Critical Stress Intensity Coefficient Research Articles

Determination of Crack Resistance of the Cover and Slide Glass by Indentation Method with the Visualization Using Atomic Force Microscopy

Crack resistance of two types of glass was studied – cover glass (0.17 mm thick) and slide glass (2 mm thick) using an improved technique through the use of the probe methods, which makes it possible to increase the accuracy of determining the crack resistance of glass. Colorless silicate glass was used. Crack resistance was determined by the Vickers pyramid indentation method. Microstructure of glasses surface and deformation region after indentation were studied using an atomic force microscope. Mechanical properties of glasses were determined by nanoindentation. Surface relief of a glass slide is rougher than that one of a cover glass. Roughness Rz for a cover glass is less than for a slide glass. Specific surface energy value of 0.26 N/m is higher for the slide glass compared to the coverslip. One elastic modulus value E of the cover glass is 48 GPa, and that one of the slide glass is 58 GPa. The microhardness value H is almost the same for by the glasses and amounts to 6.7 GPa for a slide glass and 6.4 GPa for a cover glass. Atomic force microscope images of deformation region after indentation with a Vickers pyramid show that the first cracks appear at a load of 1 N on the slide glass, and at 2 N on the cover glass. At a load of 3 N, the cover glass is destroyed. Based on the results of crack resistance calculations it was found that critical stress intensity coefficient KIC values are 1.42 MPa∙m1/2 for a glass slide, and 1.10 MPa∙m1/2 for a cover glass.

Беттік нығайтылған бөлшектердің тозуы кезінде беріктігін бағалау әдістемесі

The article considers the method of durability during wear of surface-hardened parts. The increase in the strength of equipment is primarily due not only to an increase in the reliability of its main components, aggregates, connections and parts, but also to a reduction in costs and fuel consumption during production processes. The surface quality of machine parts includes both geometric characteristics and physical and mechanical properties. The study of the influence of the roughness of working surfaces on their wear resistance has shown that the minimum surface roughness does not always ensure its high wear resistance. The relationship between wear resistance and mechanical parameters is always determined experimentally. The coefficient p is proposed as a generalized parameter, to determine which it is necessary to consider the combined effect of the parameters specified in the article. Apparently, as a generalized criterion for evaluating the effectiveness of the developed carbonitration regime, a parameter controlled by the hardness and strength of the core of the product, as well as the hardness and strength of the surface-hardened layer, should be used. In turn, the article considers a number of ways to express the critical stress intensity coefficient through the tensile strength and yield strength. The research took into account the well-known results of scientific work carried out at the Department of Mechanics and Mechanical Engineering on state budget and grant topics in the field of mechanical engineering.

The Effects of Corrugated Steel Fiber on the Properties of Ultra-High Performance Concrete of Different Strength Levels

This article describes the influence of corrugated steel fiber on the mechanical properties and fracture energy of Ultra-High Performance Concretes (UHPC) of various strength levels. Three UHPC formulations with compressive strengths of 143, 152, and 177 MPa were tested. The following parameters for the formulations without fiber and those containing 2% steel fiber by volume were determined: compressive strength, splitting tensile strength, flexural strength, modulus of elasticity, Poisson’s ratio and critical stress intensity factor. From the axial tensile test results, the following parameters were obtained: the cracking stress, tensile strength, and fracture energy of Ultra-High Performance Fiber Reinforced Concrete (UHPFRC) of different strength levels. With the introduction of steel fiber, an increase in all the investigated parameters is observed regardless of the strength of the concrete matrix. The most remarkable influence the fiber has on the splitting tensile strength, flexural strength and critical stress intensity coefficient, the increase is up to 1.6–3.2 times. There was a slight increase in compressive strength and elastic modulus—up to 5.0–7.4% depending on the composition. Poisson’s ratio was equal to 0.2 regardless of the strength of the concrete matrix and the presence of steel fiber. Based on the test results, equations were proposed to predict the properties of UHPC and UHPFRC depending on the water–cement ratio, silica fume content, cement compressive strength and the volumetric content of corrugated steel fiber. The calculated and experimental values showed good convergence with a correlation coefficient in the range of 0.885–0.997.

