Loading Cycle Asymmetry Research Articles

Influence of load cycle asymmetry on the fatigue resistance of ÉP202 and VNS-25 alloys at different loading frequencies

The results of investigation of the influence of load cycle frequency and asymmetry, test base, and stress concentration on the fatigure limit of EP202 nickelbase alloy and VNS-25 maraging steel are presented. It is shown that in symmetric apd asymmetric loadings the effective stress concentration factors do not depend upon loading frequency. It is established that the diagrams of limiting amplitudes to relative coordinates approximate the results of tests by common curves with the same parameters for all loading frequencies.

Effect of loading cycle asymmetry, stress concentration, and fretting corrosion on the fatigue resistance of alloy D16T

Experimental data are presented for the fatigue strength of smooth specimens, specimens with a stress concentrator, and specimens under fretting corrosion conditions for alloy D16AT at three levels of loading cycle asymmetry coefficient R = −1, 0, and −0.5. It is established that fatigue resistance for alloy D16AT under fretting corrosion conditions is governed by the amplitude of operating stresses but it is practically independent of changes in loading cycle asymmetry coefficient. The possibility is demonstrated of calculating the fatigue limit for specimens with a stress concentrator with different loading cycle asymmetry from known values of the threshold stress intensity factor, the theoretical stress concentration factor, and the length of an nondeveloping crack at fatigue limit. Limiting stress amplitude diagrams for smooth specimens under fretting corrosion conditions have a different nature for the relationship between amplitude and average stresses of the loading cycle and therefore they cannot be described by a general analytical expression.

Effect of loading frequency and cycle asymmetry on the fatigue resistance of alloy AMg, 6N

Results are given for experimental determination of the fatigue resistance characteristics of alloy AMg6N with symmetrical and asymmetrical loading in the frequency range 35–10,000 Hz. Dependences are suggested for describing fatigue curves and limiting amplitude diagrams taking account of structural and operating factors making it possible to predict cyclic strength characteristics for the material at low loading frequencies from data for high-frequency fatigue tests.

Effect of load-cycle asymmetry and specimen thickness on the kinetics of the plastic zone in alloy D16

Effect of load-cycle asymmetry and specimen thickness on the kinetics of the plastic zone in alloy D16

Effect of the temperature on the crack resistance characteristics of steel with different strength levels

1. On the basis of the experimental investigation of the effect of the test temperature (153–293°K) on the rate of FCG in steels IP-1, IP-2, and IP-3 with a coefficient of load cycle asymmetry R=−2, −1, 0, and 0.5 it was established that lowering of the test temperature has an ambiguous effect on the rate of fatigue crack growth in the mentioned steels. In most cases the rate of FCG is practically insensitive to the test temperature although we can see a general tendency of the coefficient m of the Paris equation increasing with the test temperature being lowered from 293 to 153°K. 2. A change of the coefficient of load cycle asymmetry in the range −2–0 does not have a substantial effect on the rate of FCG, and in the range 0–0.5 it reduces this rate (in coordinates dl/dN-Kmax) at 213 and 293°K, particularly substantially at 213°K. 3. For the investigated chrome-nickel-molybdenum steels in the temperature range 293-153°K a single dependence was established; it describes the decrease of the coefficient m with rising level of fracture toughness under static loading. 4. With the test temperature rising from 113 to 153°K, the characteristics of fracture toughness of all the investigated steels increase monotonically under static and cyclic loading, and also in the case of stopping of the crack. 5. Cyclic loading reduces substantially (to one half) the fracture toughness of steels IP-1 and IP-2 in the temperature range 113–153°K and does not change the values of K1 fc compared with KIc for steel IP-3. 6. In steels IP-1, IP-2 at temperatures of 113–153°K the fracture toughness under cyclic loading corresponding to final fracture of the specimen practically coincides with the fracture toughness at the instant of stopping of the crack. 7. In the temperature range 100–183°K of the three investigated steels steel IP-1 has the highest resistance to brittle failure under static loading and at the instant of stopping of the crack, steel IP-2 has the lowest resistance.

