Investigation into threshold stress intensity factors of materials under cyclic loading. Report 2. Prediction of fatigue limits and fatigue crack growth

On the basis of the results described in Report 1 [Probl. Prochn., No. 4, 5–16 (1998)], the author considers the possibility of using threshold stress intensity factors for the prediction of fatigue limits for titanium alloys by the formulas of linear elastic fracture mechanics and presents a substantiation of the method for predicting fatigue crack growth in structural elements. Institute for Problems of Strength, National Academy of Sciences of Ukraine, Kiev, Ukraine. Translated from Problemy Prochnosti, No. 5, pp. 5–11, September–October, 1998.     

Estimation of the effect of vibration treatment on the life of low-carbon steel welded joints

Using a complex limit stress diagram, we determine nondamaging conditions of vibration treatment of steel 20 welded joints, which ensure a decrease of residual stresses down to 0.5 of the yield strength of the material. We carry out comparative fatigue tests of specimens with high levels of residual stresses after vibration and thermal treatment and analyze the test results. Institute for Problems of Strength, National Academy of Sciences of Ukraine, Kiev, Ukraine. Translated from Problemy Prochnosti, No. 2, pp. 106–114, March–April, 1999.     

On a reliable equation for the second portion of the fatigue crack growth diagram

The fatigue crack growth kinetics on the second portion of the fatigue crack growth diagram is described using an equation that takes into account both the current crack tip opening displacement and the instant when the crack closure in a cycle vanishes and does not appear until the specimen failure. This instant corresponds to the critical effective SIF range, ΔK_fcl. Institute of Problems of Strength, National Academy of Sciences of Ukraine, Kiev, Ukraine. Translated from Problemy Prochnosti, No. 3, pp. 83–87, May–June, 1999.     

Influence of the parameters of temperature cycles on the cyclic fracture toughness of 5Kh3V3MFAS steel

We performed experimental investigations of the crack resistance of tool steel under conditions of nonisothermal loading and established the effect of the time of holding at the maximum temperature of thermal cycles on the critical size of a fatigue crack and cyclic fracture toughness. Institute for Problems of Strength, National Academy of Sciences of Ukraine, Kiev, Ukraine. Translated from Problemy Prochnosti, No. 4, pp. 34–38, July–August, 1998.     

Analysis of experimental data on low-cycle fatigue in nonproportional deformation

We have analyzed experimental data on low-cycle fatigue in biaxial nonproportional deformation of various materials. To establish a correlation with the fatigue limit, we have proposed a strain parameter-the reduced range of strain. For most of the materials under consideration this correlation dependence can be obtained from uniaxial low-cycle fatigue experiments and can be represented by a linear function. Institute of Problems of Strength, National Academy of Sciences of Ukraine, Kiev, Ukraine. Translated from Problemy Prochnosti, No. 1, pp. 13–21, January–February, 2000.     

Prediction of fracture characteristics of metals from high-frequency test data

A method is proposed for the prediction of cyclic crack resistance characteristics of metallic materials under low-frequency loading from high-frequency test data, which is based on a model of development of local plastic deformation regions during the accumulation of fatigue damages and fatigue crack growth with allowance for cyclic loading rate. We performed a comparative analysis of the results of prediction of fatigue fracture diagrams with test data for VT22, VT18U, VNS-25, and AMg6N alloys in a frequency range of 20 Hz–10 kHz. Report on International Conference “Dynamics, Strength, and Life of Machines and Structures” (1–4 November 2005, Kiev, Ukraine). __________ Translated from Problemy Prochnosti, No. 2, pp. 121–128, March–April, 2007.     

Prediction of fatigue strength characterics for cylindrical shells with an annular concentrator

We present a procedure of experimental and theoretical prediction of fatigue strength characteristics for cylindrical shells with an annular concentrator under distributed axial cyclic loads. The results of fatigue testing of segments of the shells prepared by partitioning the shells along their generating lines are used as initial data for the prediction of the fatigue characteristics of the shells. The procedure involves statistical prediction. It is based on an approach within the theory of reliability whereby the probability of failure of a system is calculated from the data on the probabilities of failure of the system components. Based on the calculation results, we plot the curves of distribution of the fatigue lives of the shells versus cyclic load. S. P. Timoshenko Insitute of Mechanics, National Academy of Sciences of Ukraine, Kiev, Ukraine. Translated from Problemy Prochnosti, No. 6, pp. 23–31, November–December, 1998.     