Strength and crack resistance of cement composites under multilevel reinforcement

The working hypothesis is confirmed that the required fracture toughness of structural concrete can be provided by multi-level reinforcement: at the level of the crystalline joint of cement stone – carbon nanotubes, and at the level of fine-grained concrete – various macro-sized fiber fibers (steel, polymer). Reinforcement of a crystalline splice with carbon nanotubes leads to an increase in tensile strength by 20 %, an increase in Young’s modulus. With dispersed reinforcement of concrete modified with nanoparticles at the level of fine-grained concrete, the tensile strength increases by 109 %, the critical stress intensity coefficient (crack resistance index) increases by 280 % at normal separation, and by 48 % at transverse shear.

Композиты системы Al2O3/Yb-TZP, модифицированные катионами кальция, стронция и бария

The paper discusses the possibility of implementing two strengthening mechanisms in a ceramic system of Al2O3 and ZrO2 simultaneously: namely, transformational, due to the tetragonal form of zirconium dioxide and dispersion, due to the in situ formation of hexaaluminates of alkaline earth elements. The concentrations of modifying additives of calcium, strontium and barium, providing the formation of the corresponding hexaaluminates, have been established. The influence of the formed phases on the microstructure and strength characteristics of the obtained composites is considered. On composites including strontium cations, high strength parameters were obtained, flexural strength — 700 MPa, the critical stress intensity coefficient K1c reaches 10 MPa·m1/2, which is due to the optimal effect of a combination of transformation and dispersion hardening effects.

Application of the ultrasound control to evaluation of the crack-resistance of low-carbon martensite steel 07Kh3GNMYuA

The goal of the study is to reveal the impact of change in the structural state of steel 07Kh3GNMYuA after heat treatment on the values of the critical stress intensity coefficient (K1c) obtained at a temperature of 50°C and on the velocity of ultrasound wave propagation, as well as to determine a correlation between them for rapid assessment of the crack resistance using acoustic characteristics. The mechanical characteristics of the material and the critical stress intensity coefficient K1c were obtained on the test machine «Inspekt 100 Table». The tangent method is used for determination of K1c. Three samples per K1c value were used in the experiment for a three-point bending scheme at the operating temperature T = –50°C. Acoustic parameters were measured using the echo-pulse method. The results of ultrasonic scanning of heat-strengthened samples made of steel 07Kh3GNMYuA demonstrated the possibility of non-destructive quantitative evaluation of the critical stress intensity coefficient. New data on the mechanical properties of steel 07Kh3GNMYuA and on the correlation between the velocity of longitudinal elastic waves and the values of the critical stress intensity coefficient of structures were obtained. Deviation of the calculated values of K1c obtained using acoustic measurements from the experimental values does not exceed 10%. The proposed model, which explains change in the acoustic characteristics of steel 07Kh3GNMYuA on the basis of phase changes occurring in the steel structure upon tempering, provides conducting of similar studies for other modes of heat treatment and other steel grades. The method is a low labor- and time-consuming procedure for determination of the mechanical characteristics of the products made of steel 07Kh3GNMYuA, since it does not require the manufacturing of samples and their testing. The developed procedure can be proposed for manufacturing application, as the main or additional method for evaluation of the mechanical parameters of materials after various modes of heat treatment.

