Mechanics and physics of quasibrittle fracture of polycrystalline metals under the conditions of stress concentration. Part I. Experimental results

In the present work, which consists of three parts, we make an attempt to formulate the fundamental ideas of the theory of quasibrittle fracture of polycrystalline metals and structural alloys on the macroscopic and microscopic levels. Within the framework of the usual approach, we consider physical aspects of the problem of analysis of the elementary acts of fracture of the crystal lattice and regularities of macroscopic fracture under conditions of stress concentration in connection with the problem of structural integrity. We propose simple and convenient practical criteria of quasibrittle fracture under conditions of stress concentration and formulate physically substantiated concepts of the ductility of metals. In the first part of the work, we present the results of the experimental investigation of the fracture of specimens with stress concentrators of various geometries and fatigue cracks, establish the regularities of changes in the local characteristics of fracture, and analyze the principal factors affecting the value of the critical cleavage stress σ_f. Institute of Physics of Metals, National Academy of Sciences of Ukraine, Kiev, Ukraine. Translated from Problemy Prochnosti, No. 4, pp. 5–16, July–August, 1997.     

A new approach to the evaluation of danger of short fatigue cracks

We demonstrate the possibility of using new physically substantiated characteristics of toughness of metals and the embrittling action of stress concentrators proposed by the authors somewhat earlier in the analysis of brittle fracture initiated by short cracks. We describe a procedure for the experimental determination of a parameter characterizing the embrittling action of these defects and obtain an approximate expression for its evaluation in the presence of a short crack. For typical structural steels, we establish the dependence of the toughness margin of steel with a short crack on the level of strength of this material. We also formulate the requirements on the level of toughness of steels guaranteeing that defects of this type are not dangerous. Institute of Physics of Metals, National Academy of Sciences of Ukraine, Kiev, Ukraine. Translated from Problemy Prochnosti, No. 3, pp. 106–114, May–June, 2000.     

Mechanics and physics of quasibrittle fracture of polycrystalline metals under conditions of stress concentration. Part 2. Theoretical basis

We propose a physical model of quasibrittle fracture of polycrystalline metals as applied to the most general case of the complex stressed state and nonuniform distributions of stress and strain fields. Within the framework of this model, on the basis of the analysis of elementary acts of the process of initiation and loss of stability of incipient cracks, we explain, for the first time, the basic regularities of the variation of the local stress of quasibrittle fracture σ_c, of a metal under the conditions of stress concentration. It is shown that the value of σ_c depends not only on the length of incipient cracks but also on their number in the local plastic zone at the notch tip and on the law of their distribution over orientations. On this basis, we explain the causes of the, influence of the acuity of the notch and testing temperature on σ_c. It is shown that the application of the proposed model in combination with the finite-element method makes it possible to predict (with sufficiently high accuracy) both the mean value and the spread of the fracture load of a specimen with stress concentrator of any acuity, including fatigue cracks. Institute of Physics of Metals, National Academy of Sciences of Ukraine, Kiev, Ukraine. Translated from Problemy Prochnosti, No. 3, pp. 5–16, May–June, 1999.     

Fracture toughness diagrams of a notched body

To describe fracture toughness diagrams of notched bodies, a model of the cohesion zone near the notch root and an averaging criterion of stresses in this zone were employed. The geometric stress concentration factor and biaxiality coefficient affect greatly the shape of fracture toughness diagram. The notch root critical stress intensity factor is a decreasing function of geometric stress concentration factor. __________ Translated from Problemy Prochnosti, No. 5, pp. 142–148, September–October, 2006. Report on International Conference “Dynamics, Strength, and Life of Machines and Structures” (1–4 November 2006, Kiev, Ukraine).     

Simulation of the limit state of thin-walled pipes with repeating axial defects

The existing standards for simulation of multiple defects are, as a rule, created for the analysis of brittle strength of cracked bodies. At the same time, it seems much more important to be able to determine the ultimate load of ductile fracture P_LL. We propose a general analytic procedure for the evaluation of P_LL for pipes with two equal surface defects (cracks or concentrators) or through cracks. Analysis of the accumulated data shows that the generally accepted rules for simulation of defects are nonconservative and may lead to qualitative errors. Scientific and Engineering Center of Materials-Science Support of the Production and Certification of Equipment of Nuclear Power Plants at the Institute for Problems of Strength of the National Academy of Sciences of Ukraine, Kiev, Ukraine. Translated from Problemy Prochnosti, No. 2, pp. 172 – 180, March – April, 1998.     

