The effect of frequency on the cyclic strain and fatigue behavior of cast rené at 1600° F

Controlled strain fatigue tests have been carried out for two heat treatments of cast René 80 at 1600°F at several frequencies and strain ranges. Results were fitted to frequency-modified fatigue equations for elastic and plastic strains using regression analysis. Save for a more pronounced cyclic softening, little difference in fatigue behavior was found in the two heat treatments. This material exhibits a normal frequency behavior for the elastic strain component of the total strain, and an inverse frequency effect for the plastic strain component, resulting in a negligible frequency effect on total strain in the high strain, short life regime. This behavior is attributed to the persistence of a transgranular mode of crack propagation in preference to the more usual intergranular mode in all tests including those at frequencies as low as 0.03 cpm. On the other hand, crack nucleation was more characteristic of cast nickel-base superalloys at 1600°F, that is, by oxide ridging at surface grain boundaries. A low transition fatigue life was found, such that for fatigue lives greater than 1000 cycles, elastic behavior essentially prevails.     

The effect of one slow-fast strain cycle on the fatigue crack growth behavior of SUS 304 stainless steel at elevated temperature

Fatigue crack growth behavior of SUS 304 stainless steel when one slow-fast strain cycle was introduced to the fast-fast strain cycles was investigated at 600 °C. It was found that the fatigue crack growth rate in the fast-fast strain cycles was accelerated by the introduction of one slow-fast strain cycle. It was also found that there was a linear relationship between the size of the region where the acceleration occurred and the value of creepJ-integral range, ΔJ_C, which was produced just when one slow-fast strain cycle was introduced. To investigate the above acceleration, the longitudinal section through the specimen was observed. It was shown that the wedge-type cracks were produced at the grain boundaries and that they contributed to the above acceleration. Based on the results thus obtained, the predicting procedure of crack growth curve when one slow-fast strain cycle was introduced to the fast-fast strain cycles was proposed. As the result, it was shown that the predicted crack growth curve was in good agreement with the experimental one.     

Low cycle fatigue behavior of René 80 at elevated temperature

Low cycle fatigue of René 80 was studied at 871 and 982 °C. It was found that when the data were represented on the basis of plastic strain, the life increased with decreasing frequency and imposition of a 90 s hold at maximum strain. Transmission electron microscopy studies showed that the ′ coarsened and an interfacial array of edge dislocations developed. The density of dislocations in the matrix was very low. Light optical microscopy revealed that cracks generally inititiated at oxide spikes in surface connected grain boundaries. A crack initiation criterion based on the maximum stress and oxide depth at the time of crack initiation was found to represent the data very well. Based on that representation, an expression for the initiation fatigue life was developed. That expression includes temperature, frequency and cyclic stress strain parameters as variables. S. Liu, formerly Research Associate at the University of Cincinnati.     

The effect of frequency on the elevated temperature fatigue of a nickel-base superalloy

In this investigation, the effect of a frequency variation between 2 cpm and 6 × 10⁴ cpm on the 1400°F fatigue properties of wrought Udimet 700 was determined at a constant stress range of 85 ksi. It was found that a peak existed in the cyclic life vs frequency curve such that a) an increase in frequency from 2 to 600 cpm increased the fatigue life 100 times and b) an increase in frequency from 600 to 6 × 10⁴ cpm reduced the fatigue life sevenfold. The peak in the cyclic life vs frequency curve is the result of two competing processes: 1) there is a reduction in the effects of creep and oxidation with increased frequency that tends to increase the life and 2) there is an increase in the heterogeneity of deformation with increased frequency that tends to reduce the life. At low frequencies, crack initiation occurred at surface-connected grain boundaries. Crack propagation was initially intergranular and then proceeded noncrystallographically normal to the stress axis (Stage II mode). Crack initiation at high frequencies occurred at subsurface brittle phases located at grain boundaries or at the intersection of coherent annealing twin boundaries. Crack propagation was entirely transgranular, proceeding initially along twin boundaries or slip bands (Stage I mode) and then changing to the Stage II mode. The statistical nature of the fracture process, the significance of subsurface crack initiation, and the relation of these results to existing high temperature fatigue models are discussed.     

