Characteristics Of Cyclic Crack-growth Resistance Research Articles

Specific Features of the Fatigue Fracture of Welded Joints of 34KhN2MA Steel Formed by Electrodes with Different Phase Compositions

We study the cyclic crack resistance of the metal of welded joints of 34KhN2MA thick-plate steel obtained with the help of ferritic and austenitic electrodes. It is shown that the weld metal has a higher crack resistance than the base metal only within the second part of the kinetic diagrams of fatigue fracture. In the first and third parts, the characteristics of the weld metal are worse. The results of fractographic investigations demonstrate that the decrease in the fatigue crack-growth resistance in pure austenitic welds is explained by the presence of hot cracks in the zone between the near-root metal of the welded joint and the base metal. As for the mixed ferrite-austenitic welds, the decrease in the fatigue crack-growth resistance is explained by inadmissibly large slag inclusions creating additional stress concentration and deteriorating the characteristics of cyclic crack-growth resistance.

Fatigue Crack-Growth Resistance of Thermally Hardened 65G Steel Under Transverse Shear

We plot the kinetic diagrams of fatigue fracture of 65G steel under the conditions of transverse shear after different modes of heat treatment within the range of fatigue crack-growth rates covering six orders of magnitude with the use of the stress intensity factor for the evaluation of the stress-strain state in the vicinity of the crack tip. The main and auxiliary characteristics of cyclic crack-growth resistance are determined with regard for the friction of the lips of a fatigue crack. We construct the diagrams of structural strength and confirm the hypothesis that the fatigue thresholds increase with the yield strength of 65G steel under the conditions of shear.

On the concept of selection of steels for high-strength railroad wheels

On the basis of the results of statistical analysis of the in-service defects of high-strength railroad wheels and the data of related laboratory investigations of wheel steels, it is shown that the major part of these defects are pockmarks and flats formed on the rolling surface. Therefore, it is necessary to change the concept of selection of steels for high-strength wheels in order to take into account not only the characteristics of strength and hardness (wear resistance) but also the characteristics of crack resistance under the conditions of contact fatigue. The new concept should be based on the approaches of the structural mechanics of fatigue fracture and aimed at the production of high-strength wheel steels with lowered carbon contents and higher characteristics of cyclic crack-growth resistance under the conditions of mode I and mode II fracture.

Service life and fracture of solid bodies under the conditions of cyclic contact interaction

A numerical-analytic model is formulated for the investigation of fracture processes and evaluation of the residual service life of solid bodies under the conditions of cyclic contact interaction. The model is based on the criteria of fracture of materials in the process of growth of fatigue cracks and the algorithms of step-by-step construction of the paths crack propagation developed by the method of singular integral equations for two-dimensional contact problems of the theory of elasticity for bodies with curvilinear cracks. The model is realized for contact interactions of rolling and fretting fatigue. The causes and specific features of the formation of typical defects in rolling bodies of revolution, such as pitting, spalling, squat defects, and cracking are analyzed depending on the operating parameters of a rolling couple. It is shown that, both in the case of rolling and under the conditions of fretting fatigue, the character of fracture of the contact surface and the sizes of debris particles are strongly affected by the value of the friction coefficient, the distribution of tangential forces between contacting bodies, the characteristics of cyclic crack-growth resistance of the material, and the service or random products in the contact zone (water, lubricants, powder fretting products, etc.). We present examples of evaluation of the residual service life of rail (RSB12) and roller (9KhF and GH) steels.

On the Kinetics of Elastoplastic Growth of Through Fatigue Macrocracks in Metals under Extreme and Natural Operational Conditions

The relationship between the empirical parameters C 1 and C 2 established earlier by the author makes it possible to avoid the necessity of conducting, under extreme and natural conditions, of a complex experiment aimed at the evaluation of the effective characteristics of cyclic crack-growth resistance of a structural material and the upper bound ΔK fcl,X of growth of a through fatigue macrocrack in the presence of crack-tip closure. The results of the basic tests performed to determine the nominal effective characteristics of cyclic crack-growth resistance and the indicated relationship reliably describe the kinetics of growth of macrocracks in the entire range of the kinetic diagram of fatigue fracture with the help of an equation taking into account the regularities of its behavior in a broad range of action of various factors under extreme and natural conditions and in the presence of crack closure. Under the indicated conditions, we determine the durability of the material of a cracked element and its dangerous limiting state. The analysis of the kinetics of growth of a through fatigue macrocrack shows that the stability of its propagation is guaranteed by the presence of residual compression stresses along its lips.

Description of the kinetics of growth of a fatigue crack on the basis of the criterion of current crack tip opening displacement

We suggest a microprocessor-based data-processing and measuring system and a special device aimed at the determination of current values of the crack tip opening displacement. They are used to establish current values of the crack tip opening displacement at working frequencies of loading of the specimen and describe the kinetics of fatigue crack growth. A decrease in the loading frequency of the specimen from 15 to 0.1 Hz in the course of measuring the current values of the crack tip opening displacement does not affect the reliability of determination of the characteristics of cyclic crack-growth resistance. The reliability of the results is affected only by the inertia of the mass of the gauge case.

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 Kop 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 ΔKeff in the case of its simple determination from the kinetic diagrams of fatigue fracture together with Kop. 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.

Effect of the microstructure on the low-temperature cyclic crack-growth resistance of high-strength cast irons

The regularities of the influence of temperature on the cyclic crack-growth resistance of high-strength nodular cast irons unalloyed and alloyed with nickel are studied in the temperature range 77–293°K for matrices of different structural types (ferritic, ferrite-pearlitic, and pearlitic). It is shown that ferritic high-strength nodular cast irons subjected to high-temperature annealing and high-strength nodular cast irons with the structure of granular pearlite subjected to hardening and high tempering are characterized by satisfactory low-temperature fatigue crack-growth resistance (up to 213°K). The results of mechanical tests are in good agreement with the data of microfractographic analysis. It is demonstrated that the characteristics of cyclic crack-growth resistance of the investigated types of high-strength nodular cast iron are in no case worse than the corresponding characteristics of cold-resistant structural steels frequently used in mechanical engineering.