN43MBTYu (EP-915VD) alloy modifications with different heat treatment modes and chemical composition has been studied. It has been found that the critical stress intensity factor KIc in the presence of hydrogen, just as the ductility characteristics of smooth specimens, depends on the deformation rate, reaching minimum values at rates of less than 0.1 mm/min. The fracture toughness decreases under the action of hydrogen by a factor of 2.5, and the plane strain state occurs at a much smaller specimen thickness. An optimal combination of high strength, ductility, short- and long-term static crack resistance in air and hydrogen has been achieved in a fine-grained alloy with low carbon and sulfur content. Based on the results of long-term static crack resistance tests at the predetermined maximum fatigue test duration of 300 h, the invariant characteristics of crack resistance, the threshold values of stress intensity factor in hydrogen, have been determined, which vary from 23 to 48 MPa ∙ m1 / 2 depending on the alloy heat treatment mode.

Resistance of high-strength medium-alloy steel to brittle fracture and its connection with structural state parameters

Tests were carried out on the static crack resistance of sheet metal for experimental melts of highstrength martensitic-bainitic steel. They showed significant differences in the quality of the metal in this characteristic with relatively small differences in the content of alloying elements and production technology. A comparative metallographic analysis of the structural state of the metal, which differs in crack resistance, is performed. Based on the results of this analysis, the main microstructural factors are identified that correlate with the static crack resistance of the investigated material.

On the Role of Gamma-Alpha Transformation in the Increase of the Fracture Toughness of Maraging Steel

Based on the results of the static crack resistance tests, the contribution of the strain-induced γ→α-transformation into the fructure toughness of steel N18K9M5T with two-phase (α+γ)-structure has been assessed.

2024 aluminum alloy ultrahigh-strength sheet due to two-level nanostructuring under cryorolling and heat treatment

Abstract The effect of rolling to a total effective strain of 2 at the liquid nitrogen temperature and subsequent natural and artificial aging on the structure and service properties of the pre-quenched hot-pressed 2024 aluminum alloy was investigated. Using optical and electron microscopy, and X-ray analysis, it was found that the cryorolling did not qualitatively change the type of the initial coarse-fibered microstructure, but produced a well-developed nanocell substructure inside fibers. Further aging led to decomposition of the preliminary supersaturated and work-hardened aluminum solid solution and precipitation of strengthening phases in the statically recovered and/or recrystallized matrix. As a result, the rolled and naturally aged alloy demonstrated the yield and ultimate tensile strengths (YS= 590 MPa, UTS=640 MPa) much higher than those in the pressed and T6-heat treated alloy at equal elongation to failure (El˜6%). Artificial aging at a temperature less than conventional T6 route could provide the extra alloy strengthening and the unique balance of mechanical properties, involving enhanced strength (YS=610 MPa, UTS=665 MPa) and ductility (El ˜10%), and good static crack resistance (the specific works for crack formation and growth were 42 and 18 kJ/m2, respectively) and corrosion resistance (the intensity and depth of intercrystalline corrosion were 23% and 50 μm, respectively).

Кинетика и механизм разрушения в области малоцикловой усталости и трещиностойкость магниевого сплава Mg6Al после отжига и равноканального углового прессования

Кинетика и механизм разрушения в области малоцикловой усталости и трещиностойкость магниевого сплава Mg6Al после отжига и равноканального углового прессования