Effect of the loading cycle asymmetry on the cyclic cracking resistance characteristics of heat-resisting steels

1. In the representation of the experimental data on the relationships of fatigue crack growth in the da/dN-KI max coordinates, the increase of the stress ratio from 0.1 to 0.75 greatly reduces the rate of fatigue crack propagation in medium- and high-amplitude sections for all the steels examined and may exert an ambiguous effect on the relationships of fatigue crack propagation in the near-threshold region. 2. The Paris dependence satisfactorily describes the kinetics of fatigue crack propagation at various values of the stress ratio only in the medium-amplitude section of the da/dN-ΔKI diagram. Use of this dependence at high values of SIF is restricted for 15Kh2MFA (I) steel by the presence of the section of the insensitivity of the rate of fatigue crack propagation at the variation of SIF, whereas for 08Kh18N10T steel it is limited by the development of nonconstricted plastic strains at the tip of the fatigue crack preventing application of the parameters of linear fracture mechanics. 3. An increase in R from 0.1 to 0.75 for the 15Kh2MFA (I) and 15Kh2MFA (II) steels has no effect on the critical stress intensity factor in cyclic loading, but it greatly reduces the critical rate of fatigue crack propagation (da/dN)c. The quantity da/dNc can be accepted as the characteristic of the transition from fatigue to brittle failure and is sensitive to the load cycle asymmetry. 4. The dependence of the rate of fatigue crack propagation on the range of the opening displacement of the crack tip for all the specimens examined is invariant to the load cycle asymmetry both in the elastic and elastoplastic region.

Effect of high temperature and load cycle asymmetry on the growth rate of fatigue cracks in stainless steel, nickel, and titanium alloys

Effect of high temperature and load cycle asymmetry on the growth rate of fatigue cracks in stainless steel, nickel, and titanium alloys

Effect of the dimensions of specimens and of cycle asymmetry on the regularities of unstable crack propagation under cyclic loading

1. In unstable fatigue crack propagation in embrittled steel 15Kh2MFA (II) the dimensions of the specimens and load cycle asymmetry do not affect the critical values of the stress intensity factors, the length of the section of stable crack growth, and the length of the brittle jump of the crack. 2. The dimension of the brittle jump of the crack in embrittled steel 15Kh2MFA (II) is controlled by the zone of fatigue damage to the material at the crack tip. There the dimension of the zone of fatigue damage is determined by the maximal stress intensity factor in the cycle, and the number of load cycles necessary for fatigue damage to the zone is determined by the amplitude of the stress intensity factor.

Influence of crack closure and loading cycle asymmetry on kinetic fatigue failure curves at normal and low temperatures

Influence of crack closure and loading cycle asymmetry on kinetic fatigue failure curves at normal and low temperatures

Influence of load cycle asymmetry and overloading on fatigue crack growth rate in stainless steels

Influence of load cycle asymmetry and overloading on fatigue crack growth rate in stainless steels

Prediction of the effect of the asymmetry of the load cycle on the cyclic fracture toughness of structural alloys

1. The results show that the increase of the coefficient of load cycle asymmetry from 0.1 to 0.95 for the 15Kh2MFA (I) and 15Kh2MFA (II) steels at 293°K leads to a large reduction of the rate of fatigue crack propagation, if the results are presented in the dl/dN-Kmax coordinates. 2. In the examined range R(0.1–0.93), 15Kh2MFA (II) steel is characterized by unstable fatigue crack propagation which precedes final fracture of the specimen. 3. The cyclic fracture toughness (Kfc1)min of 15Kh2MFA (I) steel at 293°K does not depend, in the given scatter range, on load cycle asymmetry for the value of R varying from 0.1 to 1 and is equal to K Q max . For 15Kh2MFA (II) steel, the value of (Kfc1)min remains constant up to R=0.87. With a further increase of the coefficient of cycle asymmetry, the value of (Kfc1)min increases linearly to the level of KIc. 4. An increase of the coefficient of load cycle asymmetry from 0.1 to 0.95 is accompanied by the increase of threshold SIF Kth for the 15Kh2MFA (I) and 15Kh2MFA (II) steels. The value of Kth for 15Kh2MFA (II) steel at R≥0.87 coincides with the critical SIF (Kfc1)min. 5. An increase of the coefficient of load cycle asymmetry from zero to one at the identical values of Kmax reduces the critical rate of FCP in the 15Kh2MFA (I) and 15Kh2MFA (II) steels and at R=0.87 the rate of FCP in 15Kh2MFA (II) steel is equal to 10−10 m/cycle. 6. The rate of reduction of fracture toughness in cyclic loading depends greatly on cycle asymmetry. For example, while at R=0.1 the reduction of the fracture toughness of 15Kh2MFA (II) steel from KIc to (Kfc1)min takes place approximately within 4.104 cycles, at R=0.83–0.85 the identical reduction takes place within 3.106 cycles. 7. The results of the investigations of the cracking resistance of the 15Kh2MFA (I) and 15Kh2MFA (II) steels were used to proposean approach to predicting the effect of load cycle asymmetry on fracture toughness in cyclic loading.