Prediction of the fracture of metals in the processes of cold plastic deformation. Part 1. Approximate model of plastic deformation and fracture of metals

We propose an approximate semiphenomenological model of the joint process of cold plastic deformation and fracture of metals. Within the framework of this model, for 10kp, 20kp, 20G2R, and 38KhGNM steels, we show that moving dislocations overcome barriers through a force process. The formation of nascent microcracks is also realized through a force process, i.e., local stresses in the “head” of an arrested dislocation pileup attain the levels of theoretical strength. We also suggest a general algorithm of the application of the proposed model to the prediction of fracture of metals in technological processes of plastic metal working. Ufa State Aviation Technical University, Ufa, Russia. Translated from Problemy Prochnosti, No. 1, pp. 76–85, January–February, 1999.     

Investigation of structural conditions for improving the characteristics of strength, plasticity, and fracture toughness of maraging steels

The structural parameters governing the embrittlement and plasticizing of stainless (Kh12N10MT, Kh11N10M2T, and Kh13N9D2MT) and high-strength (N18M3T and N18K9M5T) maraging steels are analyzed; they are varied by thermal-kinetic conditions, the sequence and multiplicity of various heat-treatment processes such as austenitization, age hardening, and α↔γ-phase transformation in the subcritical temperature range. We justified both the individual and joint influence of austenite grain size and volume fraction of particles of intermediate phases on the regularities of variation and correlation of characteristics of strength, plasticity, and fracture toughness during thermal hardening. We revealed the role of the austenite which is formed during the α→γ-phase transformation in the increased deformability and fracture toughness of the steels under study, taking into account the character of their alloying. Based on the revealed shift in the temperature range of the α→γ-phase re-transformation toward the temperature range of age hardening, we determined the structural conditions for implementing its plastizicing effect on the high-strength state of the maraging steel. Timoshenko Institute of Mechanics, National Academy of Sciences of Ukraine, Kiev, Ukraine. Translated from Problemy Prochnosti, No. 2, pp. 51–61, March–April, 2000.     

Fracture toughness anisotropy of partially stabilized ZrO2 crystals in the plane (001)

Partially stabilized ZrO₂ crystals doped with Tb₂O₃ exhibit fracture toughness anisotropy in the plane (001) in Vickers indentation. The anisotropy is determined over a range of loads that give rise to fully developed radial cracks around the indenter impression. Extensive lateral cracking and surface chipping obscure the observed anisotropy of fracture toughness at relatively high loads. Institute of Problems of Strength, National Academy of Sciences of Ukraine, Kiev, Ukraine. Translated from Problemy Prochnosti, No. 5, pp. 86–95, September–October, 1999.     

Effect of crack closure on the experimentally measured characteristics of cyclic crack-growth resistance of steels

On the basis of the proposed approach and an original procedure for determination of the current value of crack tip opening displacement, we obtain new characteristics of cyclic crack-growth resistance for which the effect of crack closure disappears. We experimentally discovered a phenomenon of natural pulsations of crack closure with constantly decreasing amplitude and duration of pulses to the minimum stress intensity factor K _min in a cycle for any positive asymmetry of cycles. We construct a model of damped pulsations of crack closure depending on the maximum stress intensity factor K _max in a cycle and asymmetry of cycles. This model is used to show that the kinetic diagrams of fatigue fracture are invariant for any positive asymmetry of cycles within the limits of the presence of the effect of crack closure. The observed effect of asymmetry quantitatively takes into account the range of pulsating stress intensity factors of crack closure ΔK _cl ^op in the range of stress intensity factors ΔK in a cycle. Practical experience shows that the procedure for quantitative investigation of the kinetics of growth of fatigue cracks extensively used in linear fracture mechanics and based on analysis of the coefficients of crack opening displacement U and γ and functional dependences of the stress intensity factor of crack opening displacement K_op on k _max is inconsistent. We propose a new experimental approach to the qualitative analysis of the kinetics of growth of fatigue cracks based on evaluation of the effect of crack closure and expressed in terms of the effective range of stress intensity factors ΔK_eff in the case of its simple determination from the kinetic diagrams of fatigue fracture together with K_op. We hope that the accumulated experimental data and the proposed model of damping pulsations of crack closure will lay a foundation for a new understanding of the natural resistance of materials to fracture under cyclic loading. Scientific and Engineering Center of Materials-Science Support of the Production and Certification of Equipment of Nuclear Power Plants at the Institute for Problems of Strength of the National Academy of Sciences of Ukraine, Kiev, Ukraine. Translated from Problemy Prochnosti, No. 2, pp. 161 – 171, March – April, 1998.     