The effect of strain rate and depressed temperature on the low cycle deformation behavior of alpha iron

The low cycle deformation saturation stress in Ferrovac-E a-iron was studied using diametral plastic strain (0.001 ≤ Δε_dp/2 ≤ 0.0135) as the control variable. Increasing strain rate (6 × 10^•5 s^•1 • 4 × 10^•3 s^•1) and decreasing temperature (295 to 173 K) increased the saturation stress levels. The cyclic work hardening coefficient decreased from 0.18 at 295 K to 0.10 at 173 K, which is consistent with previous studies of monotonie deformation. The temperature dependence of both the saturation stress and the strain rate sensitivity, as measured during cyclic deformation, were similar to that measured during monotonic tensile tests. The temperature dependence of the dislocation velocity indexm* was in good agreement with published values from high cycle fatigue and monotonie tensile tests. Thus the same deformation mechanisms are believed to occur in both monotonie and large plastic cyclic deformation (Δε_dp/2 ≥ 0.001) of a-iron.     

Influence of time and temperature dependent processes on strain controlled low cycle fatigue behavior of alloy 617

Strain controlled low cycle fatigue tests have been conducted in air to ascertain the influence of strain rate(ε = 4 × 10^-6'to 4 × 10^-3 s^-1) and temperature(T = 750/850/950 °C) on LCF behavior of Alloy 617. A strain range of 0.6 pct and a symmetrical triangular wave form were employed for all the tests. Crack initiation and propagation modes were studied. Microstructural changes that occurred during fatigue deformation were evaluated and compared with the results obtained on isothermal aging. Deformation and damage mechanisms which influence the endurance have been identified. A reduction in fatigue life was observed with decreasing ε at 850 °C and with increasing temperature at ε = 4 × 10^-5 s^-1. Cyclic stress response varied as a complex function of temperature and strain rate. Fatigue deformation was found to induce cellular precipitation of carbides at 750 and 850 ‡. Dynamic strain aging characterized by serrated flow was observed at 750 °C (ε = 4 × 10^-5 s^-1) and in the tests at higher ε at 850 °C. Strengthening of the matrix due to dynamic strain aging of matrix dislocations by precipitation of M₂₃C₆ carbides led to fracture of grain boundary carbide films formed at 750 °C, producing brittle intergranular crack propagation. At 850 °C transgranular crack propagation was observed at the higher strain rates ε≥4× 10^-4 s^-1. At 850 and 950 °C even at strain rates of 4 × 10^-5 s^-1 or lower, life was not governed by intergranular creep rupture damage mechanisms under the symmetrical, continuous cycling conditions employed. Reduction of endurance at lower strain rates is caused by increased inelastic strain and intergranular crack initiation due to oxidation of surface connected grain boundaries. formerly Guest Scientist at the De-partment for Reactor Materials of the Nuclear Research Centre, Juelich (IRW/KFA),     

The high temperature low cycle fatigue behavior of the nickel base alloy IN-617

Fully reversed strain controlled LCF tests were performed on 4.5 mm thick sheet specimens IN-617 at 1033 K and 1144 K in air. The strain-life and cyclic stress-strain data were analyzed parametrically. While the inelastic strain amplitude-life relationships are similar at the two temperatures, the softer cyclic stress-strain relationship observed at 1144 K produces inferior fatigue resistance to that at 1033 K when comparisons are based on stress or total strain amplitude. Transmission electron microscope observations suggest that grain boundary sliding was the primary mechanism of deformation at 1144 K while at 1033 K intragranular slip produced a dense dislocation substructure which was stabilized by fine scale precipitation of M₂₃C₆. At 1144 K, grain boundary migration occurred within the specimen interiors, producing a cellular precipitation of M₂₃C₆ as well as the intragranular M₂₃C₆ observed at 1033 K. Cellular precipitation was not observed in the near surface regions. This is attributed to oxygen penetration along grain boundaries. Comparison of the data with mean stress-HCF data indicates that the mean load significantly reduces fatigue resistance at both temperatures.     

The room temperature fatigue behavior of nickel-base superalloy crystals at ultrasonic frequency

Previous investigations have invariably observed strain rate related deformation effects as the fatigue frequency is raised to the ultrasonic range. Through room temperature tests on strain rate insensitive nickel-base superalloy single crystals of Mar-M200, we have shown that another effect of increasing the fatigue frequency to the ultrasonic range is in the suppression of the deleterious influence of environment. It was found that above a stress amplitude of 30,400 psi the fatigue lives of crystals ultrasonically fatiguedin air increase with decreasing stress in a manner which is functionally similar to, that of crystals conventionally fatiguedin vacuum. Similarly, the fracture surfaces of ultrasonically fatigued crystals have a dimpled appearance over most of their areas which is characteristic of locally ductile failure and identical to, the appearance of crystals failed at conventionally frequency in vacuum. These results, along with a kinetic analysis of gaseous adsorption, indicate that the major effect of increasing the fatigue frequency to the ultrasonic, range is in the suppression of the influence of oxygen in enhancing the rate of crack propagation. In addition, the short test times involved in running large numbers of cycles have allowed for the determination of the fatigue limit in a nickel-base superalloy. This is the first indication of no-fail behavior in this type of alloy.     