Evaluating the Static Cracking Resistance of Thin-Walled Pressure Vessels

The failure criteria and limit states for thin-walled pressure vessels are analyzed taking into account the influence of plastic deformations. The introduction describes the main problems in operation of thin-walled vessels working under internal excess pressure associated with technological defectiveness and reduction of the residual life. Typical technological and operational defects in welded joints of vessels and statistical data on their number and types are presented. The share of welding defects is 62% of the total number of defects, and the share of remaining types of defects is significantly less. Histograms of the dimensions of welding defects are drawn, and the distribution laws are determined: the length of undercuts is described by a lognormal distribution law, whereas the depth of undercuts is described by a normal distribution law. Further, the limiting states and fracture criteria of vessels with defects and cracks under the conditions of elastoplastic deformation of material are indicated. The advantages of using the generalized equations of the form of J-curves are shown for calculation of the cracking resistance. A formula is given for calculating the J-curves that associate the dimensionless J-integral with the dimensionless load. The stress-strain state of a thin-walled vessel with a surface half-elliptical crack and an internal elliptical crack is analyzed in a volumetric setting. The peculiarities of the stress and strain fields in the local region of the crack zone under elastoplastic deformation are investigated. For the vessel model with a surface half-elliptical crack and an internal elliptical crack under elastoplastic deformation, the energy criterion of fracture mechanics, the J-integral, is calculated, and the results of calculations are presented. The results are presented in the form of graphs of the dimensionless J-integral versus the geometrical dimensions of the vessel and crack. The equations of J-curves are obtained and the limit load for thin-walled vessels is determined which depends on geometrical dimensions, loading parameters, strength properties of the material, and characteristics of cracking resistance and deformation. From the J-curves and the deformation curve, a formula is derived for determining the dependence of the limit load on the crack size, loading parameters, and characteristics of the material. Using this formula, the dependences of the limiting pressure of the vessel under elastoplastic deformations on the ratio of crack length a to vessel wall thickness S (a/S) is plotted for surface and internal cracks with different ratios R/S (R is the shell radius) and Jc which allow estimating the limiting pressure levels of the safe operation of thin-walled vessels.

Microstructural evolution and corrosion behavior of friction stir processed fine‐grained AZ80 Mg alloy

AbstractFriction stir processing (FSP) and subsequent aging heat treatment were used to modify the microstructure of AZ80 magnesium alloy. The influence of FSP and aging heat treatment on the corrosion behavior was systematically studied by using potentiodynamic polarization, immersion, and slow strain rate tensile tests. The results revealed that FSP led to grain refinement, rapid dissolution of β‐phase, and the deflection of c‐axis from transverse direction (TD) and processing direction (PD) by approximately 55° and 25°, respectively, improving the static corrosion and stress corrosion cracking (SCC) resistance. The aging heat treatment rendered a little influence on the grain size and slightly affected the grain orientation. The content of β‐phase in FSP‐5 and FSP‐24 samples was 8.1 and 21.8 wt.%, respectively. Static corrosion and SCC resistance of FSP‐5 and FSP‐24 samples were lower than those of the FSP samples. Compared with FSP‐5 sample, the amount of β‐phase and the proportion of Al2O3 increased in FSP‐24 sample, leading to enhanced static corrosion and SCC resistance. SCC behavior was controlled by anodic dissolution, whereas the presence of hydrogen accelerated the SCC.

Comparison of the characteristics of modern aluminum alloys

The results of experimental studies of the static strength, fatigue and crack resistance of modern improved aluminum alloys 1163ATV, 1163RDTV, 1441RT1, 1163T, 1163T7, 1161T, V95ochT2, B96-3pchT12., 1973T2 developed at the All-Russian Scientific Research Institute for Aviation Materials (VIAM, Russia); and 2524-T3, 6013-T6 HDT, 2324-T39, C433-T351, 7055-T7751 developed at ALCOA (USA) are presented. Those materials are used in the construction of modern operated and designed aircraft. The experimental data were obtained in testing standard specimens on electro-hydraulic machines MTS (USA), Instron (Great Britain) and Schenk (FRG). The tested specimens were cut from semi-products manufactured according to serial technologies. The mechanical properties of materials under tension (σb, σ0.2, δ), fatigue characteristics, fatigue crack growth rate, stress crack propagation curves under static loading (R-curves), conditionally critical stress intensity factors are determined according to domestic standards. To ensure high weight efficiency combined with a high resource and high performance characteristics of the aircraft structures, aluminum alloys must have the following set of necessary characteristics: high resistance to variable loads, low rate of fatigue cracks growth, required residual strength, and good corrosion resistance. The obtained results of experimental studies clearly demonstrate the advantages and disadvantages of the strength properties of the materials under study. The results of experimental studies provide the possibility to compare the strength properties of the materials under study to optimize their use for the specific zone of the structure and thus increase the life and safety of the aircraft design.

