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.     

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.     

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.     

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.     

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.     

Thermal fatigue crack propagation in massive cylinders during their cyclic heating

Processes that reduce the level of maximum unsteady stresses during thermal fatigue crack propagation in massive bodies during their convective cyclic heating are considered. It is shown that during crack propagation, the stress intensity factor K₁ increases initially to a certain limit K _1max and then decreases. The factor K _1max depends on the initial and boundary conditions of heat exchange as well as on the rates of decrease in the maximum stresses during thermal fatigue crack propagation. It is shown that the asymmetry of boundary and initial conditions of heat exchange can contribute to either acceleration or retardation of crack propagation depending on the material properties and heating conditions. Deceased. Institute of Problems of Strength, National Academy of Sciences of Ukraine, Kiev, Ukraine. Translated from Problemy Prochnosti, No. 4, pp. 68–80, July–August, 1999.     

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.     

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.     

The influence of porosity on the fatigue crack growth behavior of Ti–6Al–4V laser welds

The effect of porosity––a common welding defect––on the fatigue crack growth rate (FCGR) in Ti–6Al–4V laser welds was investigated. The experimental results reveal that porosity was present in partial penetration welds over a narrow fusion zone (FZ) with martensite structure. The FCGR of the FZ was lower than that of the base plate. The fracture surface morphology of weld metal was much rougher as compared to that of the base plate. Randomly oriented martensite in the FZ led to local cleavage fracture along a preferred plane, thus, altering the crack growth direction significantly out of the primary crack plane. The zigzag crack path in the FZ resulted in a reduced FCGR at a given ΔK compared to the base plate. Besides, the porous weld showed a serration on the crack growth curve, and behaved the similar crack growth characteristics as the defect free one. SEM fractography revealed that the deflection of crack path around porosity together with local notch blunting as the crack tip pierced into porosity, balanced the increased FCGR for the occurrence of instant crack advance as the crack front reached the porosity at a low stress ratio. In contrast, the serration and drop in FCGR occurred sparingly at a high stress ratio as the crack front met the porosity.     

Fatigue fracture of AMg6N alloy under loading at sound frequencies

Cyclic crack growth resistance tests of AMg6N alloy under loading at frequencies of 20 to 10 kHz have demonstrated that the rate of fatigue crack propagation decreases with increasing frequency and the threshold stress intensity factors increase exponentially with the frequency of strain cycling. Fractographic observations of fracture surfaces of the specimens have revealed that an increase in the loading frequency is accompanied by a decrease in the fatigue striation spacing and in the size of the striation microzones by intnsifying the processes of secondary cracking and the formation of fretting products. This leads to a decrease in the rate of fatigue crack propagation and an increase in the threshold values of the stress intensity factors. An increase in the asymmetry of the loading cycles reduces the contribution of delamination and the formation of fretting products to the process of fracture of the alloy and results in a smaller fraction of the striation relief and in an earlier occurrence of the elements of quasistatic fracture by dimples, which is the cause of the reduction in characteristics of the cyclic crack growth resistance under asymmetrical loading. Institute for Problems of Strength, National Academy of Sciences of Ukraine, Kiev, Ukraine. Translated from Problemy Prochnosti, No. 2, pp. 94–105, March–April, 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.     

Statistical description of the effect of Zr addition on the behavior of microcracks in Cu–6Ni–2Mn–2Sn–2Al Alloy

As possible substitutes for high-strength Cu–Be alloys, Cu–6Ni–2Mn–2Sn–2Al alloys have been developed. To clarify the physical background of the effect of trace Zr on the fatigue strength of such alloys, the initiation and propagation behavior of a major crack that led to the fracture of the tested specimens was monitored. When the stress amplitude was less than σ _a = 350 MPa, the fatigue life of the alloys with Zr was about 2–2.5 times larger than that of the alloy without Zr. When σ _a > 350 MPa, the effect of Zr addition on the fatigue life dramatically decreases as the stress amplitude increases. The increased fatigue life due to Zr addition resulted from an enhancement of the crack initiation life and microcrack growth life. The enhanced crack initiation life was mainly attributed to the strengthening of grain boundaries due to the precipitation of SnZr compounds. A statistical analysis of the behavior of multiple cracks was made to quantitatively evaluate the scatter in fatigue behavior. The statistical analysis supported the conclusions obtained from the behavior of a major crack.     

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.     

On investigation of the kinetics of small surface cracks in light alloys under high-frequency loading

For the case of cyclic loading of metal materials in the frequency range from 100 to 10,000 Hz, we propose a method of studying small crack growth using a system of measuring the response of a defect section of material to the action of a high-frequency electromagnetic field. We present approaches to the construction of a measuring system for determination of the length of small surface cracks propagating from an introduced initial defect and formulate requirements for the choice of geometrical parameters of an initial concentrator. With the cyclic stress level being equal, short fatigue cracks (about 0.1 to 0.2 mm), have been found to originate sooner (with respect to the number of cycles to fracture) at a higher loading frequency. Institute of Problems of Strength, National Academy of Sciences of Ukraine, Kiev, Ukraine. Translated from Problemy Prochnosti, No. 2 pp. 67–74, March–April, 2000.     

Influence of oxygenation time on crack growth in titanium alloy under cyclic bending

We present the results of fatigue crack-growth tests for Ti–6Al–4V titanium alloy. Plane specimens 4 mm in thickness with a unilateral notch were tested. The analysis of crack growth was carried out for specimens without and after oxygenation process for 2 and 4 h. The fatigue crack growth rate was determined. It follows from the test results that the fatigue crack growth in the specimens without thermochemical treatment differs from the fatigue crack growth in the specimens after oxygenation. It has been also found that the applied surface treatment increases the fatigue life of the considered alloy.     

Effect of removal of crack lips on the characteristics of the crack resistance of structural alloys

We develop a special procedure and perform experimental investigations of the influence of crack lips on the effect of crack closure under cyclic loading. Results of testing two types of steel and an aluminum alloy for cyclic crack resistance are analyzed for the cases of symmetric and asymmetric loading cycles and the case of symmetric cyclic loading accompanied by removal of material affecting the degree of crack closure from the crack lips. Institute of Problems of Strength, National Academy of Sciences of Ukraine, Kiev, Ukraine. Translated from Problemy Prochnosti, No. 5, pp. 70–77, September–October, 1999.     

Determination of the characteristics of distribution of the fatigue limits of materials on the basis of the energy approach

We propose a method for the determination of the mean value of the fatigue limit of a material and its mean-square deviation according to the results of standard fatigue tests. The interpretation of these results from the viewpoint of energy enables us to establish the relationship between the individual durability of specimens and their potential ability to withstand fatigue fracture in the form of individual fatigue limits. Institute for Problems of Strength, National Academy of Sciences of Ukraine, Kiev, Ukraine. Translated from Problemy Prochnosti, No. 4, pp. 28–33, July–August, 1998.     

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.     

On some criterial problems of fatigue crack initiation and growth in polycrystals

We address a relationship between mechanisms and direction of a fatigue crack growth at stages I and II. The criteria for the fatigue crack growth direction at those stages are substantiated based on a study of physical and mechanical features of the fatigue crack propagation in polycrystals. We discuss the conditions under which microcracks may initiate brittle fracture. __________ Translated from Problemy Prochnosti, No. 4, pp. 5–24, July–August, 2008.