Low cycle fatigue behavior of Ni-Nr lamellar eutectic composites at elevated temperatures

Low cycle fatigue properties of unidirectionally solidified lamellar eutectic Ni-51 Cr alloy were determined and compared with those of the cast microstructure in the temperature range of 300° to 760°C. Both materials exhibited an initial cyclic strain hardening followed by saturation over most of the temperature range. The rate and the amount of cyclic work-hardening decreased with temperature above 600°C. Rapid softening due to macro-crack propagation occurred at later stages of the fatigue process, which occupied an increasing portion of the fatigue life in the lamellar material as the strain amplitude was decreased. At Δ∈_T = 0.0190, the lamellar material exhibited longer fatigue life over the entire temperature range which has been related to the ability of Cr-rich lamellae to deflect fatigue cracks. At 625°C, the fatigue life (Nf) of both materials was related to the plastic strain range ( Δ∈_P) through the relationship (Δ∈_P/2 =K(2N_f)^c wherec andK are -0.39 and 0.068 for the lamellar, and -0.45 and 0.074 for the cast structure, respectively. At this temperature with decreasing strain amplitude lamellar material became more resistant to fatigue than as-cast structure, which has been related to the more efficient deflection of fatigue cracks by Cr-rich lamellae at lower strain amplitudes . Formerly with the Dept. of Metallurgical and Materials Engineering, University of Pittsburgh, Pittsburgh, Pa. Formerly Visiting Scientist, Department of Metallurgical and Materials Engineering, University of Pittsburgh Formerly Professor and Chairman, Department of Metallurgical and Materials Engineering, University of Pittsburgh     

The effect of high temperature low cycle fatigue on the corrosion resistance of austenitic stainless steels

The effect of high temperature (650 °C) low cycle fatigue on the corrosion behavior of five austenitic stainless steels (Types 304, 316L, 321, and Incoloy Alloys 800 and 800H) has been investigated. For comparison, corrosion tests were also performed on samples of as-received material as well as material which had been solutionized and material which was sensitized at 650 °C. It was observed that cyclic loading at high temperature reduces the corrosion resistance to a much greater extent than does just the exposure of unstressed material to elevated temperatures. Formation of chrome carbides during cycling and depletion of chromium from the matrix is responsible for the decrease in corrosion resistance. Of the alloys tested, Type 304 exhibited the lowest corrosion resistance. Superior corrosion resistance of the other alloys was due to the following: (a) a lower carbon content, (b) a higher chromium content, and (c) the presence of a strong carbide forming element (stabilized material).     

Crack propagation of steels during low cycle thermal-mechanical and isothermal fatigue at elevated temperatures

The surface and through crack propagation of three high temperature steels in low cycle thermal-mechanical and isothermal fatigue at elevated temperatures was investigated. The rate of crack propagation obtained was correlated with the range of cyclicJ-integral, ΔJf. It was found that there is a linear relationship on alog-log plot regardless of materials, test conditions, and crack configurations. Furthermore, fatigue life predicted by integrating the equation of crack propagation was compared with experimental results.     

The effect of high vacuum on the low cycle fatigue law

Push-pull fatigue tests have been conducted on several materials at various frequencies and temperatures in air and high vacuum (10⁻⁸ torr) and the fatigue life determined in terms of the cyclic plastic strain. In contrast to a changing exponent of the Coffin-Manson law with increasing temperature in air, in high vacuum this exponent is found to remain nearly constant at a value of about 0.5. Further, the temperature sensitivity of this exponent and of life at a specific plastic strain range in high vacuum is slight. Pronounced plastic instability (specimen shortening and fattening) was observed for the ductile metals investigated and crack nucleation was retarded. In all cases crack propagation was transgranular in vacuum. It is concluded that for the materials, temperature, and frequencies investigated, the degradation of fatigue life at elevated temperature is due to environmental enhancement of intergranular fracture. Materials investigated include A286 at room temperature and 593°C, nickel A at 550°C, 304 stainless steel at 816°C and 7075T6 aluminum alloy.     