Procedure and results of the investigation of full-scale lock joints of blades of compressors of gas-turbine engines for fretting fatigue

We analyze the experimental procedures aimed at testing specimens which simulate contact pairs and full-scale lock joints of blades of compressors of gas-turbine engines with grooved disks for fretting fatigue. The experimental results corroborate the efficiency of these procedures and enable one to choose coatings and procedures of treatment of contacting surfaces which guarantee the attainment of higher values of the endurance limit than those exhibited by blade roots under the conditions of fretting. The indicated modes of treatment are characterized by high technological and economical efficiency. Institute for Problems of Strength, National Academy of Sciences of Ukraine, Kiev, Ukraine. Translated from Problemy Prochnosti, No. 4, pp. 137–144, July–August, 1997.     

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. Institute for Problems of Strength, National Academy of Sciences of Ukraine, Kiev, Ukraine. Translated from Problemy Prochnosti, No. 4, pp. 60–67, July–August, 1998.     

A machine for fatigue testing under the joint action of pure bending and torque moment

This paper describes a machine for fatigue testing of specimens of materials and structural elements under the joint action of pure bending and torque moment. We discuss the loading setup of the machine and the design and kinematic features of the mechanisms, which eliminate the mutual influence of the loading components. Performance specifications of the machine are given. Timoshenko Institute of Mechanics, National Academy of Sciences of Ukraine, Kiev, Ukraine. Translated from Problemy Prochnosti, No. 4, pp. 132–135, July–August, 1999.     

Procedure for the investigation of crack resistance of materials of specimens with a corner crack under cyclic loading

We propose a computational and experimental procedure for the investigation of the influence of thermomechanical loading on the propagation of a corner fatigue crack under cyclic loading. The taring dependence of the stress intensity factor on the relative radius of the crack is plotted. The limited kinetic diagrams are presented for KhN73MBTYu (éI698VD) high-temperature chromiumnickel alloy for temperatures of 20 and 600°C. Institute for Problems of Strength, National Academy of Sciences of Ukraine, Kiev, Ukraine. Translated from Problemy Prochnosti, No. 4, pp. 116–125, July–August, 1998.     

Investigation into threshold stress intensity factors of materials under cyclic loading. Report 1. Methods and experimental results

On the basis of the results of studying threshold stress intensity factors of stainless and heat-resistant steels and titanium and nickel alloys, the author considers the correlation between numerical values of the threshold stress intensity factors and the yield stress, ultimate strength, and fatigue limit, as well as the influence of temperature, environment, cycle asymmetry, and overloads on the threshold stress intensity factor. Institute for Problems of Strength, National Academy of Sciences of Ukraine, Kiev, Ukraine. Translated from Problemy Prochnosti, No. 4, pp. 5–15, July–August, 1998.     

Investigation of the effect of surface treatment on the cyclic strength characteristics of nickel alloy by the acoustic emission method

Using the acoustic emission method, we study deformation processes in a plastic structural material with different technological inheritance properties under static and cyclic loading condition. The dependence of the total acoustic emission on applied stresses is shown to contain a special point characterizing the moment of change in the stress-strain state of the surface layer of the material. We derive relationships for the evaluation of fatigue limits of structural materials by acoustic emission inspection results. Zaporozh'e State Technical University, Zaporozh'e, Ukraine. Translated from Problemy Prochnosti, No. 3, pp. 139–145, May–June, 1999.     

Prediction of the fatigue life of a notched body by the local stress-strain state. Part 2. Determination of a parameter representing the fatigue life of bodies with stress concentrators

We have studied the dependence of the fatigue life of smooth and notched specimens on various parameters of the notch-tip stress-strain state in the region of high-cycle fatigue prior to crack initiation. The tests were carried out on specimens of 30KhGSA steel and D16AT and AMtsM alloys with various stress concentrators. Local strains were measured using strain gauges with a small nominal length by the hysteresis loop method. It is shown that in the case of equal values of residual-stress amplitudes, the values of fatigue life coincide. We have proposed a relationship for calculating the residual-strain amplitude and have compared the calculated results with the experimental data. Kharkov Aviation Institute, Kharkov, Ukraine. Translated from Problemy Prochnosti, No. 3, pp. 46–55, May–June, 2000.     

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.     