Critical Stress Intensity Coefficient at Transverse Shear for Nanofibrobeton

Critical Stress Intensity Coefficient at Transverse Shear for Nanofibrobeton

Crack Resistance Increase of Bushings while Assembling Joints with Tension

In order to restore worn parts during repair work, it is often used to press the repair bushings. While assembling joints, cracks may form in the spanning part (bushing), as it may have surface and structure defects. Therefore, an urgent task is to increase the crack resistance of joints with tension during its assembly. The paper examines the influence of the geometry of the spanning part on the process of cracking. To assess the effect of defects on the crack formation, the method of modeling defects with a surface crack of a semi elliptical shape was used. The crack resistance is estimated using the force criterion of fracture mechanics – the stress intensity coefficient. As a result, it was found that the ratio between the wall thickness of the enclosing sleeve and its outer diameter has a significant effect on the crack resistance of the joint with tension, which allows reducing the origin probability and crack development during its pressing by varying these parameters. While assembling joints, the following methods are often used: mechanical-using a press and thermal with heating of the covering part. When the method of joint with cooling of the covered part, the greatest strength of the joint is achieved, but the probability of cracking increases. This is because when the temperature of the enclosing sleeve decreases in contact with the cooled shaft, the critical stress intensity coefficient (fracture toughness), which is a mechanical characteristic of crack resistance, decreases. To reduce the effect of cooling the sleeve, you can use a combined method of assembling the joint, in which the covered part is cooled and the covering part is heated. It is shown that to assess the fracture resistance at low temperatures, it is optimal to conduct full-scale tests, where the cooling and heating temperatures of the press joint parts are the experimental factors.

RELIABILITY ANALYSIS OF REINFORCED CONCRETE FLEXURAL ELEMENT ON CRACKING BASED ON FRACTURE MECHANICS

The article describes a probabilistic approach to the reliability analysis of a flexural reinforced concrete element by the cracking criterion using the provisions of fracture mechanics. Two mathematical models of limit state are proposed for reliability analysis: with the evaluation of the critical stress intensity coefficient directly and through the design concrete compressive strength. On the basis of regression analysis, the relationship between the critical stress intensity coefficient and the design concrete compressive strength is established which can be used in the inspection of reinforced concrete structural elements. The influence of the design concrete compressive strength on the failure (cracking) probability of the flexural reinforced concrete element is analyzed. The numerical example of reliability analysis is given for the reinforced concrete beam by the criterion of cracking. It is noted that the required level of reliability should be set for each structural object individually based on the acceptable risk value using economic and non-economic losses.

A PROBABLISTIC APPROACH TO EVALUATION OF THE ULTIMATE LOAD ON FLEXURAL RC ELEMENT ON CRACK LENGTH

The fracture mechanics of concrete and reinforced concrete is a promising direction in the development of methods for reinforced concrete structural elements design and inspection. At the same time, probabilistic methods of design and behavior analysis of structural elements are of particular interest. The article describes a probabilistic approach to load-bearing capacity and reliability analysis of flexural reinforced concrete elements based on the crack length criterion. The functional relationship between the critical stress intensity coefficient of concrete and the design compressive strength of concrete is given. The article presents a method for the reliability analysis of flexural reinforced concrete elements at the operational stage with limited statistical data about the critical stress intensity coefficient of concrete. The ultimate value of the failure probability (or reliability index) should be set for each object individually based on the value of the acceptable risk.

Predicting the critical stress intensity in structural steels

A method is proposed for predicting the critical stress-intensity coefficient K 1c of structural steels (their crack resistance) on the basis of uniaxial tensile tests of standard samples. The derivation of this method relies on the concepts of mechanical stability and embrittlement when nonuniform force fields act on a metal. The strain-hardening index n of the metal at the critical transition temperatures from the plastic state to the quasi-brittle state (T c or T 0) and from the quasi-brittle state to the brittle state (T k2) plays a key role here. The proposed method may prove effective in monitoring the crack resistance of steels certified in scientific and plant laboratories.

Sr-anorthite glass ceramic with enhanced crack resistance, reinforced with silicon nitride particles

It is demonstrated that composite materials based on Sr-anorthite glass ceramic can be synthesized with finely dispersed α-Si3N4 and β-Si3N4 powders introduced as fillers. A study of the influence exerted by the polymorphic forms of fillers, their morphology, dispersity, and concentration and also by the nature of the matrix on the synthesis, structure, and properties of the composites demonstrated that the mechanism and temperature range of sintering in syntheses of materials are determined by the nature of the matrix and the degree of densification and the structure of the composite depend on the concentration, dispersity, and shape of filler particles, their phase composition, and wettability of the glass-phase matrix. In syntheses of glass-crystalline composites by the hot compaction method, raising the filler content from 10 to 70 vol % leads to a substantial decrease in the density of the materials (by 20%); the maximum content of the filler should not exceed 30 vol %. The use of both α-Si3N4 and β-Si3N4 in an amount of 30% made it possible to raise the critical stress intensity coefficient of the strontium-aluminosilicate matrix by more than a factor of 2. A study of the thermal properties of the composites demonstrated that the glass-crystalline matrix can protect nonoxide fillers from oxidation by atmospheric oxygen at elevated temperatures, with the oxidation onset temperature increased by 300°. The developed class of composites is promising for application in airspace technology, chemical industry, and motor-vehicle construction.