Evaluation of crack resistance of 38KhN3MFA-Sh structural steel by fracture properties and elastic wave velocities

The results of the study on the effects of tempering temperature on 38KhN3MFA-Sh steel static strength and crack resistance are given. The fractured surface texture has been studied after various heat treatment modes using electron-fractography analysis. Relationships between the fracture properties and the critical stress intensity factor have been established. The effect of tempering temperature on the velocities of ultrasonic bulk waves has been investigated. A linear relationship has been found between the velocity of elastic waves and the critical stress intensity factor of 38KhN3MFA-Sh steel. The dependence discovered allows us to estimate the changes in crack resistance of steel using a non-destructive test method with variations in tempering temperature. The method of electron fractography has been used to analyze specimen fractures having a crack in structural high-quality 38KhN3MFA-Sh steel. Investigations of the fractured surface texture subjected to various heat treatment modes have shown that the microrelief is represented by flattened cone-shape pits. An increase in tempering temperature is accompanied by an increase in the diameter of flattened cone pits on the fractured surface. A quadratic dependence has been established between the crack resistance parameter and the diameter of the pits. It is shown that the contribution of ferrite matrix structural condition to crack resistance value is much more significant than the contribution of isolated carbides. The velocities of elastic waves in steel have been measured; their values increase with the growth of tempering temperature. The characteristics of strength and crack resistance of structural steel exposed to hightemperature tempering have been predicted based on the values of transverse wave velocities. Deviation of the predicted values of crack resistance K1С and ultimate strength σb from the experimental values does not exceed 5.4% and 12.6%, respectively.

CRACK RESISTANCE OF CENTRIFUGAL-CAST STEAM P I P E S MADE OF STEEL 1 5 K h l M l F

Significance of assessing the characteristics of crack resistance of centrifugal-cast (CFC) tubes made of steel 15KhlMlF is substantiated. The characteristic features of the metal under study are considered along with the methodology of crack resistance tests. Three modifications of the metal of the CFC pipes in the initial state, that differ in the degree of developed segregational heterogeneity, and two modifications of the metal with almost the same degree of structural heterogeneity, which differ in the running time as a part of the operating equipment are studied. It is shown that a decrease in the intensity of the segregational damage of the metal structure has almost no effect on the level of the strength characteristics, but is accompanied by an increase in the plastic properties and toughness of the metal. We present the results of studying crack resistance of centrifugal-cast pipes (steel 15KhlMlF) under static, cyclic and prolonged static loading in conditions of high-temperature creep. It is shown that an increase in the degree of structural heterogeneity of the CFC steel adversely affects the characteristics of static crack resistance and cyclic crack resistance as well, but to a far lesser extent and has little if any effect on the crack resistance of the CFC steel at creep. Cyclic crack resistance slightly decreases with increase of the operating time of the metal in functioning pipelines, whereas the characteristics of the prolonged static crack resistance get worse significantly. It is shown that welded joints of the CFC pipelines have a substantially lower crack resistance under creep conditions compared to the base metal. Statistical processing of the experimental data is used to get kinetic diagrams of cyclic and prolonged static crack resistance of 15X1M1F CFC-steel in creep conditions. The results of practical applications of the obtained crack resistance characteristics of the metal of the CFC pipes are demonstrated with a focus on the problems of ensuring the reliable operation of the steam pipelines of thermal power plants at the end of their service lifetime.

ПОВЫШЕНИЕ ТОЧНОСТИ И ОБЕСПЕЧЕНИЕ НАДЕЖНОСТИ ОПТИЧЕСКИХ СИСТЕМ ПРИ ПРОВЕДЕНИИ ИЗМЕРЕНИЙ

This article examines the vibration-proof devices, essentially improving accuracy of measuring. A new thermographic method for definition of static crack resistance characteristics of material is offered. This method allows define these characteristics more precisely and quickly, increasing the reliable of optomechanical devices.