Comparative estimation of mechanical properties of cemented carbides by the kinetics of penetration of a rigid punch under contact cyclic loading

We suggest a method of contact fatigue testing of cemented carbides. It involves cyclic loading of a specimen (half-space) with a flat circular punch by a pulsating compressive stress. Both the loading punch and the test specimens are made of cemented carbides. It is found that the parameters of contact fatigue of a cemented carbide include the depth of the punch penetration into the specimen, σ, after a given number of cycles, the form of the dependence of the penetration depth on the number of cycles, the number of loading cycles between abrupt changes in the growth of the σ values, and the number of these changes. With these parameters determined by laboratory tests on small specimens, we can predict the durability of large carbide components subjected in operation to contact cyclic loads. V. N. Bakul Institute for Superhard Materials, National Academy of Sciences of Ukraine, Kiev, Ukraine. Translated from Problemy Prochnosti, No. 3, pp. 28–37, May–June, 1998.     

Special features of high-frequency loading of coated materials

We study the features of high-frequency loading of coated components: different combinations of stresses in the adhesive contact zone, which are caused by dynamic deformation of a base material and the action of body inertial forces, and the effect of cyclic loading on the adhesive and cohesive strength of a coating. We introduce the notion of “vibration strength” of a coating and show that it is essential to ensure an equal fatigue life of the base-coating system under cycling in an operating environment. Institute for Problems of Strength, National Academy of Sciences of Ukraine, Kiev, Ukraine. Translated from Problemy Prochnosti, No. 5, pp. 134–138, September–October, 1988.     

Prediction of the fatigue life of a notched body by the local stress-strain state. Part 1. Determination of stresses and strains in a notch under elastoplastic cyclic deformation

We have carried out an experimental investigation of local strains in specimens with various stress concentrators under cyclic and monotonic loading conditions. The strains in notches were measured by a strain gauge with a small nominal length. Using the test results for, smooth specimens, we have plotted cyclic stress-strain diagrams for steel and two aluminum alloys. The values of the full strain amplitude calculated by the known approximate methods were compared with experimental data. The Neuber formula with the Makhutov correction function has been found to ensure the best agreement with the experimental data. We have given recommendations regarding the application of this function for calculating the local strain amplitudes under loads corresponding to high-cycle fatigue and to a region of transition from low- to high-cycle fatigue. Kharkov Aviation Institute, Kharkov, Ukraine. Translated from Problemy Prochnosti, No. 3, pp. 37–45, May–June, 2000.     

Efficiency of the method of spectral vibrodiagnostics for fatigue damage of structural elements. Part 4. Analysis of distortion of harmonicity of vibration cycle of beams with closing transverse cracks

Using an asymptotic method of nonlinear mechanics and concepts of linear fracture mechanics, we derive analytical expressions that relate the constant component and amplitudes of even harmonics, and the ratio of maximum deviations and half-periods of displacements of opposite signs in a vibration cycle of prismatic beams to the length of a closing transverse crack. The analytical solution is compared to the data of experimental determination of these parameters in bending vibrations of rectangular cantilever beams with an edge transverse crack. Analysis of the results of analytic and experimental investigations has shown that harmonic analysis of the strain waveform of beam-type elements in resonance vibrations can be used as an efficient diagnostic method for detecting fatigue damage such as closin cracks. Institute of Problems of Strength, National Academy of Sciences of Ukraine, Kiev, Ukraine. Translated from Problemy Prochnosti, No. 1, pp. 5–12, January–February, 2000.     

Fatigue strength and life of compressor blades for marine gas turbine engines

An experimental procedure has been developed for the investigation of fatigue and crack growth resistance of materials and real compressor blades. Methods for the determination of stress intensity factors in specimens and in blades with cracks have been justified. Investigations have been performed on the influence of manufacturing residual stresses and surface defects in the form of simulators of dents, corrosion pits, and nonmetallic inclusions on the fatigue strength of steels and a titanium alloy. The characteristics of the material crack growth resistance have been studied taking into account the effect of the medium (sea water), stress ratio in a cycle, and programmed mode of loading. The authors also consider fatigue strength of newly-manufactured blades and those in operation, as well as specific features of fatigue crack propagation in blades. They have substantiated a method for predicting the life of blades with cracks. Institute for Problems of Strength, National Academy of Sciences of Ukraine, Kiev, Ukraine. Translated from Problemy Prochnosti, No. 1, pp. 28–40, January–February, 1999.