Low cycle fatigue behavior of polycrystalline Ni3Al alloys at ambient and elevated temperatures

The low cycle fatigue (LCF) resistance of polycrystalline Ni₃Al has been evaluated at ambient, intermediate (300 °C), and elevated (600 °C) temperatures using strain rates of 10⁻²/s and 10⁻⁴/s. Testing was conducted on a binary and a Cr-containing alloy of similar stoichiometry and B content (hypostoichiometric, 200 wppm B). Test results were combined with electron microscope investigations in order to evaluate microstructural changes during LCF. At ambient and intermediate temperatures, the cyclic constitutive response of both alloys was similar, and the LCF behavior was virtually rate independent. Under these conditions, the alloys rapidly hardened and then gradually softened for the remainder of the life. Initial hardening resulted from the accumulation of dislocation debris within the deformed microstructure, whereas softening was related to localized disordering. For these experimental conditions, crack initiation resulted within persistent slip bands (PSBs). At the elevated temperature, diffusion-assisted deformation resulted in a rate-dependent constitutive response and crack-initiation characteristics. At the high strain rate (10⁻²/s), continuous cyclic hardening resulted from the accumulation of dislocation debris. At the low strain rate (10⁻⁴/s), the diffusion of dislocation debris to grain boundaries resulted in cyclic softening. The elevated temperature LCF resistance was determined by the effect of the constitutive response on the driving force for environmental embrittlement. Chromium additions were observed to enhance LCF performance only under conditions where crack initiation was environmentally driven. Formerly Postdoctoral Research Fellow, School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA     

GH4720Li合金中温区低周疲劳行为研究

奥氏体沉淀强化GH4720Li合金用于制备航空发动机涡轮盘,其服役条件下的疲劳性能是当前研究的热点.研究了GH4720Li合金在中温区(450和550 ℃)不同应变条件下的低周疲劳行为.结果表明,GH4720Li合金的剪切模量和杨氏模量随着温度的升高而不断降低,但是中温区的抗拉强度和屈服强度不随温度而剧烈下降.GH47...     

The effect of alloy chemistry and heat treatment on the low cycle fatigue behavior of 7000 series aluminum alloys

The purpose of the present investigation is to determine the relative importance of minor variations in alloy chemistry and thermomechanical treatment on the low cycle fatigue behavior of 7000 series aluminum alloys. Two types of alloying variations are considered: changing the alloy purity level by controlling the iron and silicon content, and changing the grain refiner from chromium to zirconium. The effects of these alloying variations, with regard to mechanical properties other than low cycle fatigue, have been discussed elsewhere.^1-4The purpose of thermomechanical processing is to provide increased strength over 7075-T7351 with equivalent fracture toughness and corrosion properties.^5-7 The effect of the dislocation substructure introduced by thermomechanical processing (TMP) on the high cycle fatigue behavior of 7075 was documented by Reimann and Brisbane.⁸ The present work was undertaken to determine the relative importance of purity level, dispersoid type, and dislocation substructure (TMP) on the low cycle fatigue behavior of 7000 series aluminum alloys. formerly with the Air Force Materials Laboratory, Wright-Patterson AFB, OH     

The cyclic deformation and fatigue behaviour of the low carbon steel SAE 1045 in the temperature regime of dynamic strain ageing

The cyclic deformation behaviour of normalized SAE 1045 steel (german steel grade Ck 45) has been investigated over a range of temperatures between 20 and 375°C. Special attention has been paid to the effects of dynamic strain ageing, which are most pronounced around 300°C. Different types of deformation tests (tension tests, incremental step tests, and constant amplitude cyclic deformation tests under stress control with a stress amplitude of 400 MPa as well as under plastic strain control with a plastic strain amplitude of 0.5%) were carried out to observe the influence of temperature on the macroscopic mechanical behaviour. These tests were followed by TEM studies on microstructural features. In the temperature range of maximum dynamic strain ageing, the material was found to show maximum strength in unidirectional as well as in cyclic deformation tests. While the fatigue life is maximum at the temperature of maximum dynamic strain ageing in stress-controlled tests, it is minimum in plastic strain controlled tests. At the temperature of maximum dynamic strain ageing around 300°C, the dislocations are arranged in dense dislocation tangles and parallel dislocation walls, whereas at room and at higher temperatures (375°C) mainly dislocation cell structures are observed.     

Low cycle fatigue hold time behavior of cast rené 80

Strain-time programs involving continuous cycling, equal hold times in tension and compression, tensile strain hold periods, and compressive strain hold periods are compared for cast René 80 at 1600°F. It was found that the predictive equations for high-temperature fatigue give good agreement with experiments in which the hold times are equal. Contrary to expectation, however, tensile strain hold experiments gave longer liver than predicted, wile compressive strain hold period experiments were shorter than the predictions. Concurrently, the hysteresis loops from these two sets of experiments exhibited pronounced and opposite mean stresses, the tensile strain hold tests showing a compressive mean stress shift. These shifts were explainable in terms of the times spent in tensile and compressive loading, and could account qualitatively for the observed life behavior.