Using a quantum-chemical approach for the investigation of corrosion on surfaces of metals and alloys

We analyze results of using quantum chemistry methods for simulating and calculating the interaction of surfaces of metals and alloys with a corrosive medium in the cluster approximation. Models of corrosion dissolution of brass and intermetallics of an aluminum alloy are constructed, and energy characteristics of the interaction of components of a chloride-containing medium with their surfaces are determined. We determine energy barriers of ionization of clusters of the components of brass and the CuAl₂ intermetallic in a medium, which enables us to propose the mechanism of their fracture. A model of the contact interaction of dissimilar metals Al–Fe, Al–Cr, Cu–Al, and Cu–Fe in the presence of particles of the corrosive medium is considered, the values of the adhesion energy of the corresponding clusters of dissimilar metals is computed, and its dependence on the composition of the medium is found. The prospects and efficiency of the quantum chemistry methods for the study of corrosion fracture of metals and alloys are shown.     

Crack resistance of the near-weld region of welded joints of titanium and aluminum alloys

We studied the crack resistance K_c (K_Ic) of the near-weld region of welded joints of VT3-1 and VT14 titanium alloys and AMg6NPP, 12401, and 1420 high-strength aluminum alloys under fatigue and static loading. By the methods of fracture mechanics and according to the results of testing cylindrical specimens with external circular cracks initiated in the weld metal, fusion zone, zone of thermal influence, and parent material, we established that the minimum crack resistance is observed in the fusion zone and the maximum crack resistance in the parent material. The weld metal and the zone of thermal influence are characterized by intermediate values of crack resistance both under fatigue and static loading. By using 1402 high-strength aluminum alloy as an example, we studied the influence of the texture, procedure of welding, modes of thermal hardening, and scaling factor on the character of brittle fracture of the metal in the near-weld region. To enhance the characteristics of crack resistance of the near-weld region of welded joints in its weakest zones (weld and fusion zone), we propose efficient methods for welding these alloys, e.g., electron-beam welding together with the efficient choice of the modes of thermal hardening instead of argon-arc welding. This enables one to improve the purity and homogeneity of the structure of the metal in the molten-metal bath in the process of welding by decreasing its porosity and the number of inclusions and cracks appearing in these zones. “L’vivs’ka Politeknika” State University, L’viv, Ukraine. Translated from Problemy Prochnosti, No. 5, pp. 89–99, September–October, 1998.     

The equation of the second portion of the fatigue fracture kinetic diagram for various additional load ratios

The second portion of the fatigue fracture kinetic diagram is described by a reliable equation with additional load ratios R in the case of the crack-tip closure effect and by the Forman equation for the values of load ratio R^* in the case without the crack closure effect. We have obtained the analytical dependence of the effective range of the stress intensity factor ΔK_eff on the nominal range ΔK_R for the first part of the second portion of the fatigue fracture kinetic diagram (ΔK_1-2,R≤ΔK_R≤ΔK_fcl,R) with various load ratios. Institute of Problems of Strength, National Academy of Sciences of Ukraine, Kiev, Ukraine. Translated from Problemy Prochnosti, No. 2, pp. 75–80, March–April, 2000.     

Influence of the pliability of a testing machine on the resistance of metals to deformation in the case of jump-like development of their elastoplastic deformation

We analyze the influence of the stiffness of a testing machine and the loading mode on the mechanical characteristics and resistance of metals to deformation in the case of nonmonotonic development of their elastoplastic deformation caused by cooling to 4.2 K and (or) the action of pulses of electric current. We propose an approach that enables one to reduce the value of a jump of loading caused by the action of a pulse of electric current for an arbitrary loading mode to the maximum possible value (for a constant strain rate and a small amount of elastic energy accumulated by a loading device). Institute of Problems of Strength, National Academy of Sciences of Ukraine, Kiev, Ukraine. Translated from Problemy Prochnosti, No. 3, pp. 125–132, May–June, 2000.     