Origins of brittle microcracks in crack resistance tests of steels of VVER-1000 vessel in various states

In fractographic study of steels of nuclear reactor vessels after tests for crack resistance, two major sources of brittle destruction are detected: nonmetallic impurities and intergranular or subgranular boundaries. X-ray spectral microanalysis reveals that these are manganese sulfides and silicon oxides. The probable size of nonmetallic impurities (∼1 μm) which can initiate brittle destruction is detected. The interrelation between critical stress intensity coefficient KJc and distance from “leader” to fatigue crack point CID (cleavage initiation distance) is considered. The influence of irradiation conditions on crack resistance of steels is demonstrated.

Evaluation of the effect of perforations in the collector vessel of a steam generator on the critical stress intensity coefficient

An approach to calculating the effective critical stress intensity coefficient for mainline cracks in the perforation zone of the first-loop collector of a VVER steam generator is examined. The energy criterion is chosen as the condition for crack stability: the decrease of the elastic deformation energy which is due to the opening of a growing crack should not exceed the change of the surface energy of the crack in the process. The difference between the critical stress intensity coefficient for a mainline crack in the continuous and perforated regions is analyzed. Relations making it possible to lower the critical stress intensity coefficient as a function of the parameters of the perforation (spacing, diameter of openings) as well as the dimensions and orientation of the mainline crack in the perforation zone are presented.

Brittle failure resistance in modified epoxide compounds

Finely divided inorganic fillers are used in the epoxide compounds to increase the hardness and wear resistance and to reduce the level of thermal deformations, and particularly to reduce the cost. The critical stress intensity coefficient for normal detachment KIC was less than a substantial factor than the critical stress intensity coefficient for antiplanar shearing KIIIC for all the materials. These data can be used to define filled composites for particular working conditions on the basis of cracking resistance.

Properties of AMg5 Alloy Welded Joints in Cryogenic Engineering

Measurements have been made on the temperature dependence of the critical stress intensity coefficient and stress concentration coefficient as regards the strength of the alloy, the few-cycle fatigue, and the heating time under stress as affecting the mechanical properties.

Estimating working life for aircraft gas turbine engine disks at the stage of fatigue crack development. Part 1

Studies have been performed on the static and cyclic cracking resistance (failure ductility) of aircraft gas turbine disk materials at normal and elevated temperatures and at various frequencies and for various thicknesses. When the conditions of planar strain are violated for these materials and these geometrical dimensions, one can estimate the critical stress intensity coefficient K{sub 1C} from the critical loading P{sub Q} obtained on intersection of the 5% secant with the strain diagram.

Fracture mechanics of metal matrix-metal fibre composites

A model of the metal matrix-metal fibre composite has been constructed. The critical stress intensity coefficient has been estimated taking into account the increase of the energy absorbing capacity of a fibre surrounded by a plastic matrix due to the increase of the elongation at rupture of the fibre under these conditions. The results of experiments carried out on aluminium matrix-steel fibres composites support the validity of the model. The effect of conditions at the fibre-matrix interface on the fracture toughness of the composite has also been studied.

Correlation between the rupture resistance and low-cycle fatigue of samples and real structures of ferritic ? Pearlitic steel

1. The critical stress-intensity coefficient for the brittle failure of samples of steel 09G2S is no lower than 247 kg/mm3/2; for real constructions the value of KIc may fall to 175 kg/mm3/2. 2. The life of butt welds based on the criterion of the first crack exceeds 105 loading cycles; in the region of undercuts or in the case of extreme angularity of the shell welds, fatigue failure may occur after 5·104 cycles. 3. The methods tested may prove extremely useful in determining the rupture resistance and low-cycle fatigue of structures made from low-carbon and low-alloy steels.