A fatigue damage sensor and the basis of its use. Report 1

1. A design and method of preparation of a fatigue damage sensor have been developed and the basic characteristics of the sensor in regular loading considered. 2. It was shown that the possibilities of use of the sensor may be broadened by the use of deformation multipliers. The design of the deformation multipliers developed was considered and their characteristics are given. 3. The influence of loading frequency, temperature, and cycle asymmetry on the sensor readings was investigated. It was established that load cycle asymmetry in the investigated range of stresses does not influence the basic characteristics of the sensor. The ranges of change in frequency and temperature in which their influence on the sensor readings is insignificant were determined. 4. The possibility was shown of the use of the fatigue damage sensor for structural elements in the presence of stress concentration.

Influence of temperature and loading cycle asymmetry on the crack resistance characteristics of heat-resistant steels

1. The fatigue crack development rate in steels II, III in off-center tension of compact samples is practically the same. In the 213–673°K temperature range the fatigue crack development rate in off-center tension of samples of steel III (f=15 Hz) is higher than in cantilever bending of samples of steel II (f=25 Hz). Obviously, the reasion causing the decrease in the fatigue crack development rate in steel II in comparison with steel III is the difference in the loading methods — plane bending and off-center tension. 2. An increase in the test temperature from 293 to 673°K has an insignificant influence on the fatigue crack development resistance of steels I and III, but increases by 1.5–2 times the crack development rate in steel II in the medium-amplitude portion of kinetic fatigue failure curve. 3. At low temperatures recurrence of loading significantly reduces the fracture toughness characteristics K1fc and Kkfc of steels I, II, and III in comparison with the fracture toughness in static loading. For example, for steel II in the 183–213°K temperature range, KIc/K1fc=1.6–1.8. For steel I at room temperature, the value of K1fc agrees with the critical stress intensity factor Kc measured based on the maximum force Pmax. 4. At 213°K steel III has a higher brittle fracture resitance in static loading than steels I, II. From an analysis of literature data and also of microstructural investigations made in this work, it follows that the obvious reason for the reduction in KIc is the presence in steels I, II of larger nonmetallic inclusions than in steel III. 5. At room temperature steels I, II, and III have practically the same fatigue crack development resistance (in the whole range of its rate) and satisfy the ASME cyclic crack resistance requirements for shell materials.

Effect of strain history and load cycle asymmetry on the fracture toughness characteristics of alloy VT9

1. It was shown that the unidirectional accumulation of plastic strain during low-cycle loading leads to a reduction in the critical fatigue fracture toughness of alloy VT9. 2. The process of sign-changing reversible deformation does not change the critical value of the fatigue fracture toughness of alloy VT9, regardless of the strain amplitude. 3. The load-cycle asymmetry coefficient Rσ significantly affects both the amount of unidirectionally accumulated plastic strain and the critical stress intensity factor in the low-cycle fatigue region, lowering the value of the latter in the transition from pulsating loading (Rσ = 0) to symmetrical loading (Rσ = -1).

Effect of loading frequency, temperature, and cycle asymmetry on the endurance of heat-resistant steels 1Kh2M and Kh18N9. II

1. The application of a mean tensile load in tests of low-alloy steel causes a reduction in the endurance of the specimens throughout the investigated ranges of frequency and temperature. With an increase in loading frequency, endurance remain constant or increases slightly at all values of asymmetry. In tests of an austenitic chromium-nickel steel which undergoes strain hardening, the opposite effects are seen: At room and elevated temperatures, the application of a mean tensile stress of (0.2–0.4)σu may increase endurance by 15–20% compared to the case of symmetrical loading. The strain-hardening effect is especially evident with a reduction in loading frequency, so that the effect of the latter on endurance is inversed: in symmetrical loading, an increase in frequency increases endurance at room and elevated temperatures, while in asymmetrical loading endurance may decrease with an increase in loading frequency at elevated temperatures. 2. To evaluate endurance for the asymmetrical loading of low-alloy and austenitic steels at normal and high frequencies and room (in air) and elevated temperatures, the following well-known linear relation (Goodman curve) may be used: $$\sigma _\alpha = \sigma _{ - 1} \left( {1 - \frac{{\sigma _m }}{{\sigma _u }}} \right)$$ . The error which is possible with such an estimate increases the safety factor. 3. The fatigue strength of the investigated materials is lower in an air-water medium or in tap water than in air. The application of a mean tensile load within the range (0.15–0.25)σu sharply increases the damaging effect of the medium. The rate of decrease in endurance drops with a further increase in the mean load.

Effect of loading frequency, temperature, and cycle asymmetry on the endurance of heat-resistant steels 1Kh2m and Kh18N9. Part 1

Effect of loading frequency, temperature, and cycle asymmetry on the endurance of heat-resistant steels 1Kh2m and Kh18N9. Part 1