On the connection between the adhesive strength and temperature drops in heat-protective coatings

We propose a procedure and results of experimental determination of the dependence of adhesive strength of a ZrO₂ plasma-sprayed heat-protective coating with NiCr sublayer on the value of the stationary temperature drop in it. The dependences of the adhesive strength on the temperature of the surface of adhesive contact are compared for homogeneous and inhomogeneous temperature fields. The results obtained enable us to substantiate the choice of temperature drop in heat-protective coatings with regard for the adhesion safety factor. Institute for Problems of Strength, National Academy of Sciences of Ukraine, Kiev, Ukraine. Translated from Problemy Prochnosti, No. 4, pp. 126–130, July–August, 1998.     

Abnormal manifestation of the high-temperature degradation of the weld metal of a low-alloy steel welded joint

We have shown that hardness, impact toughness, mechanical properties in tension, and the local parameters of fracture mechanics (static and cyclic crack resistance) are sensitive to the operating degradation of weld metal of steam pipelines of thermal power plants made of 15Kh1M1F steel. The simultaneous decrease in the resistance to brittle and plastic fracture (hardness, strength, and impact toughness) represents a phenomenon of the operating degradation of weld metal. We have established a specific correlation between the characteristics of plasticity and other mechanical parameters of operated metal: the increase in °₅ of operated weld metal is in good agreement with the decrease in its strength, whereas the reduction of Ψ correlates with the lowering of resistance to brittle fracture. Electrolytic hydrogenation decreases the characteristics of strength and plasticity of operated weld metal much stronger than in the initial state. The absence of ferritic edgings on the boundaries of primary austenitic grains makes for a low resistance to brittle fracture, and the change in acicular ferrite deteriorates the mechanical properties. The ductile fracture of nonoperated metal is replaced by brittle intercrystalline failure in operated metal. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 43, No. 1, pp. 73–79, January–February, 2007.     

Correlation between the fracture roughness and fatigue threshold of high-temperature degraded steel

We analyze the influence of degradation of 12Kh1MF heat-resistant steel under in-service and laboratory conditions on the characteristics of surface roughness after fatigue fracture. It is shown that the behavior of the threshold stress intensity factor ΔK _{th cl} characterizing the effect of crack closure is similar to the behavior of the amplitude characteristics of surface roughness in the direction perpendicular to the direction of crack propagation. This enables to make a conclusion that the decisive role in the process of crack closure in the degraded metal is played by longitudinal shear. It is shown that hydrogen absorbed by the degraded metal determines the characteristics of its crack resistance and affects the process of crack closure. We propose a model of crack closure that additionally takes into account the influence of longitudinal shear. Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv; Technical University of Wrocław, Poland. Published in Fizyko-Khimichna Mekhanika Materialiv, Vol. 35, No. 6, pp. 37–42, November–December, 1999.     

Fracture toughness studies on V-notched ceramic specimens

The fracture toughness of silicon nitride, alumina, and zirconia ceramics was compared by the SEVNB method. This method, easily realized in practice both for compact and for composite ceramics, is shown to provide small scatter of data. The K_lc values determined by this method correspond to those obtained by the SEPB method. Institute of Problems of Strength, National Academy of Sciences of Ukraine, Kiev, Ukraine. Translated from Problemy Prochnosti, No. 1, pp. 120–127, January–February, 2000.     

Propagation of a through crack in a sheet material under biaxial tension

We present the results of investigation of the effect of the form of the stressed state and low temperatures on the growth of plastic strains at the crack tip in sheet materials subjected to repeated static biaxial loading. Information of this sort enables one to obtain adequate estimates of the load-carrying capacity of plates with cracklike concentrators by determining the plastic component of strains at the crack mouth and to introduce the required corrections in the existing models of fracture mechanics. Institute for Problems of Strength, National Academy of Sciences of Ukraine, Kiev, Ukraine. Translated from Problemy Prochnosti, No. 4, pp. 52–59, July–August, 1998.